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Increasing the static compression ratio increases power via an increase in thermal-conversion efficiency. Increasing boost pressure increases power via an increase in mass-air flow rate. There’s less gain in thermal-conversion efficiency (and hence power) via an increased static CR compared to the power gain by increasing the mass-air flow rate via an increase in boost pressure. For example, increasing the static CR from 8.5 to 9.5 results in an increase in thermal-conversion efficiency (for an ideal Otto cycle) of about 3.2%. On the other hand, increasing the boost pressure from just 14 psi to 15 psi, increases the mass-air flow rate by about 3.5%. If boost pressure is increased by 2 psi, (from 14 to 16 psi), the increase in mass-air flow rate will now be more than twice that compared to the increase in thermal-conversion efficiency, (~7% vs ~3.2%), and ΔT and ΔP still won’t be as great as they are when increasing the static CR from 8.5 to 9.5. Therefore, not only can it be “safer” from the knock point of view, but a little more power is gained as well, (relatively speaking that is).
Please find me a factory turbocharged 4valve per cylinder car older than 5 years and with CR less than 9? I would guess that there are none or some with ancient drivetrain.
LOL Show me a modern 8V, 4 cylinder turbo engine with a very large bore.
So I guess JET951 made a big mistake running 8.0 CR on his 4V 3L motor? Maybe that is why he has been so unsuccessful? And I also made a mistake by running 8:0 CR on my 8V motor. I guess that is why my car is so slow?
Originally Posted by Thom
Corleone ran an unusually sharp ignition profile with a reasonable boost level to make high power from a responsive engine.
I run very close to Corleone's ignition curve. Sid runs pretty much Corleone's curve also.
I also run lots of numbers and I love to model but I never let theory trump empirical data. I have ran a lot of dynos and looked at hundreds of dynos. I have researched builds on our motors over twenty years and talked to a lot of great builders. Our motors, especially the 8V motors, are vulnerable at the head, not just do to knock but also head lift. IMO this is got to be a heavy consideration on a big power build on our motors if you want the build to be reliable.
Why would I increase CR if I can increase pressure for 2 PSI and get more, safer?
Is ~3% better off boost efficiency really worth the trade off safety and higher boost ability?
+3
CR may nominally help responsiveness and efficiency, but there's about a 1mpg difference in fuel economy between the NA and turbo despite a 1.5 CR difference..
and youd be hard pressed to find any more "response" between say a 9.5 NA vs a 10.2 NA, turbos should be the same concept.
im playing with building a super-low CR engine (8:1 or even less) to really blow the **** out of with the big supercharger because who needs off-throttle power when there's a supercharger bigger than some other engines strapped on top?
+3
CR may nominally help responsiveness and efficiency, but there's about a 1mpg difference in fuel economy between the NA and turbo despite a 1.5 CR difference..
and youd be hard pressed to find any more "response" between say a 9.5 NA vs a 10.2 NA, turbos should be the same concept.
im playing with building a super-low CR engine (8:1 or even less) to really blow the **** out of with the big supercharger because who needs off-throttle power when there's a supercharger bigger than some other engines strapped on top?
Indeed, but the approach results in lower peak cylinder pressure at the same power level.
Yes!
The difference in engine dynamics is shown in the different shapes of the torque curves. Corleone's was remarkably flat on the whole usable rpm band, while yours has a more typical "mountain" shape with a clear ramp up to peak torque, then slightly falling down.
Again, different ways to skin a cat.
Lol for a treet car I would not go lower than 8.5 with 2 valves and lower than 9.5 with 4 valves per cylinder.
SDS article was written some 10+ years ago or something? Please find me a factory turbocharged 4valve per cylinder car older than 5 years and with CR less than 9? I would guess that there are none or some with ancient drivetrain.
Gen 1 997 Turbo has 4v, around 1.3 HP/l & CR of 9:1... whereas here we are talking about 8v & 2+ HP/l & significantly higher boost levels.
Yes!
The difference in engine dynamics is shown in the different shapes of the torque curves. Corleone's was remarkably flat on the whole usable rpm band, while yours has a more typical "mountain" shape with a clear ramp up to peak torque, then slightly falling down.
Again, different ways to skin a cat.
It is hard to fully understand Corleone's curves as posted in his thread. It is clear that he kept making power past the "typical" roll-off point. I am sure I am biased but I do not think my torque curve is typical as it allows me to keep making power at the top end.
Quote:
Originally Posted by Thom View Post
Yes!
The difference in engine dynamics is shown in the different shapes of the torque curves. Corleone's was remarkably flat on the whole usable rpm band, while yours has a more typical "mountain" shape with a clear ramp up to peak torque, then slightly falling down.
Again, different ways to skin a cat.
Originally Posted by refresh951
It is hard to fully understand Corleone's curves as posted in his thread. It is clear that he kept making power past the "typical" roll-off point. I am sure I am biased but I do not think my torque curve is typical as it allows me to keep making power at the top end.
'
Thom, are you talking about Corleone's dyno posted in his thread on 10-09-2007, 10:57 PM @ #134? I had seen it before but not any others.
If so, Shawn could simply adjust his boost curve so as to maintain around 330 ft lbs of TQ (which he could clearly do from around 3,700 rpm based upon his own dyno since he is holding well above that at peak rpm) and he would have a similarly flat TQ curve.
I was talking about street car where off-boost reponse matters.
And yes, 1 point of CR matters quite a bit, just compare US and ROW 944 engines and then apply turbo on top of that.
When you have decent CR, your car is more fun.
For a track car the goals and methods for getting the power are different, that's all.
It's your engine, do whatever you want with it I was just talking from my experience.
Just for the reference, a bone stock 924 2.0 euro engine with its 9.3:1 CR and crappy 2valve Heron combustion camber runs happily at 1.8 bar boost and 350+rwhp with E85. 1.2 bars and 300hp with regular 98octane pump gas
Please find me a factory turbocharged 4valve per cylinder car older than 5 years and with CR less than 9? I would guess that there are none
Subaru Impreza WRX has a compression ratio of 8.0:1 with 4 valve per cylinder.
Originally Posted by Raceboy
I was talking about street car where off-boost reponse matters.
And yes, 1 point of CR matters quite a bit, just compare US and ROW 944 engines and then apply turbo on top of that.
When you have decent CR, your car is more fun.
For a track car the goals and methods for getting the power are different, that's all.
It's your engine, do whatever you want with it I was just talking from my experience.
Just for the reference, a bone stock 924 2.0 euro engine with its 9.3:1 CR and crappy 2valve Heron combustion camber runs happily at 1.8 bar boost and 350+rwhp with E85. 1.2 bars and 300hp with regular 98octane pump gas
Imagine what 924 VW power plant could do with lower CR
I think there are ways to achieve better off boost without scarifying CR. Adding displacement, removing barn doors, catalysts, tuning intake, etc.
I agree with Raceboy. There is no way to get "good" off boost performance using a large turbo (and a cam timed to suit the compressor's best efficiency rpm bandwidth) without a high CR.
No one is going to make 500wN.m with a GT3582R 1.06 at 3k rpm without a high CR, even on a 3L, and still keep the torque flat up to peak rpm.
And the benefits of higher CR can't really be shown to its full extent on a dyno chart. More displacement and higher CR both give more off boost power and more exhaust gases which is exactly what drives the turbo.
So even if two engines have similar power on the dyno the one with higher CR will have a better response and get on boost quicker and be quicker everywhere where corners. Higher CR and lower boost also means lower IAT and usually higher timing which lowers EGT.
It's a balancing act. No such thing as "high CR is bad" or vice versa.
I built a 9.5:1 3.0l 16v in the past that made 514 rwhp on 1.15 bar on regular pump gas. With a GT3582R 0.82 T3 housing that thing was on boost just by looking at the throttle! It was superb. Like a well tuned factory high displacement engine with very linear power delivery and broad powerband.
Now with my latest engine which is a 100% race engine I run lower CR, around 8.6:1. For a race engine power response is also very important (at least if you run on smaller tracks). But given you are using higher rpms most of the time it is not as sensitive as on the street.
06-11-2015, 05:47 AM
supercharger
but I do not think my torque curve is typical as it allows me to keep making power at the top end. 
