jthorn9

OK, I had my compression ran today and here's what I got:

1 - 150
2 - 150
3 - 155
4 - 155
5 - 150
6 - 150
7 - 150
8 - 150

Now here's my question is this? Such as 9.5:1, 10.5:1 etc? I asked them what unit of measure they used and they told me pressure (if that means anything). I'd really like to know the ratio number if anybody has that. Also, is that normal to have a little variation in 3/4 or do I have a problem.

Demon 340

when you say "i have a 10 to 1 motor", that means that when your piston as at the lowest point in its travel (bottom dead center) it has 10 times more volume than it does when it is at the highest point in its travel (top dead center). This is why when you buy pistons, they list the compression ratio based on a certain combustion chamber size of cylinder head, because if you have a head with a small CC, there is less volume at TDC, thus increasing your compression ratio. in short, when your piston as at BDC, the cylinder has 10 times more volume than when at TDC.

The compression check that you had done does not really relate to your compression ratio. The compression check that they performed basically tells you how "tight" your engine is...engines lose compression due to a handful of issues such as bad rings...or in my case, a bad cam...

It has been my understanding that less than a 10% variation in compression from cylinder to cylinder is not an indication of an issue. Im not 100% sure of that figure though.

Im not certain how to figure the compression ratio of your engine, but if you know what piston you have, and the size of the CC in the head, you should be able to figure it out...or at least get a ballpark. if your engine was a stock 9.5 to 1 engine with a certain sized CC, and you kept the same pistons but have heads with a smaller CC, you have a higher compression ratio, but ive never had to figure out my CR. the pistons that i have for my white car now are 13.2 to 1 with a 60cc head, but im going to run a bigger CC head to get that ratio down a bit.