Bypass cooling for the 6.2/6.5

[More Power]

The concept for “bypass cooling” has been around for years, and it usually comes up here in this BB forum about once a year. I’m not a fan of the idea. Let me explain...

http://www.thedieselpageforums.com/photopost/data/500/bypasscooling05c.jpg

First, what is "Bypass Cooling"? The 6.2/6.5 cylinder heads are interchangeable right for left. Among other features, this provides a convenient coolant port with a block-off plate at the inside rear of each right & left cylinder head. GM produced a somewhat different block-off plate (pn-14028949) for use with their 6.2L diesel engine. These 6.2L block-off plates incorporate a 1/2" NPT pipe thread fitting, which was used for a Glow Inhibit switch on the passenger side cylinder head and for the 1982-83 screw-in glow controller located at the rear of the driver's side cylinder head.

Some feel these 6.2L block-off plates could be installed on a 6.5L diesel, and would provide a good place to mount a pair of 1/2" hose barbs to pull coolant out of the head, and re-route that coolant to somewhere near the thermostat housing. The arrangement allows a portion of the coolant to "bypass" traveling through the cylinder head. Some have installed such a modification and saw a reduction in engine coolant temperature, as seen by the engine coolant temperature sensor located in the coolant crossover at the front of the engine near the thermostat housing. This is a problem. If the coolant isn't picking up as much heat in the heads as it should, of course, the ECT (engine coolant temperature) sensor will report a decrease.

GM’s “6.2 Liter Diesel Engine” product service training manual #16015.05-1D says this about the Cooling System Schematic.

“Coolant is drawn from the lower radiator hose by the water pump. The pump pushes coolant into both sides of the cylinder block. Coolant flows around the cylinders and up into the heads, where it circulates around the exhaust passages and fire deck areas, then flows forward toward the thermostat housing.”

For the following images and discussion, let's assume we're looking at the passenger (right) side of the engine.

http://www.thedieselpageforums.com/photopost/data/500/bypasscooling02.jpg

This head gasket image shows the ports (highlighted in green) where coolant can pass from the block, through the gasket, and up into the heads. The rectangular passage shown on the right is not used at the front of the engine (ahead of cyl 2 on the right bank, ahead of cyl 1 on the left bank). The cylinder block does not contain a coolant passage at that location.

The rectangular coolant port on each head gasket (behind cyl #8 on the right bank & behind cyl #7 on the left bank) each passes approximately 50% of the coolant flow for that side of the engine, which then travels forward through the heads. The remaining 50% of the head coolant flow comes from the other smaller round ports in the gasket – again, highlighted in green.

There are a series of tiny holes in the head gasket (green dots) that allow a small fraction of the coolant to pass into each head, which are there primarily to allow trapped air and steam to escape.

http://www.thedieselpageforums.com/photopost/data/500/bypasscooling03.jpg

Head gaskets used on the 6.2/6.5 are identical for both right & left banks. If the above cylinder head was to be installed on the passenger side of the engine, only those coolant passages outlined in red are allowing coolant to pass from the block and into the head.

http://www.thedieselpageforums.com/photopost/data/500/bypasscooling01.jpg

This photo shows the front portion of the passenger side block deck. Cylinder #2 is at the right side of the photo. You can see that there is no rectangular coolant passage matching the head gasket (red X).

I believe most people who think taking coolant out at the back of the heads improves engine cooling come to that conclusion after seeing the several large coolant passages in both the block and head decks. Problem is, most of them are blocked off by the head gasket. Those large passages are there to facilitate manufacturing the sand-cast heads & blocks.

http://www.thedieselpageforums.com/photopost/data/500/bypasscooling04.jpg

This photo shows the passenger side block deck cylinders #6 & #8. The larger rectangular cooling passage is behind cylinder #8, which matches the port in the head gasket. Some of you have noticed the scored cylinder wall in cylinder #6, and have asked how that occurred. I used this photo primarily to show the cooling passages, but that picture does have a story... The condition of #6 (and #5 on the opposite bank) was the result of 1500 degrees EGT and 250 degrees engine coolant temperature. As you can see, high coolant temperatures do not always affect the rearmost cylinders, which is an argument used by those who promote bypass cooling.

In conclusion: Taking coolant out at the back of the heads reduces the amount of coolant flowing forward and through the heads - plain and simple. That cannot be argued. The cylinder heads generate most of the heat in a running engine, and the cooling system has to move enough coolant through them to prevent damage. If a bypass cooling strategy is used, you may see a lower engine coolant temperature, which is what you'd expect if the coolant isn't picking up as much heat as it should in the cylinder heads. Not removing heat from the cylinder heads could result in hot spots that develop in the area around the exhaust valves and exhaust runners, which won’t be reflected by the engine temperature sensor. I’m open to third party data and test results, but till then my opinion is that bypass cooling may actually contribute to cylinder head cracking - engine damage...

Jim

[Kennedy]

I saw the title and thought oh boy here we go new product:rolleyes: . But then again, I knew better.:cool: Excellent writeup. You really did your homework here! No bypass BS for me...

[DA BIG ONE]

I can't think of anything that would have laid it out (exposed) any better to understand.

A BIG Thanks

[Robyn]

Great writeup !!!
Another thing to think about??? After some time and miles many of the small holes can become plugged. When I took my 94 apart this summer there were several of the tiny holes that were clogged and so what does that do to the overall battle plan?
Seems that the front 2 cylinders would see more failure due to heat rather than the rear. The cooler water is at the rear and as it flows forward it heats up more and this would translate into hotter firedecks for 1-3 and 2-4
Is this the case???

Again great tech
Robyn

[grape]

x marks the spot you have to drill in the block to make it work.........lol.

[DmaxMaverick]

x marks the spot you have to drill in the block to make it work.........lol.

Nope. Lol.

The 6.5's, especially TD's, have issues with rear cylinder heat anyway. Evidenced by the rear cylinder overbore and increased clearances, and they usually seize/score before the others. Moving coolant from front to rear would just exaggerate that.

[Subzilla]

Administrators, why do I get no picture and "www.thedieselpage.com" in white squares? An inquiring mind wants to know.

[DmaxMaverick]

Administrators, why do I get no picture and "www.thedieselpage.com" in white squares? An inquiring mind wants to know.

Simple solution. PM or email me with your OS and browser versions (XP, 98, MacOS-X, firefox, Internet Explorer, etc), and the link (address) you use to access the website.

[damork]

I agree with not bypassing and had first hand experience with another fleet of GM engines - the 53 & 71 series 2 cycles. I worked across the middle east during the oil boom of the late 70's and early 80's and the 2 cycle Detroit Diesels were very common, we had several 4-53, 6V-53 and 8V-71 engines in our fleets.

When cooling became an issue our mechanics started pulling thermostats and rerouting coolant from the back of the heads. Just like what is mentioned here - the heat was not getting carried out of the engine as designed and made the problem worse. Coolant passages are designed to utilize the hardware in a certain way and if bypass was necessary it would have been done at the factory.

[moody]

Any chance the origanal post could be saved as a sticky on the 6.2 tech forum or made into an artical for the diesel page? Great work and pictures.

[grape]

Nope. Lol.

The 6.5's, especially TD's, have issues with rear cylinder heat anyway. Evidenced by the rear cylinder overbore and increased clearances, and they usually seize/score before the others. Moving coolant from front to rear would just exaggerate that.


you act like i'm putting the water from the water pump in the block in the stock location..........lol

and $50k engines have the cylinders closest to the fresh water entering the block set looser to make up for the cooler water hitting the OD of the sleeve..........guess it's always different on these high tech diesels.......

[DmaxMaverick]

you act like i'm putting the water from the water pump in the block in the stock location..........lol

and $50k engines have the cylinders closest to the fresh water entering the block set looser to make up for the cooler water hitting the OD of the sleeve..........guess it's always different on these high tech diesels.......

I'm familiar with the design you refer to, and it is warranted for the application they are intended for. But.....Those engines are of different design than what we are talking about here. That, and the high dollar engines use the same material for the block and pistons. The reason the greater clearance is needed on a 6.5 cylinder bore is the dissimilar metals. Aluminum (piston) expands at a much higher rate and extreme than the iron cylinder wall and deck when heated. The greater the heat, the greater the expansion differential.

Good theory, wrong argument, IMO.

[grape]

a $50K SB2.2 has a GM iron block and sleeves and doesn't have iron pistons......lol.

[DmaxMaverick]

OK. I was refering to a different $50K block. Lol. Anyway, they are of different design and use than what we are discussing here. A sleeved engine also has different characteristics, and they vary depending on wet/dry sleeves and head design, among other things. The fuel type is also a consideration. #2, even in our very modest GM V-8 Diesels, have a considerably higher combustion tempurature than a high revving gasser burning gasoline or methanol/nitro. The duty cycles and environments also vary. The heat needs to be managed differently.

I'd love to engage a discussion along the lines you bring up. Start another thread, and you can educate some of us, seriously. The similarities and differences between daily driver and towing truck engines compared to professional competition engines could use some definition. Some pricipals apply to both, and others are unique. It wouldn't hurt to separate them.

[More Power]

On a related note: The fourth image is of a 6.5 block that had a problem running the hill during our Pull-Off in 2002. You can see that the piston in cylinder #6 was damaged along with the cylinder. What is also visible is that #8 was undamaged.

Whether it was a competitive driver or just too much going on to keep track of, the engine temperature crossed 250 degrees and EGT went above 1500 during that run. Also, one of the other photos used here was of the very same rebuilt shortblock with new ceramic coated 18:1 pistons... :)

Jim

[ronniejoe]

I'm pretty sure what Grape is doing is pulling water out of the pump, piping it down the outside of the block and bringing it into the block in the center of each bank, probably through a freeze plug hole or other newly machined inlet port. Then, by opening up the transfer passage in the front of each deck surface, he can allow the coolant to flow from the center of each bank forward and rearward, then up through the heads and out at both ends. He's probably added a larger passage to pass coolant to the heads in between the center two cylinders on each side as well. This way he gets nearly parallel flow, rather than the series loop that the factory has. This approach has to be carefully metered to balance all the flows to avoid hot spots. If done properly, it will maintain more even temperature distribution throughout the enigne.

Just guessing.

[aloharovers]

So why doesn't the head gasket have holes in it to match up these coolant passages? (in blue)

[HammerWerf]

Aloha Rover,

After reading your post, I took a really close look at the picture of the gasket that MP highlighted the coolant passeges in green. They are hard to see, and very small, but there are holes that corrispond to the cleanout holes in the block and head.

MP indicates that these tiny holes are to allow air and steam to pass from the block to the head for removal from the block.

HammerWerf

[More Power]

HW, you are correct. And, those little holes are usually plugged with stuff not all that long after the engine is put into service.

Jim

[aloharovers]

ok I see them now. So would it make any sense to make them a little bigger? Like maybe run a 1/8 or 3/16 drill bit through them?

[More Power]

I wouldn't modify (enlarge) the small holes in the head gaskets we're talking about there. Larger holes could alter coolant flow patterns through the heads in unexpected ways.

Incidentally, any air pockets that remain in an engine will slowly be absorbed by the water/coolant mixture. Over a few days or a couple weeks, the air will disappear. This assumes the air pocket is below the uppermost coolant level in the cooling system.

Jim

[grape]

i make the holes bigger but that's cause i'm moving water up, not back-up-then forward.

[Cowracer]

I think that removing water from the heads is a big mistake. There are others that disagree.

this will do NOTHING to help prevent a cracked block. The cracks form at the bearing journal, which is far way from the cooling jackets.

the blocks aint cracking because they are getting hot.

Tim

[rustyk]

"Bypass cooling" is, to me, an oxymoron. GM engineers might not be the brightest lights in the chandelier, but I'd go with their cooling scheme anyway.

[toyboxrv]

I have done this particular modification well over 100k miles ago with only positive results. I did not do this because I felt the large holes could pass water. I saw that besides the large passage at the rear of the block, only the small holes between the cylinders pass water to the heads. The water is coolest in the block closest to the water pump inlets. The water is hottest when it passes through the heads and exits at the thermostat crossover. The small holes helps to balance the temperature in the heads by using the coolest water in the block with the hotter water in the heads.

It was explained to me that the larger passage in the back flows more of the hot water, which like all hot things is trying to expand. That limits flow between the head and block at the smaller holes. Reducing flow of the hotter water allows more of the cooler water to pass from the block upwards to the heads.

Before installing this setup I was seeing temps on the dash gauge of better than 210 on a hill coming into Santa Fe, NM, an elevation of 7,000, with outside temps of around 90. I also had a gauge installed on the passenger head at the rear and temps there were showing as high as 235. After making the change I could keep temps at 210 on the dash gauge and around 220 on the other one. Every place where I towed and saw higher temps, I now have lower temps and less differential between the front and back of the engine.

It seems that most of those opposed to this idea are doing so based on their theories. When they have some real experience with the idea and can show it will negatively affect engine life, I may consider their experiences. Right now I can only go with what I have observed myself and it doesn't seem to conform to the theories and speculation.

I spent around $200 doing the setup using braided lines and having fittings welded to the thermostat crossover. I have an extra crossover so that I could have put things back to stock if I felt it wasn't working. Haven't seen a need to change so far.

[More Power]

GM increased the water pump flow rate from 87-gpm to 130-gpm primarily to increase coolant flow through the heads in an effort to reduce the incidence of cylinder head cracking. That's not a theory. By-pass cooling reduces coolant flow through the length of the heads. That's not a theory.

There are a couple of different vendors who produce these kits. I spoke to one of them a couple of years ago about by-pass cooling. I offered to travel to them, at my own expense, to witness an instrumented test using at least three thermocouples on one head. I was not invited. If I sold and believed in these kits, I would welcome/encourage an opportunity to prove their worth.

I would like to see the cyl head temperature between the valves on each of the end and center cylinders during a sustained full load (long grade, heavy trailer, full pedal or 1250 degrees EGT). If the temperature(s) could be shown to be lower at each thermocouple with by-pass cooling, I'd write a story and advertise that product at no cost to the vendor. We are, after all, here to help 6.5 owners. ;)

Jim

[DA BIG ONE]

Jim, I've been wondering if those openings at back of heads are there for marine applications where the body of water is the cooling medium, or?

[More Power]

The coolant crossover uses the openings at the front of each head. If you swapped the heads right for left on the block, the opposite end of each head would now be at the front. So, the heads contain openings at both ends to allow them to be used on either bank.

The single most important coolant temperature is that temperature within the cylinder heads. Bypassed coolant isn't picking up heat in the heads. So, of course, the coolant will show a lower temperature.

Jim

[toyboxrv]

I do have the higher output water pump and dual thermostat on my 94 as well as the Kennedy fan clutch and Dmax fan. My temps are more under control than when the rest of the engine was stock and much better than after doing some mods.

My observation was that the output from the water pump is a larger hole than the output from the crossover. The flow from the crossover and bypass lines, in my opinion, is not more than the flow from the water pump. The crossover itself is a restriction to flow and as I stated earlier, the hotter water in the front cylinders of the heads can reduce transfer of cooler water from the block into the hotter end of the head.

I'm not trying to get into a big argument with anyone, but my results have been positive and I have been running my truck this way for over 100k miles. I only have a 1/2 ton, so I have not had loads as extreme as some might, but I do have a 5th wheel that has a lot of wind resistance and do run up some steep hills in thin air with high outside temps. I try and avoid 1250* egt's and lift when I get close. My destination will still be there a few minutes later.

The only way a test would really indicate something is do run it with the bypass and again without it. It would be interesting to see if testing could show a benefit or not, but I do feel that long term results show something as well.

[More Power]

I'll say it again.....


Bypassed coolant isn't picking up heat in the heads. So, of course, the coolant will show a lower temperature.

[rustyk]

Can we have a BIG "AMEN"?!

Sometimes I wonder where these Bright Ideas come from. The only useful bypass (which most engines have) is to route coolant to the heater core to provide cabin heat as quickly as possible, until the 'stat opens...

[toyboxrv]

I don't see where you can prove that the heat is not transferring to the water. If hotter water is being used to "cool" the heads, the temperature of the head will be higher. Increasing the flow of hot water through the head isn't beneficial.

If it's such a bad idea I would expect to have seen a problem with it by now. The only thing I have seen is lower overall temps and less variance between the temps at the rear of the head versus the front. The temps at the rear of the head where water is just entering the head is lower under tow conditions. It is not because it hasn't picked up heat from the head, because it hasn't passed through much of the head yet.

Again, running the same hills with the same outside temps and the same load has shown lower temps and quicker return to normal temps. Over 100k miles of testing hasn't shown a problem.

[More Power]

The bypassed coolant should be traveling through the cylinder heads, but in your case, it's not. So, only a fraction of what should be traveling through the heads actually is.

By bypassing, the bypassed coolant will move faster (less resistance when compared to traveling through the heads), which produces an artificial lower temp where you're measuring it at the back of the heads.

If you had a thermocouple located at the mid-point inside the cylinder head, it would show a higher than normal temperature when using a bypass strategy. This is exactly where the GM engineers wanted to reduce temperatures. This is why they upgraded to a 130-gpm water pump.

In your case, perhaps you haven't run the engine at a high enough load and subsequent ECT/EGT temperature level to cause cylinder head problems. Your guess is as good as mine. I know a guy who reported 450,000 miles on his original FSD module too.

I'd love to be invited to participate in a bypass cooling test (with/without)where an instrumented 6.5 head was used. We'd use a 10K trailer, a 6% grade and full pedal for a minimum of 1 mile - longer the better. Then compare with/without bypass cooling. If the bypass strategy could show a lower temperature at mid head, I'd write an article singing its praises and I'd promote whatever vendor sells it - at no cost... Till such a time, I'm going with what the GM 6.5 cooling system engineers say about what works best for the 6.5... More flow t h r o u g h the heads - to reduce the incidence of cylinder head cracking...

The Diesel Page doesn't sell any parts for your engine. There is no financial incentive for me to promote any part (or suggest not using a part). We just want to help folks be successful 6.5 owners. ;)

Jim

[Jason "Toby" Tobias]

I guess this is a little of subject, But does anyone no if the 130 gpm water pump will bolt directly to a 1994 truck?

[More Power]

The 1996 and earlier factory 6.5 water pumps were rated at 87-gpm, which were designed to operate with a single thermostat.

The uprated 1997/98 130-gpm water pump is a direct bolt-on, and your existing fan-clutch will bolt on. We recommend using the twin thermostat setup as well. Several vendors sell the complete kit using all genuine GM parts.

The uprated water pump is also available in the 1999/00 style, which contains an integral water pump pulley that includes a threaded fitting for a screw-on style fan-clutch. Whether 1997/98 or 1999/00 style, each produce 130-gpm at 3000 engine rpm.

These are not "heavy-duty" water pumps. They are "High-Output" water pumps. Only the 1997/98 style water pump is marked "HO".;)

Jim

[phantom309]

wonder why the australians are so fond of this mod ? something about steam pockets, steam bubbles and cavitating water pumps,. so more flow works better,. they run their stuff in the desert,. and when the pump is pumping water around the block thru the heads and also thru the bypass lines, if it comes to a closed thermostat, then the water gets sent around again, till it is hot enough to open the t-stat,. the theories mentioned here seem to be based on the water heading straight back into the rad,.
australians also drill the fire deck in the spots needed to increase the flow up thru the head,.
"soak" time is not the best way to remove heat, "flow' works better,.
JMHO,

nick

[More Power]

wonder why the australians are so fond of this mod ? something about steam pockets, steam bubbles and cavitating water pumps,. so more flow works better,. they run their stuff in the desert,. and when the pump is pumping water around the block thru the heads and also thru the bypass lines, if it comes to a closed thermostat, then the water gets sent around again, till it is hot enough to open the t-stat,. the theories mentioned here seem to be based on the water heading straight back into the rad,.
australians also drill the fire deck in the spots needed to increase the flow up thru the head,.
"soak" time is not the best way to remove heat, "flow' works better,.
JMHO,

nick

placebo effect.... who knows? Hot is hot, boiling is boiling, doesn't matter whether you're crossing the Outback or pulling the Grapevine out of LA at max GCVW in the middle of August. HMMWVs don't use bp cooling in the deep sand and 130 degree heat of the Iraqi desert.

There are lots of theories about engine cooling, but not a lot of verifiable data collection out there - except by the GM 6.5 cooling system engineers who wanted dual t-stats used with their HO wp.

Generally, the biggest proponents of bp cooling are vendors. I don't sell any part you can bolt onto your truck. I have no financial stake in whether a product helps or not. In fact, I've turned away vendors and their advertising who had crappy products or had ads that were over-the-top unprovable BS.

I would hope that vehicle owner/proponents of bp cooling would be just as diligent in asking the vendors for hard verfiable data and photos of the testing equipment, and not just take their word for it that they tested it. Those vendors have a stake in selling you hardware.

Through the years I've tested quite a few aftermarket products that were advertised to improve one thing or another. Some did, and I could usually prove it using data collection (i.e. performance products, using dynos, elapsed times). For those tests, I always included photos of the testing equipment and included pics of the truck on the hill or the dyno (plus indicated when & where it was dyno'ed so others could verify what I said).

I've made the offer before.... Any vendor who will instrument a cylinder head using three calibrated thermocouples (end, center, end) to measure coolant temperature and can prove to me that bypass cooling lowers overall engine temperature or otherwise improves cooling, I'll write a story about it and give that vendor a free year of banner advertising. I'll travel anywhere in the continental US at my own expense to witness the data collection. Of course, and to be fair, if he's blowing smoke or the stuff doesn't work, you'll hear about that as well. My purpose for creating this web site 14+ years ago was to help owners improve GM diesel reliability, performance and ownership satisfaction. Still is..... ;)

[DmaxMaverick]

I'm sure I've posted this before, but I don't feel like digging through the posts to find it. So.....

Heat is a product of consumed energy, and can be measured absolutely by many scales (BTU, KW, Calories, etc.). During operation, an engine consumes fuel and produces heat. That heat is present, regardless of the methods of removing it. It must be removed. In most all motor vehicle engines, this is done with a liquid-air heat exchange. Before the heat can be removed from the engine, it must first be transfered to the coolant, then moved to the liquid-air heat exchanger (the radiator). Any successful method of a more efficient heat removal will (all else being equal) indicate the additional heat being removed. There are several vendors selling a bypass cooling kit, as well as available published instructions. Each I've seen has claimed immediate cooling results, by an indicated engine coolant temperature decrease. Problem is, if the cooling method were in fact more efficient, the result would indicate the exact opposite: HIGHER indicated coolant temperature. If the method was actually removing additional heat, it would be indicated by an ECT increase, not a decrease. There should also be a more active thermostat and fan clutch with an increase of coolant temperature. A decrease in indicated ECT is evidence of LESS heat actually being removed from the engine.

If your 6.5 is overheating, it needs to be fixed. Not bypassed.

[phantom309]

http://www.4wdaction.com.au/forum/showthread.php?t=23850

interesting read for the first few pages,.pay attention to 'LES ADDISON'
he says the bypass cooling is not really that, but they are air bleeds to help release the bubbles caused by the sonic vibrations,. large diesels have the same sort of technology,.it stops cavitation at the water pump,.
Interesting to see his modified waterpump,.
shame that some of the pics aren't available,.

Nick

[tomtaylor]

Hey guys, since I am reading this for the first time I have to say how interesting it is since I was just thinking after repairing my 6.2 head gaskets that I should run a line from the back of the head to the crossover. Obviously I won't do that now as I can see that this will cause more problems than it will solve! I didn't know there was any discussion about it or products for that matter, I just thought I would try it.

HOWEVER if flow is the only true solution please advise me about running that same line but instead, from the pressure side of the water pump to the back of the head, in an effort to "add lower temp water flow" to the back of the head rather than removing the hotter water flow from that spot with a bypass? is there a fundamental flaw in this idea.

[DmaxMaverick]

Hey guys, since I am reading this for the first time I have to say how interesting it is since I was just thinking after repairing my 6.2 head gaskets that I should run a line from the back of the head to the crossover. Obviously I won't do that now as I can see that this will cause more problems than it will solve! I didn't know there was any discussion about it or products for that matter, I just thought I would try it.

HOWEVER if flow is the only true solution please advise me about running that same line but instead, from the pressure side of the water pump to the back of the head, in an effort to "add lower temp water flow" to the back of the head rather than removing the hotter water flow from that spot with a bypass? is there a fundamental flaw in this idea.

Bad idea, and right on par with the bypass method. Increasing flow at the rear of the head will decrease coolant flow upstream. The dual stat and HO water pump upgrade is about as good as it gets. Assuming the rest of the system is healthy (radiator, stats, jackets, etc.).

[tomtaylor]

So you are saying that any flow diverted will have will have a neg effect because the current design is as good as it gets without going to the HO pump and dual stats. correct?

Do the dual stats simply double flow back to the rad? I guess you need a rad modification for two return hoses? I've never seen one before.

I also noticed here that the GM engineer said if you flow to fast through the engine then you will not be absorbing enough heat from the metal surfaces and you will produce a false reading of lower (ECT)engine temp. So I guess that means that they have already figured out that the HO pump and dual stats is probably the maximum flow you would want. Have I got this right?
thanks for your feedback

[JohnC]

If you add flow to the back of the heads at sufficient pressure to overcome the flow from the block then you cool the heads at the expense of flow through the block. which will get hotter.

If you could figure out a way to remove the hot water and replace it with cooler water then it would help.

[DmaxMaverick]

So you are saying that any flow diverted will have will have a neg effect because the current design is as good as it gets without going to the HO pump and dual stats. correct?

Correct. Short of making sure the system is as healthy as it can be (clean and operating well), that's about it. Optimizing injection timing and boost levels will reduce temps under load as well as any other method.


Do the dual stats simply double flow back to the rad? I guess you need a rad modification for two return hoses? I've never seen one before.No. The dual stat upgrade is OEM equipment for 1997+ models (and late 1996). The HO water pump is the same as your original, except for the output volume. It's a parts swap.


I also noticed here that the GM engineer said if you flow to fast through the engine then you will not be absorbing enough heat from the metal surfaces and you will produce a false reading of lower (ECT)engine temp. So I guess that means that they have already figured out that the HO pump and dual stats is probably the maximum flow you would want. Have I got this right?
thanks for your feedbackHogwash! Any engineer who would say that is no engineer.

[More Power]

I also noticed here that the GM engineer said if you flow to fast through the engine then you will not be absorbing enough heat from the metal surfaces and you will produce a false reading of lower (ECT)engine temp. So I guess that means that they have already figured out that the HO pump and dual stats is probably the maximum flow you would want. Have I got this right?
thanks for your feedback


To add to what DM said, "dwell time" is important for either absorbing heat in the engine or releasing heat in the radiator, but there is a fairly wide range that'll work fine. A longer dwell time in the radiator is important too.

It's important to understand the reason why the GM engineers increased coolant flow rate through the engine in the late 1996-early 1997 engines - to help prevent cylinder head cracking... Increasing flow rate through the heads helps to sweep the steam vapor layer off the exhaust runners in the heads and allow the coolant to better absorb the heat directly from the cast iron heads.

The coolant passages in the cylinder heads and head gaskets were all designed to move coolant in certain directions and at certain volumes. Any cooling system mod that reduces flow through the cylinder heads works against what the GM 6.5 cooling system engineers learned in the laboratory and ultimately applied to engine production.

[Robyn]

I will chime in here for a bit of input.

Lest say right off, I agree 100% with Maverick and More Power.

The Bypass gimick is just that, a GIMICK TO MAKE SOMEONE $$$$$$$$$$$$$$ and to try and fool people into believing that all is well.

The coolant flow through the 6.2/6.5 engines must work as it was designed in order to keep the engine cooling properly.

Head gaskets do go away on these engines, but usually its not until around 200K miles or so.

The gaskets used on the older 6.2 engines (80's) were of a different design than the ones used on the 6.5 later in the production sequence.

Currently the best gasket to use in a 6.2 is the late style 6.5 gaskets produced by Felpro.

One of the most overlooked items in the cooling system is the radiator, followed closely by the fan clutch.

The original equipment fan clutches were a POS. GM raised the temperature at which the clutch "Hooked UP" after the myriad of whinning complaints they had on the mid 80's pickups/Burbs/Blazers about FAN NOISE

I had a customer once that swore his tranny was slipping when he was in the mountains and pulling hard.

I checked the tranny and all was fine. He insisted that I rebuild the tranny, install a cooler, a shift kit and a special torque converter.

OOOOOOOK, so we did all that. Two weeks later the fellow came back, madder than a boiled owl, tranny was slipping again :eek:

Nope, that was not the case. I kept the rig for a weekend and took it over the mountains for a family outing.

As soon as the engine temp would reach about 200 (gauge) the fan would howl like crazy and the temps would drop right off

I took the rig back and explained the situation to the owner, he was not convinced so I installed a Tach in the vehicle and had him watch that.

Only after seeing that when the engine fan was howling, the RPM did not climb was he convinced.

I digress

Now GM was fed up with all the whiners, so they raised the temps at which the clutches engaged plus the fan designs were changed and the overall fan speed (clutch connection) was altered so the noise level was dropped off to a less noticeable level.

Soooooooo these are some of the reasons why the cooling issues have happened.

The radiators also are of concern, as the size and cooling capacity is marginal when new, and after 100K miles or more the cores tend to "SILT" in down in the areas that have a slower flow rate.

The cooling air passages also tend to plug up with Bugs, dirt and other crap that gets ignored.

The results are an engine that continually runs too hot.

The cooling system upgrade with the new HO water pump and dual stats is a great addition, but the radiator should be changed out too (high milers)

The other issue in the radiators is the crap that sticks to the inside of the cooling water tubes, this being deposits from minerals in "TAP" water used in the coolant mix.

These deposits through time and heat collect and insulate the coolant from the metal tubes, thus blocking the rejection of heat.

I have replaced radiators in 6.5 trucks that had heating issues and the problems go away, instantly.

The raiators looked fine, until you cut them open and took a good look inside.

A 6.5 rig with a fresh radiator, a good water pump and Stat/stats plus a fan that will hook up when the air coming through the radiator gets to about 210F, will cool just fine.

Now if your talking H1 Hummers, thats a horse of a totally different color.

The airflow through the radiator on these must pass such a torturous path that the rigs are prone to cooling issues.

The radiator mounting position is part of the problem, but we are not going to redesign the Hummer here.


Clean radiator, good fan clutch, upgraded water pump and dual stats and the thing will cool fine without the need for add on hoses, fittings and other such nonsense.

In fact, just a clean radiator and a good fan clutch and most rigs will do fine without the HO pump and dual stats.

Missy

[tomtaylor]

Hey everyone thanks for sharing the excellent info, learning alot here, still not fully clear on how the dual stat unit makes a difference. What exactly does this unit do that the old one does not? Is the turbo creating different temp characteristics on each side of the engine? thanks.

[DmaxMaverick]

Hey everyone thanks for sharing the excellent info, learning alot here, still not fully clear on how the dual stat unit makes a difference. What exactly does this unit do that the old one does not? Is the turbo creating different temp characteristics on each side of the engine? thanks.

The dual thermostat is not different than a single, other than having 2 stats in parallel. The coolant crossover contains 2 thermostats, with 2 inlets and 1 outlet, same as the single. This also offers an option for thermostats with different temp standards, which can be beneficial, and is OEM on Duramax engines.

[tomtaylor]

Thanks, I guess what I was looking for was in one of Jim's articles about system flow rates. The single stat doesnt keep up with the flow rate of the HO pump so a quick & cheap fix was to design a dual stat X-over that flows high enough for that pump, simple as that.

[Robyn]

The dual Xover has a major increase in flow rate through the bypass port so the water in the block does not stagnate and allow steam pockets to form and result in Hot spots.

The other benefit is an increased flow through he radiator.

The radiator flow increase is not huge, but beneficial.

The major difference is the overall coolant flow rate through the block.

A much faster flow stirs up the entire cooling area in the heads and keeps stagnant areas with little or no flow from forming.

The coolant flows through the block, in and around the cylinders and then out the rear block port and up into the rear of the head. From here the water flows forward through the heads and up into the crossover.

During closed thermostat running the coolant flow rate on the old single stat system was much much slower, allowing the stagnation of coolant in various portions of the heads and block.

Velocity is the key here to keep these areas moving and steam pocket formation to a minimum.

The original single stat system was not bad, If the stock type stat was used.

A non stock stat without the proper base on it would alter the bypass flow rate and adversely affect the cooling ability.

The original stat had a baffle of sorts on the bottom of the stat that modified the flow back into the pump during the time that the coolant was flowing to the radiator.

The baffle on the stat would close of the bypass port to a great degree and keep coolant going to the radiator.

The wrong stat would allow too much coolant to back feed into the pump instead of going to the radiator.

The new dual system is a good setup.

Missy

[tomtaylor]

Great thanks for adding in those details, so the purpose of the bypass circuit is to keep coolant on the move until the stat opens and this helps prevent those hot spots/steam pockets from forming, correct?

I seem to recall seeing a coolant product advertised here a while back that claimed to reduce or eliminate this steam layer and then I saw a comment from Jim about it maybe didn't live upto it's claim or something like that, does anyone recall this?

[DmaxMaverick]

Great thanks for adding in those details, so the purpose of the bypass circuit is to keep coolant on the move until the stat opens and this helps prevent those hot spots/steam pockets from forming, correct?

No. The "bypass" is there to circulate coolant to the thermostat to keep it "in touch" with actual engine/head coolant temperatures. This is necessary when the thermostat is "detached" from the engine. Meaning, because it is in a crossover manifold, the thermostat may not be exposed to actual coolant temperatures occurring in the engine. circulating a small amount of coolant at all times keeps the thermostat in contact with engine temperature coolant. Many other engine designs (such as the traditional SBC/BBC) don't have this problem, because the thermostat is actually housed in the engine.

The HO water pump creates a condition of higher coolant velocity, which helps reduce the condition of "film boil", which causes the heat/steam pockets.


I seem to recall seeing a coolant product advertised here a while back that claimed to reduce or eliminate this steam layer and then I saw a comment from Jim about it maybe didn't live upto it's claim or something like that, does anyone recall this?"Water Wetter" is the product. Its purpose is to reduce the condition of film boil to prevent cylinder wall detonation. This is a serious condition with, primarily (in the light truck world), Navistar 6.9 and 7.3 IDI engines. The "wet" cylinder sleeves would actually erode to the point of failure without the correct coolant additive. None of the GM engines have ever suffered this condition. The additive has never proved, that I've seen, to help with actually cooling the engine.

If you are concerned with steam pockets/layers, use NPG coolant. No water, so no boil. It works very well, but is expensive. If you never have a leak, it could be your last coolant service on that vehicle. Ever.

[tomtaylor]

Thanks again for the great details, I will definately look up the NPG product although I don't think I'm in need of it. I am just trying to understand as much as I can about these engines.

I would like to ask though with all the discussion here about lowering coolant temp is there not an ideal engine operating temp to maintain thermal efficiency to get the most out of your power and economy? Is this operating temp a narrow or broad range?

[DmaxMaverick]

The ideal engine temperature to achieve optimal thermal efficiency, power and economy far exceeds the abilities of traditional engine cooling system components, coolant fluid, and engine components. Essentially, the coolant medium is the limiting factor at the lowest level. The use of NPG coolant removes this factor, and steps the process to the next level: the components. 240°F is about the maximum with EG/water coolant at 15 PSI. The ~200°F operating temperature standard for almost all passenger vehicles allows for a bit of flexibility to keep the temperature at a safe level, with a buffer for overheat conditions. Above that, components such as hoses and seals become vulnerable to heat damage.

The problem isn't so much what temperature can be withstood, but what temperature can be maintained and/or managed. Lowering an engine's operating temperature (by installing lower rated thermostats, for example) to address a frequent overheat condition is most often a band-aid to mask a problem of a less than optimal cooling system, or exceeded mechanical/physical limits (too much power). The lower operating temperature only increases the range of the buffer. In most cases, the problem isn't the maximum temperature, but the ability to control the temperature once it reaches a point of no return. This is why we hear of some trucks that can do a job without ever overheating, while others boil over, all else being equal. Heat generated by burning fuel is used in two ways, either by converting it to forward motion, or shedding it to the environment (cooling system). A 100% efficient powerplant would remain at ambient temperature, all the time, always have power limited only by the fuel consumed, and be absolutely silent. Unfortunately, we can't achieve that, so we have to settle for a middle ground. Some heat energy is used to move you down the road, and the rest is returned to the environment, one way or another.

[scarmic]

Just a quick comment on cooling system performance. I have found a relationship between engine blow by and cooling system performance.
This has possibly been related to efficiency as with modern ring, piston design oil consumption is usually excessive long after engine inefficiency has caused problems with cooling. Also it seems to me that when ever I have a "hot" running engine the damn thing seems to go forever.

[More Power]

Some t-stat facts....

GM installed 180 degree t-stats in the 1980s 6.2L diesel engines.

GM upped the temp to 195 in the 1990s.

Peninsular Diesel installed (and may still) 160 degree t-stats in their marine 6.2/6.5 diesels.

Jim

[Robyn]

The main reason that the Manuf's use the 195F stats is to maintain emission levels.

The higher temps allow for a little better burn and a little more efficiency.

The cooler temp stats allows the engine to run a lot cooler and overall with place less thermal stress on the engine components.

More heat equals more expansion and contraction of the block and heads.

As these components expand and contract (They are Moving) :eek:

All this movement eventually causes issues, or at least it can.

After having been inside many many 6.2/6.5 engines, I have seen some of the damndest messes of cracks.\

Cracks in cast iron are cuased or at least can be caused by many things.

A poorly cured block, when subjected to high temperatures moves around a lot duirng its operational cycles, and in many cases, due to the fact that everything is all bolted up tight, can't move enough to relieve itself.

I have seen mainline cracks at the register fit area that have spread open as much as .030". Now this is not at all a good thing.

A hairline crack can occur due to stresses, but to have a crack open up means that stuff is moving, changing shape and "Normalizing"

A 6.2/6.5 block thats crack free and has many many miles on it will likely be fine for an almost indefinate time period.

The heat level at which these engines operate can certainly effect the whole picture.

I prefer the 180F stat as a good compromise in temp level.

Good enough to allow the oil to lose any moisture and flow well, and also a level that the thermal stresses are manageable.

If you look at many of the 6.5 equipped trucks, you will see a dash gauge (coolant tamp) that has a RED line of 250F ++

This is crazy.

I have spoken with a few owners who tow trailers and tell of long periods of towing in the heat of summer with the gauge at or near the 240 mark :eek:

This sort of temperature is just cooking the engine.

Expansion is at its maximum and the stress on the components is off the charts.

A properly set up and maintained cooling system in these rigs can and will maintain 200 F under most conditions.

Even in the extreme heat of the desert, the cooling system should be able to deal with the heat and manage it at near the 200 F mark


Missy

[scarmic]

In the far distant past I had a lot toto do with cat engines and the best examples I can come up with to illustrate my findings regarding the relationship between higher "controlled" engine temps and operating under lower temps are as follows.
3512, 3516 in 785b and 789b trucks at only 12/14 thou hrs rhs liner wear (water pump side) in excess of 14 thou lhs liners less than 5/6 thou.
3516 gen sets 1.2 Meg operating at coolant temps of 100c / 110c 18000hrs +.
Less than 10 thou on average both banks.
D7g dozers doing seismic work ie hi speed low load 2,500 hours valve guides u/s. Same dozers pushing gravel 8,000 hrs +.
Why does this not apply with the 6.5?
I am not convinced that sulpher is the sole culprit.
Would really appreciate your thoughts.


Steve.

[More Power]

In the far distant past I had a lot toto do with cat engines and the best examples I can come up with to illustrate my findings regarding the relationship between higher "controlled" engine temps and operating under lower temps are as follows.
3512, 3516 in 785b and 789b trucks at only 12/14 thou hrs rhs liner wear (water pump side) in excess of 14 thou lhs liners less than 5/6 thou.
3516 gen sets 1.2 Meg operating at coolant temps of 100c / 110c 18000hrs +.
Less than 10 thou on average both banks.
D7g dozers doing seismic work ie hi speed low load 2,500 hours valve guides u/s. Same dozers pushing gravel 8,000 hrs +.
Why does this not apply with the 6.5?
I am not convinced that sulpher is the sole culprit.
Would really appreciate your thoughts.


Steve.

Discussing the ideal thermostat temperature setting is a little off topic to this thread, but... I've thought about it too....

First, there is a big difference between a constant load operating cycle (i.e. genset) and a variable duty-cycle (i.e. light-duty pickup).

1- A case can be made where the hotter the operating environment, the more failures the original DS4 PMD suffered.

2- If the thermostat setting was nearer to the critical level where the electronic DS4 system went into limp mode (programmed de-fueling), the more often that might occur and it would present the driver with less time to prevent an overheat situation.

3- A normal and healthy diesel engine and cooling system in a light-duty pickup doesn't reach the level of the t-stat setting unless it's hot outside or there is a load on the engine - especially in cool weather. A higher t-stat setting would produce larger engine temperature swings. Large and frequent thermal cycles affect the longevity of the gaskets and other engine components. A Genset, on the other hand doesn't have to deal with the same frequency or magnitude of thermal cycles that a pickup engine does.

That said, there's nothing stopping anyone from using whatever temp thermostat they choose or have available to them. I prefer a 180 degree t-stat, but that's just me... ;)

[tomtaylor]

The ideal engine temperature to achieve optimal thermal efficiency, power and economy far exceeds the abilities of traditional engine cooling system components, coolant fluid, and engine components. Essentially, the coolant medium is the limiting factor at the lowest level. The use of NPG coolant removes this factor, and steps the process to the next level: the components. 240°F is about the maximum with EG/water coolant at 15 PSI. The ~200°F operating temperature standard for almost all passenger vehicles allows for a bit of flexibility to keep the temperature at a safe level, with a buffer for overheat conditions. Above that, components such as hoses and seals become vulnerable to heat damage.

The problem isn't so much what temperature can be withstood, but what temperature can be maintained and/or managed. Lowering an engine's operating temperature (by installing lower rated thermostats, for example) to address a frequent overheat condition is most often a band-aid to mask a problem of a less than optimal cooling system, or exceeded mechanical/physical limits (too much power). The lower operating temperature only increases the range of the buffer. In most cases, the problem isn't the maximum temperature, but the ability to control the temperature once it reaches a point of no return. This is why we hear of some trucks that can do a job without ever overheating, while others boil over, all else being equal. Heat generated by burning fuel is used in two ways, either by converting it to forward motion, or shedding it to the environment (cooling system). A 100% efficient powerplant would remain at ambient temperature, all the time, always have power limited only by the fuel consumed, and be absolutely silent. Unfortunately, we can't achieve that, so we have to settle for a middle ground. Some heat energy is used to move you down the road, and the rest is returned to the environment, one way or another.

So then our issue is not so much the operating temp as it is the control of that temp (temp range) as it relates to the flow rate and quantity of coolant on reserve(rad). This is an easily solved problem in a constant load environment where stationary power is needed but more challenging in variable load environment like motor vehicle applications. So upsizing the coolant reserve(rad) and flow rate, when possible, is always a good idea as well as the air flow across the rad. Thanks for the info again.

[gsimpson]

I was reading the article and why would you make the holes in the head gasket so much smaller than the passages in the block and head?

Also wouldn't make the engine run cooler with increased head gasket passages?

Wouldn't you drill them out to match the ports in the block and heads?

[More Power]

I was reading the article and why would you make the holes in the head gasket so much smaller than the passages in the block and head?

Also wouldn't make the engine run cooler with increased head gasket passages?

Wouldn't you drill them out to match the ports in the block and heads?

There are a lot of engineering principles involved with coolant flow and heat transfer. The various holes in the head gaskets are sized to allow a prescribed amount of coolant flow. The size of the coolant passages in the head/block are the size they are for at least a couple of different reasons.... 1- Manufacturing ease and core cleanout. 2- To control coolant flow.

This is why the article was written.... Wannabe vendors just can't decide on their own to change coolant flow without some sort of dynamic testing on a fully instrumented engine to measure the effects of any change. The vendor data should also be published so it can be verified.