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914World.com _ 914World Garage _ What's the conversion for...

Posted by: iiibdsiil Feb 17 2005, 08:27 PM

PSI to the compression ratio?

Posted by: Jake Raby Feb 17 2005, 08:28 PM

Their really isn't one...

The cam and so many other factors effect cylinder filling and that effects cranking compression...

Posted by: redshift Feb 17 2005, 08:35 PM

Happy Belated Birthday ANYHOW!

Didn't see you around.

M

Posted by: iiibdsiil Feb 17 2005, 08:36 PM

Miles, thank you sir.

Jake,

You are killing me here. I guess that is the right answer though, I'm just not happy about it.

Posted by: Jake Raby Feb 17 2005, 08:39 PM

Pop off a head, measure the deck and cc the chamber...
Thats the only for sure way to know...

Posted by: TimT Feb 17 2005, 08:39 PM

Rough work around,

say 8.5:1CR

8.5*14.7=124.9psi

so you could take psi and divide by 14.7 to tell you a rough CR

if that what your looking for..

Like Jake says to many other variables. Cam Timing,profile etc.

Posted by: iiibdsiil Feb 17 2005, 08:49 PM

Thanks guys. Explains why I couldn't get a straight answer on google.

Posted by: Jake Raby Feb 17 2005, 09:18 PM

Elevation effects air density and air density effects cranking PSI....

So someone in Florida can compression test their engine, drive it to Colorado and re-test it and get two drastic different readings... The higher the altitude the LESS PSI you will see.

I set engines that are going to High Altitude 20% higher than normal as an average number.....

Posted by: iiibdsiil Feb 17 2005, 09:38 PM

That makes sense. I never thought about it being that deep. Makes perfect sense.

Posted by: john rogers Feb 17 2005, 10:19 PM

The answer about cam overlap and the affects on compression is spot on. At one of our vintage races a guy from AZ has a 1969 Z28 and it cranks over like there are no heads on it. I asked him what the c/r was and he said 13:1 but with the cam overlap he could maybe get 100# when doing a compression test! Boy it sure ran good though once it fired up!

Posted by: Gint Feb 17 2005, 11:11 PM

QUOTE
I set engines that are going to High Altitude 20% higher than normal as an average number.....

And 20% is just about the right number for the difference between sea level and Denver at 5K altitude.

Posted by: Jake Raby Feb 18 2005, 01:25 AM

You got it....

When I say that i build engines for the application- Its not BS