Compression pros and cons
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mikestertwo
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Need more info. Depends on a lot of things like how deep your wallet is.
Increasing static CR vs boost pressure - ModularFords.com
Read this thread, it will answer your questions.
Read this thread, it will answer your questions.
Some good math there but they completely leave out the cam profile and what happens when both valves are open. It changes the picture a lot. After reading that article some may be convinced to drop the CR to 6:1 and run 50-55psi
. When the engine is built around a higher CR and high boost with the proper fuel it will make more power everywhere in the operating range. Especially when out of boost or low boost like spooling. This can change the converter requirement down low making it easier to get a good coupling % up top. If you plan on running 93 octane there is little reason to increase CR.- Joined
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Actually, it brings up more questions than it answers, especially for a turbo Buick!
The original post is too general to give a proper answer. Without a specific application and planned or current build specs, no way any good reply is possible.
As far as the Ford guy, all his theoretical info has the same issue, different applications have different answers and/or approaches.
Since we have been trying to answer this question for many years in actual engine builds, our excellent track results have give us a lot of info in this area.
More boost and more compression with the proper octane will make the most power over largest range.
dr_frankenstein
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Some good math there but they completely leave out the cam profile and what happens when both valves are open. It changes the picture a lot. After reading that article some may be convinced to drop the CR to 6:1 and run 50-55psi
So, the real question should be the affect of dynamic compression on a turbo buick VS static compression right?
A.j.
The real question is what octane are you going to run and how hard are you going to run it.
What I got from the article and from my own research is that within limits, lower compression and higher boost will allow for a higher BMEP before detonation which ultimately allows more power.
Obviously, there are limits and other factors. Examples of which are turbo efficiency ranges, spool-up , converter selection, etc. Lower compression will slow your spool time, one way of compensating is with a looser converter (good point bison) which may ultimately outweigh the power gained at peak boost because of slippage.
So to build max hp using a detonation limited fuel, if you could build an engine with very low compression and were able to generate enough mass air flow to take it right up to the detonation limit, you would build more peak power than the same engine but with more compression. The detonation limit would be lower.
This is ignoring the other factors like spool-up, power curve, turbo efficiency ranges, etc
Bison, I'd like to hear you elaborate on your thoughts on cam selection with regards to this subject if you would
Obviously, there are limits and other factors. Examples of which are turbo efficiency ranges, spool-up , converter selection, etc. Lower compression will slow your spool time, one way of compensating is with a looser converter (good point bison) which may ultimately outweigh the power gained at peak boost because of slippage.
So to build max hp using a detonation limited fuel, if you could build an engine with very low compression and were able to generate enough mass air flow to take it right up to the detonation limit, you would build more peak power than the same engine but with more compression. The detonation limit would be lower.
This is ignoring the other factors like spool-up, power curve, turbo efficiency ranges, etc
Bison, I'd like to hear you elaborate on your thoughts on cam selection with regards to this subject if you would
You can still detonate on 114
Yup.:biggrin: As far as camshafts go the closer to the peak torque rpm you are the higher the bmep so that is where the detonation will first show up. So if you are raising the rpm of the engine you are giving it less time to fill the cylinder and therefore less bmep. Therefore in a solid cammed application that revs to 7000 rpm the detonation limit will be higher even with increased CR. There are a lot more things that will effect the bmep that could take to long to post about. An example would be an engine that has 8.0:1 and 700 ft lbs of torque at 3000 and one that has 8.0:1 and 700 ft lbs at 5500. The engine at that is at less rpm has a higher bmep and raising compression ratio will increase the octane requirement substantially even if both were fine running the same octane previously. The power band becomes narrower as rpm increases also. Most of the guys on here never even come close to the detonation limit of the gasolines available today. Nearly 10:1 at 7000+rpm at about 40 psi has been done.
Good post Bison
a given HP at 7000 will have a lower bmep than the same hp at 5000
So basically what you are saying is the larger the cam (higher rpm band) the more compression you can get away with.
Seems like you still have trade-offs. Run the larger cam and you lose some low end anyway even though you can boost compression. Delicate balance there between too much and too little cam (and compression) I would think.
a given HP at 7000 will have a lower bmep than the same hp at 5000
So basically what you are saying is the larger the cam (higher rpm band) the more compression you can get away with.
Seems like you still have trade-offs. Run the larger cam and you lose some low end anyway even though you can boost compression. Delicate balance there between too much and too little cam (and compression) I would think.
Yeah kind of. But theres a lot that happens in the overlap period too. For 99.5% of the guys on here they dont really need to worry about detonaiton threshold of gasoline sine none of them are even close to it assuming they are running the proper octane and fuel. The top TSM, TSO, and outlaw guys are about the only ones i see even coming close to the threshold. They run about the best fuel they can get. It really depends on the depth of the wallet and if you want to milk it for all its worth running it on the edge or close to it. Heads up racers are that type of racer. Thats why any of the heads up classes are so expensive to be competitive in. If you can even think about something another racer probably already has tried it. You really have to go into un-charted territory to be on top. I remember when back in the mid to late 90's everyone though that 22-23psi was the safe limit for boost in these cars. I let them keep thinking that while i ran 27-28psi. Whenever someone asked how much boost i ran i always told them the truth and got a response like "you will break it for sure at that boost level". I would just answer your probably right
. I ran 2 GM steel head gaskets on each side and beat a lot of peoples a$$es back then running 28psi.Similar threads
