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Thread: Bypass cooling for the 6.2/6.5

  1. #1
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    Arrow Bypass cooling for the 6.2/6.5

    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
    Last edited by More Power; 05-06-2019 at 08:48. Reason: Update - Add to

  2. #2
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    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...
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  3. #3
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    Thumbs up The bypass exposed!

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

    A BIG Thanks
    Last edited by DA BIG ONE; 11-18-2006 at 01:51. Reason: TYPO
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    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
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    x marks the spot you have to drill in the block to make it work.........lol.

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    Quote Originally Posted by grape
    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.
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    Administrators, why do I get no picture and "www.thedieselpage.com" in white squares? An inquiring mind wants to know.
    Mark Chapman DP member #653;
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    Quote Originally Posted by Subzilla
    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.
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  9. #9
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    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.
    350k+ miles of 6.2/6.5 experience

  10. #10
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    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.
    86C 6.2l gmc
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  11. #11
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    Quote Originally Posted by DmaxMaverick
    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.......

  12. #12
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    Quote Originally Posted by grape
    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.
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    a $50K SB2.2 has a GM iron block and sleeves and doesn't have iron pistons......lol.

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    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.
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    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

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    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.

  17. #17
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    So why doesn't the head gasket have holes in it to match up these coolant passages? (in blue)
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  18. #18
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    Default Tiny Holes ( and Bubbles)

    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.

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  19. #19
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    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

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    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?
    6.2 NA, SM465, Dana 18, Saturn OD, Dana 60's all crammed into a 1970 Series IIa 88" Land Rover
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