You can't use anything in the 80's in a scientific manner.. I think everything back then was a WAG 
Wild A$$ Guess
Wild A$$ Guess
Compresion ratio ?
- Thread starter TrentNell
- Start date
True that's my best guess and I'm sticking to it... Maybe had somethign to do with the injector failures of earlier models??
Sadly, compression in our engines is NOT isothermal. It is very close to Adiabatic.
So PV=NRT is true, BUT there is now added heat from the compression process.
Adiabatic compression turns out to be P2 = P1 ((V2/V1)^-1.4) for normal air.
So PV=NRT is true, BUT there is now added heat from the compression process.
Adiabatic compression turns out to be P2 = P1 ((V2/V1)^-1.4) for normal air.
FWIW, that means you only need to make up 3.52 PSI of boost to make the same peak pressures. Peak Temps is another matter.
starting with 14.7 PSA:
17.5:1 => 807 PSA
16:1 => 712 PSA
15:1 => 651 PSA
But, What do I know......
starting with 14.7 PSA:
17.5:1 => 807 PSA
16:1 => 712 PSA
15:1 => 651 PSA
But, What do I know......
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So the math doesn't work like we were saying earlier...
1 point of CR doesn't equal out to be 14.7 psi boost lost?
1 point of CR doesn't equal out to be 14.7 psi boost lost?
FWIW, that means you only need to make up 3.52 PSI of boost to make the same peak pressures. Peak Temps is another matter.
starting with 14.7 PSA:
17.5:1 => 807 PSA
16:1 => 712 PSA
15:1 => 651 PSA
But, What do I know......
:teknology:
x2 , Great info but I need it to be in laimens (SP) terms :baby:
When you lower the static compression you have to increase the boost to get the dynamic compression to match what you had or you've lost power.
Example...
A) 17.5:1 with 45psi of boost is a dynamic compression of 20.5:1
B) 15.5:1 with 45psi of boost is a dynamic compression of 18.5:1
C) 15.5:1 with 75psi of boost is a dynamic compression of 20.5:1
D) 19.5:1 with 45psi of boost is a dynamic compression of 22.5:1
Scenario D is what I like the best. We proved it on the CR Cummins that I posted links to earlier.
The old 6.9 IDI's are 21.5:1 static compression. They are non turbocharged but they have a higher dynamic compression than we do on a stock motor at 45 psi.. Food for thought
Question of the day is.. If we can make free power with compression why is the factory decompressing... I'm assuming for NOX reasons.
Great thread .... I like trying to comprehend all this .
I know I am not making the big power some of you are , but I noticed a drop in performance just from delipping the pistons down low lugging the motor loaded towing on mild tunes . I think that shows something right there .
Same grades same tunes I use to pull these grades at 1800 and now it seems I have to be at 2100 to hold the same mph's on same tunes same weight .
Truck still pulls ok but lacks what it once had down low on same tunes , it sure seems to like big tunes and being cranked up to perform like you would expect . Under 15 psi is where I notice it most . Just seems to lack some bottom end up grades now and I have to rev it to get the same torque .
I only had mine delipped and didnt take any off the top . If I had it all to do over again I probably would have left mine alone at my power levels . I was more worried about cracking one again and why I delipped them . For the trade off I still havnt decided if it was the right choice . I liked the grunt down low I remember on mild tunes towing .
I guess this is the game and trade off when trying to use it multi level HP settings and have the best of both worlds and peace of mind on cracking a piston . Just been towing so much lately and not in the dunes that I notice the difference more of what it once did with a trailer hooked up loaded . Runs hard when on big ass tunes ...... would probably run harder with more compression is my guess though . But then the cracking game ....... with the maximum egts I push with the weight and length of time I am on it in comparison to you all in same distances .
Motor is still together so .... I guess its a toss up ....
Is it me , or does this make sense ?
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If this is true then why is it when we do a compression test we get much higher values? We have compression readings that are 350-425 psi and the GM standard would be not to have a reading under 300 psi or 50 lbs difference form highest to lowest readings. It may be true but I want to understand the theory a little better. Please explain the differences then of what we see in compression test vs. applied theory of what cylinder pressures should be.
Wow, sure is alot of worry here on compression ratio . Lower compression saves parts, period. Been that way for decades, will be that way for decades to come. High compression makes torque. Lower compression sacrifices some torque, but usually gains some rpm. Regardless of compression ratio, you will have to combine the right cam, turbo, injectors, tuning, etc. to make it all come together. If your build isn't working the way you think it should, I'm not so sure compression ratio is the lone culprit. I'm thinking turbo, cam, injectors, and tuning may have something to do with it :banghead:. For instance, if I'm going into a gun fight, and I'm only allowed to have one round in my gun, I'm thinking I will take my .44 mag and leave the .22 at home. If everything goes perfect, the .22 will give you a chance, but if your off your mark, the .44 will still kick your arse everytime .
Of course, this is JMO.
. Lower compression saves parts, period. Been that way for decades, will be that way for decades to come. High compression makes torque. Lower compression sacrifices some torque, but usually gains some rpm. Regardless of compression ratio, you will have to combine the right cam, turbo, injectors, tuning, etc. to make it all come together. If your build isn't working the way you think it should, I'm not so sure compression ratio is the lone culprit. I'm thinking turbo, cam, injectors, and tuning may have something to do with it :banghead:. For instance, if I'm going into a gun fight, and I'm only allowed to have one round in my gun, I'm thinking I will take my .44 mag and leave the .22 at home. If everything goes perfect, the .22 will give you a chance, but if your off your mark, the .44 will still kick your arse everytime .Of course, this is JMO.
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I think that lower compression saves parts... but so does tuning.
On a fixed timing motor the lower compression makes sense, but to me, on a very tunable platform like ours I think the more the better.
JMO as well Good to see you around :thumb:
On a fixed timing motor the lower compression makes sense, but to me, on a very tunable platform like ours I think the more the better.
JMO as well
Good to see you around :thumb:
Reguardless a higher compression engine will make more power if you can hold it together.
Cylinder pressure is cylinder pressure and when I build an engine it sure as hell isn't getting decompressed.
Why do all of the gas guys go up in compression when they want to make power and all of the diesel guys seem to want to go down to make power? Something doesn't make sence. I am going with the gas guys.
Going down in compression only made sense when mechanical injection was around. They do have near the control that we have for when the fuel comes in. We also can get a larger shot in a smaller amount of time with bigger injectors putting the fuel in "EXACTLY" where we need it.
I didn't see anyone worrying. It was a discussion and to each their own. BTW I haven't seen much of your truck this year. What happened powermax?
Cylinder pressure is cylinder pressure and when I build an engine it sure as hell isn't getting decompressed.
Why do all of the gas guys go up in compression when they want to make power and all of the diesel guys seem to want to go down to make power? Something doesn't make sence. I am going with the gas guys.
Going down in compression only made sense when mechanical injection was around. They do have near the control that we have for when the fuel comes in. We also can get a larger shot in a smaller amount of time with bigger injectors putting the fuel in "EXACTLY" where we need it.
I didn't see anyone worrying. It was a discussion and to each their own. BTW I haven't seen much of your truck this year. What happened powermax?
Totally agree here.. It's old school thinking. If you have a motor fixed timed at 38 degrees advance, then yeah I would want to save the motor a bit by decompressing and let the boost make up for the cylinder pressure lost on the bottom end.
Say that all you want, but some one has bent after market rods by not cutting enough off their pistons and running 120 psi boost , it HAS happend , also consider having to pay the bill when a piston cracks . Its called insurance , some need it other have the coin to risk it. I dont know any gassers raising compression in turbo charged aps ?
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Most gassers will stay 9.5 and lower with boosted applications. We are still in the learning stages with the Dmax world. It is good to us though but we got a lot of tech and improvements to be had still on. Let's get some more good discussions about issues relative to improving our part of the deisel world.
I was told aftermarket rods are indistructable.:rofl:
BTW, I didn't know anyone was running 120 pounds of boost.
I would be looking at better flowing heads if I was bottle necking things that bad.:coolspot:
IDK about boosted gassers... I was just saying in general about CR they go up.
BTW, I didn't know anyone was running 120 pounds of boost.
I would be looking at better flowing heads if I was bottle necking things that bad.:coolspot:
IDK about boosted gassers... I was just saying in general about CR they go up.
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I have a thread about pistons on this site , and i stated alooot of times that i didnt want to drop any compression on my build , the reality that sets in when you are spending thousands of dollars on a engine that HAS to last you at least a full season and knowing you cant afford to fix it if it breaks , really makes you think different , at least it has me .
