03-22-2010, 09:26 PM
Join Date: Oct 2006
My Ride: can't be defined
Originally Posted by PEI330Ci
I've measured this stuff a number of times, and I'm still coming up with the same numbers. So here's what's been bothering me:
Cylinder bore = 3.307"
Stroke = 3.528"
Combustion chamber volume = 34cc
Piston/cylinder volume below deck = 17.4cc
Head gasket thickness = 0.030"
Head gasket bore = 3.346"
Plug all those numbers into a compression ratio calculator and you get 9.911:1.
BMW advertises 10.2:1 for the M54B30
The only thing I can guess is that BMW is not including the volume inside the spark plug, and the area above the first piston ring along the cylinder wall. The actual "pour volume" using fluid equals 55.723 inside the combustion chamber with the piston at TDC. Dropping that by just 2cc brings the compression ratio to 10.24:1, which might be the case if you didn't include the spark plug volume ect....
I'm sticking with real world numbers, so the M54B30 in my books is a 9.9:1 engine.
The CP pistons I have pictured above netted a compression ratio of 9.26:1 with a "pour volume" of 60.823cc.
Using a well known engine calculation software package, I was able to build a "model" of my engine to represent the actual power it made on the dyno. (252hp/229tq)
I then changed the compression ratio from 9.26 to 11.4:1 and netted a calculated 265hp/241tq @ 6400 RPM which was closer to what I was expecting when I built the engine.
I was getting something close to those numbers, with a 33cc combustion chamber volume. I may have some insight to add tomorrow when I go to the machine shop. I have some printed calculation sheets with some of the numbers that CP uses, such as compressed gasket thickness (I believe .025). Also, aren't your CPs 84.5mm? My cylinder bore was 3.328. I'm guessing you were referring to the measurements for the stock pistons?