Compression Ratio Changes with Cams

[SpoolinGSX]

What are the dynamic compression ratio changes with stock 2g pistons with 264 and 272 intake cams?

 

[4pistons]

Compression RATIO has nothing to do with cams. It is the ratio of swept cylinder volume versus unswept volume by the piston. I think you mean cylinder pressure which will change with cams, boost, intake, porting, headers, etc. There are alot of variables. I am not sure that it can be answered conclusively. It depends on your specific application.

Rick - '91 GSX

 

[Groomz]

Excellent piston dude! Cylinder pressure would change w/ the outside enviroment, so it would be difficult to have a definitive answer. Cams do net more power, so the pressure would be higher.

 

[SpoolinGSX]

Actually the intake valve is still open for the beginning of the compression stroke. So the static compression ratio would remain unchanged with aftermarket cams, but the dynamic compression ratio, using the amount of piston displacement after the intake valve has closed would be different because of the altered duration of aftermarket cams. So in a car with no porting work, what would the difference be? I assume the 264/272 is just the degrees of the valve being open, so i will just do the compression calculation myself. Just i dont know the stock duration of the intake and was wondering if anyone knew the numbers off the top of their head.

 

[a351must2]

No answer for you, but I'll back up the fact that static compression does change with cams.

I have an 11:1 compression 351W in one of my Mustangs and when I first put it together I had too mild of a cam and 225psi of static compression. Changing to a special grind cam with not much more .050" duration, but a lot more seat to seat duration knocked the static compression down to a more reasonable 175psi that would actually run on 92 octane.

 

[4pistons]

the dynamic compression ratio, using the amount of piston displacement after the intake valve has closed

SpoolinGSX is right. The DYNAMIC CR changes with the cams. I have not heard that used in a while and glossed over it. I also assume (shame on me) that most of the questions are coming from newbies. My bad!

I don't have that data or know exactly where you could find it. AMS did some extensive cam comparisons. http://www.automotosports.com/ Maybe they could supply that info. Perhaps posting the question in the Extreme Tuner section might get you an answer. I believe the DSM shop manuals have the stock cam specs in them. I don't have one handy.

Rick - '91 GSX