Compression ratios
01-24-2004, 05:50 PM
#1
User
Thread Starter
Join Date: Oct 2001
Location: Cincinnati, Ohio
Posts: 380
Likes: 0
Received 0 Likes
on
0 Posts
Compression ratios
I am very curious about other's experience with actual measurement of volumes to determine comression ratios in the 928 engines of various years. I know Tony ran some numbers for his S4 project and got similar results to what is listed below.
The 1981 USA M28.15 engine is specified with a CR of 9:1 from Porsche. When I had the JE pistons made I ccd all my original parts and ran some numbers and came up with the following:
Vhead=60cc
Vpiston=10cc
Vgasket=8.8cc
With the 95 mm bore and 78.9 mm stroke, the numbers give the following:
Vswept= 559.3cc
Vtot=638 cc
Vclearance= 78.8cc
This yields a CR of 8.09:1, or 8.1:1 for round numbers. My methods of measurement were by clay and water displacement, but I know I was very close to actual measurements.
Perhaps the CR Porsche publishes are for the engine after 0.030 has been shaved off the head? (there is a spec for how much can be milled), if you rerun the numbers taking 8.8 cc out of the calculation (about what would be lost by shaving the heads), you come up with 8.989:1, or 9:1. I did not touch my heads or block to avoid bumping the CR up any further.
Makes me wonder if all of the CR specs for these engines are on the high side of the actual numbers before any head machining has been done?...
The 1981 USA M28.15 engine is specified with a CR of 9:1 from Porsche. When I had the JE pistons made I ccd all my original parts and ran some numbers and came up with the following:
Vhead=60cc
Vpiston=10cc
Vgasket=8.8cc
With the 95 mm bore and 78.9 mm stroke, the numbers give the following:
Vswept= 559.3cc
Vtot=638 cc
Vclearance= 78.8cc
This yields a CR of 8.09:1, or 8.1:1 for round numbers. My methods of measurement were by clay and water displacement, but I know I was very close to actual measurements.
Perhaps the CR Porsche publishes are for the engine after 0.030 has been shaved off the head? (there is a spec for how much can be milled), if you rerun the numbers taking 8.8 cc out of the calculation (about what would be lost by shaving the heads), you come up with 8.989:1, or 9:1. I did not touch my heads or block to avoid bumping the CR up any further.
Makes me wonder if all of the CR specs for these engines are on the high side of the actual numbers before any head machining has been done?...
Last edited by John.; 01-25-2004 at 01:34 PM.
