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...



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.



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.



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.



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.



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 . But then again, I knew better. 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.

[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?