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Thu Feb 21, 2013 10:08 pm

I am running 150PSI in my cylinders... anyone know about what that would convert to in a compression ratio? Is there a formula for it?

Thu Feb 21, 2013 10:15 pm

I don't believe you'll be able to figure it out based off of the pressure it's holding. Now, if it were reading volume, it might work out for ya. Pop the head, you can figure it out that way.

Fri Feb 22, 2013 8:44 am

83vwbug wrote: I am running 150PSI in my cylinders... anyone know about what that would convert to in a compression ratio? Is there a formula for it?

Camshaft also comes into play when reading cylinder PSI.
Stockish cams generate more PSI than a big cam with a lot of overlap.
Have to pull a head measure the cc's in the pocket, measure deck height, plug those numbers and the bore and stroke numbers to get true compression ratios.

You can do that here:

http://www.davis-motorsports.com/engine_calculator.html

Fri Feb 22, 2013 9:09 am

Why are you needing the information? From the compression test # we can tell a few things about the motor in an educated guess sort of manner. No one can pin it down, but If you know the cam and size of engine I bet some guys get very close.

Sat Feb 23, 2013 10:34 am

This calc returns cylinder pressure, you can use it iteratively to back-calculate your DCR from a known psi:

http://www.rbracing-rsr.com/comprAdvHD.htm

Many other useful calcs on that site, too, check it out.

Sat Feb 23, 2013 11:46 am

1915 with 110 cam. that's about all I know... that calculator is pretty cool still trying to get all my number figured into it

Sat Feb 23, 2013 2:44 pm

This is a dynamic compression calculator for your engine if you pull the head to calculate the numbers.
http://www.kb-silvolite.com/calc.php?action=comp

I just imputed the numbers for a 1915cc engine we just built.
10.54 static compression, stock rod length and a W110 cam = 8.404 dynamic.

I used tencentlife's calculator and imputed the known variables on the same engine as above.
The results for it using the 150 PSI number in that calculators boost box and it came up with 8.2.
We did not measure the compression PSI on this build.

Here is your cam timing card.

Sat Feb 23, 2013 6:59 pm

thanks for the info, much appreciated!

Mon Feb 25, 2013 9:33 am

Static compression ratio of 10:1.
Effective stroke is 2.04 inches.
Your dynamic compression ratio is 7.64:1 .
Your dynamic cranking pressure is 149.73 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.64 :1.
V/P (Volume to Pressure Index) is 39

This is using wallace racings calculator using an elevation for Grayson Kentucky of 600ft.

pluging in 9.5 compression shows only 140 psi so i think you are into the 95 octane area.