Mahle Compression Ratio Inconsistencies???
02-06-2003, 07:50 PM
#1
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Mahle Compression Ratio Inconsistencies???
Ok, so I purchased Mahle 3.4l 7.5:1 compression piston and cylinders, part number PP98-006. However, in going through the process to calculate the compression ratio to help me with setting my deck height, I came to find out that these pistons in my application are actually 7.15:1, NOT 7.5:1!!! Knowing that Mahle also makes a 7.1:1 piston, I wanted to make sure I got what I ordered, so I did verify that I got what I orderd, and not the 7.1:1 pistons.
I am using a stock 930 head with only a modification for using a second spark plug. They cc'd out to be 90.6cc. Bruce Anderson's book indicates that the 930 head is 90cc. I can attribute the .6cc to the material removed for the second spark plug and is negligable. I also cc'd a set of completely stock 3.3 Turbo heads I have and they came out to be 90.1cc.
I also measured the net dome volume of the Mahle piston since Mahle did not know what it was and said to call Andial since they designed it. Andial told me to "figure it out yourself"...but thats another story. I calculated the net dome volume and found it to be 7.9cc. Therefore, when you include a deck height of .042 and plug everything into the formula, you and up with 7.15:1 compression...
Obviously, there are production differences in heads, cylinders and pistons, but I think these are conservatively rated. As an aside note, I need approximately a 12cc net dome volume to acheive a 7.5:1 compression ratio.
Additionally, the 7.5:1 RUF pistons also have a 7.9cc net dome volume so they are also really 7.1:1 in my application.
In doing some additional research, I did the calculation for my original 3.3l compression ratio and found it to be 6.58:1 compression even though the factory lists the 930 turbo as 7:1 compression. In discussing this with some engine builders it turns out that this is not unusual, in fact it is more the norm where the advertised compression is less than the actual compression. Interesting stuff for all of us who have thought we've increased our compression ratio!!!
I am using a stock 930 head with only a modification for using a second spark plug. They cc'd out to be 90.6cc. Bruce Anderson's book indicates that the 930 head is 90cc. I can attribute the .6cc to the material removed for the second spark plug and is negligable. I also cc'd a set of completely stock 3.3 Turbo heads I have and they came out to be 90.1cc.
I also measured the net dome volume of the Mahle piston since Mahle did not know what it was and said to call Andial since they designed it. Andial told me to "figure it out yourself"...but thats another story. I calculated the net dome volume and found it to be 7.9cc. Therefore, when you include a deck height of .042 and plug everything into the formula, you and up with 7.15:1 compression...
Obviously, there are production differences in heads, cylinders and pistons, but I think these are conservatively rated. As an aside note, I need approximately a 12cc net dome volume to acheive a 7.5:1 compression ratio.
Additionally, the 7.5:1 RUF pistons also have a 7.9cc net dome volume so they are also really 7.1:1 in my application.
In doing some additional research, I did the calculation for my original 3.3l compression ratio and found it to be 6.58:1 compression even though the factory lists the 930 turbo as 7:1 compression. In discussing this with some engine builders it turns out that this is not unusual, in fact it is more the norm where the advertised compression is less than the actual compression. Interesting stuff for all of us who have thought we've increased our compression ratio!!!
02-06-2003, 09:29 PM
#3
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Seems like the crank-case deck height could vary with manufacturing varinces also. This to could effect one's compression ratio to.
It may not be as accurate as your mesurement but I had one cylinder, pistion & head built up; turned the motor so the spark plug hole was up; then added ATF in a mesured amount with a hypo to confirm my compression ratio.
On my 72S we first decked the case (instead of flycutting the heads), then we used gasket/spacers to get it where we wanted it.
Thanks for the heads up!
It may not be as accurate as your mesurement but I had one cylinder, pistion & head built up; turned the motor so the spark plug hole was up; then added ATF in a mesured amount with a hypo to confirm my compression ratio.
On my 72S we first decked the case (instead of flycutting the heads), then we used gasket/spacers to get it where we wanted it.
Thanks for the heads up!
02-06-2003, 11:41 PM
#4
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Geoffrey;
I've picked through my boxes of parts and found my ni-resist flame ring. They are all measuring .1355" to .1360". I also measured my old base sealing ring. .0415" Crushed... I'm still pondering the cylinder to piston clearance.
I've picked through my boxes of parts and found my ni-resist flame ring. They are all measuring .1355" to .1360". I also measured my old base sealing ring. .0415" Crushed... I'm still pondering the cylinder to piston clearance.
