What compression ratio?

dmaxvaz

dmaxvaz

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Nov 22, 2006
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ecc_33 said:
IMHO I wouldn't ever go high compression on a modded duramax. IMHO i think this is why alot of aftermarket rods/parts have been failing is becuase of this. Good luck keeping it together with that much air and high c/p. Just remember, If it were such a good idea to go up in c/r all the tractor pullers/ turboed car guys would be doing it too:thumb: I wouldn't go any higher than what you were going to go for before.
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Can't compare with tractor pullers. Those guys run 150 psi+ and higher rpms. The guy wants to use the truck for street too. I would stay 16.5:1 cr. If I was going all out, and did just pulling and dragracing, then I would lower the cr. Otherwise you will have a slow spoooling, laggy pig for street duty. Think of what you really want and go with it:thumb:
 
ecc_33

ecc_33

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Aug 10, 2006
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jneal said:
With that much air I'd just stay around the 16.5:1 mark. Higher compression pistons have there place in air restricted setups like sled pullers. But in your case you won't have to worry about air. Lol
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Im curious. Have you tested your theory with high cp pistons in a 2.6 or 3.0 engine? I think this is very interesting! Say engine dynoing with lower compression then switching? I know this would be alot of work but seems like you have done some kind of testing?

dmaxvaz said:
Can't compare with tractor pullers. Those guys run 150 psi+ and higher rpms. The guy wants to use the truck for street too. I would stay 16.5:1 cr. If I was going all out, and did just pulling and dragracing, then I would lower the cr. Otherwise you will have a slow spoooling, laggy pig for street duty. Think of what you really want and go with it:thumb:
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Ummmm no, Not all tractors run 150psi+ of boost. And not all classes run high rpm's IIRC a couple of the smaller tractor classes run a rpm limit thats not very high like around 3000ish rpms. I understand this is going to be a street truck thats why above i recommended 16 to 1 compression.

The only things i can see a high compression diesel engine doing is for one killing engine hard parts. I am sure that it would help a big turbo truck to stay lit at lets say the end of a sled pull. I could possibly see it helping with spooling chargers also. What i DONT SEE is a truck making more peak power with high compression. I HAVE NO HARD DATA FOR THE STATEMENTS ABOVE. This is just from my sled pulling experience so take it for what its worth :D
 
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juddski88

juddski88

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ok, to give you a hint, think about what can be done to the injectors in relation to the head to make lower compression pistons work well for them (also works really well for cummins motors, although not popular yet). Also think about what they have to do to contain the pressure in the motor, compared to a duramax...it's apples to oranges
 
ecc_33

ecc_33

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No, I dont agree. They are all Direct Injected diesel engines at the end of the day. You can also get blank duramax nozzles and degree the spay pattern on them.
 
Rhall

Rhall

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If a higher compression ratio is the answer to making more power, show me the results. I'll stay low myself.
 
jneal

jneal

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ecc_33 said:
Im curious. Have you tested your theory with high cp pistons in a 2.6 or 3.0 engine? I think this is very interesting! Say engine dynoing with lower compression then switching? I know this would be alot of work but seems like you have done some kind testing.
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Back to back testing, no. I have pulled very similar builds, almost identical except pistons. The higher compression engines light the charger easier and stay on top for longer at the end than the lower. but without doing a back to back test on the same engine I'd be hard to say if there is actually a power difference. As far as being harder on parts. I've done inspection tear downs on 2 after a year of pulling and they looked fine. No increased bearing wear or anything like that. Only downside I did find of raising the CR is the main caps fret. But billet mains solved that issue.
 
juddski88

juddski88

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ecc_33 said:
No, I dont agree. They are all Direct Injected diesel engines at the end of the day. You can also get blank duramax nozzles and degree the spay pattern on them.
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and the same has been done on the cummins without having the drastic difference like changing injector protrusion. don't ask me why, I have no idea, I have just seen the difference in my buddy's motor after everyone told him he was crazy, except curt haisley, he said it was the only thing he hadn't tried yet. I think that lower compression pistons allow for cheaper fastening options in terms of head-to-block, with the appropriate tuning. only a select few people have had the privilege to monitor real time cylinder pressure, I think that there are many many more uncontrollable variables that determine CP than just compression ratio and heat and boost. some can be mitigated, obviously, as guys like Rob have shown. But if I were to put brand new pistons in tomorrow I think I would use either the Arias 17:1 or the TTS forged Mahles in 17:1
 
ecc_33

ecc_33

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jneal said:
Back to back testing, no. I have pulled very similar builds, almost identical except pistons. The higher compression engines light the charger easier and stay on top for longer at the end than the lower. but without doing a back to back test on the same engine I'd be hard to say if there is actually a power difference. As far as being harder on parts. I've done inspection tear downs on 2 after a year of pulling and they looked fine. No increased bearing wear or anything like that. Only downside I did find of raising the CR is the main caps fret. But billet mains solved that issue.
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Thanks jneal. I expected two of your anwsers. I had thought you have done some testing between the two and track times or sledpulls is just as good to me as a dyno. Like you said. Might not of made more power but lit the charger easier and stayed on top for longer. That is were most run lost of R's to componsate for lower cp. Makes sense.

juddski88 said:
and the same has been done on the cummins without having the drastic difference like changing injector protrusion. don't ask me why, I have no idea, I have just seen the difference in my buddy's motor after everyone told him he was crazy, except curt haisley, he said it was the only thing he hadn't tried yet. I think that lower compression pistons allow for cheaper fastening options in terms of head-to-block, with the appropriate tuning. only a select few people have had the privilege to monitor real time cylinder pressure, I think that there are many many more uncontrollable variables that determine CP than just compression ratio and heat and boost. some can be mitigated, obviously, as guys like Rob have shown. But if I were to put brand new pistons in tomorrow I think I would use either the Arias 17:1 or the TTS forged Mahles in 17:1
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I def agree on the cylinder pressure testing. That would be very interesting to see the differences between A and B compression. Fwiw my truck should be around the high 14's compression wise. I will post whenever i get it running agian how it acts in cold weather.
 
paint94979

paint94979

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ecc_33 said:
Thanks jneal. I expected two of your anwsers. I had thought you have done some testing between the two and track times or sledpulls is just as good to me as a dyno. Like you said. Might not of made more power but lit the charger easier and stayed on top for longer. That is were most run lost of R's to componsate for lower cp. Makes sense.



I def agree on the cylinder pressure testing. That would be very interesting to see the differences between A and B compression. Fwiw my truck should be around the high 14's compression wise. I will post whenever i get it running agian how it acts in cold weather.
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Let me see if I can get Mike(Redneck Texan) in this thread to elaborate and give some insight
 
Fingers

Fingers

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As you folks know, Cylinder Pressure and Compression Ratios are of a great deal of interest to me.

I am running 17.5:1 oval cuts in my truck now. I have air issues, but I would characterize the engine as very "bright" in it's response once it gets air. Much more so than my low CR engine it replaced.

If you have the parts to handle the Cylinder pressures, I would recommend 16.5:1 and up.
 
Rhall

Rhall

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Theres better ways to light big turbos than raising compression. I have no problem lighting a clipped 2.6 turbo that most would laugh at trying to light in native form, hell the only reason I ever unlocked my converter this year was to save the driveline, it had no problems lugging down to the 2k range. I run 15 to 1. I see some of the theories, but theres no way I'm putting more stress on my engine parts.
 
GeorgiaDieselGTO

GeorgiaDieselGTO

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I have seen a truck start around 0* at 15/1 compression and all there was was more white smoke at startup. otherwise lit off like a champ. My 06 had 15.5/1 and i think i would go back with a little higher, because i think it hurt my mpg a little. Otherwise for performance i would go around 15/1. Just my humble opinion :)
 
S

S Phinney

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Aug 15, 2008
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J had 15.98 on my first build a d went with 17/1 on it the second time around. It starts just like my stock trucks. It has 65 over injectors also. Lower c/r does have a major effect on start ups and responsiveness

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