[flightlead404]

Originally Posted by Coultl

I posted this in my engine rebuild thread but since it’s directly related to the turbo system I’ll post here too.

To lower the compression ratio I took some material off the top of the pistons. I ended up pulling exactly 10 grams off each piston. I would have liked to take a little more off but it felt like I should quit while I was ahead. To get those 10 grams I pulled about 6mm off the top of the piston.

With Aluminum’s density of 2.75 grams per cc, the 10 grams I pulled off is equivalent to 3.6cc of aluminum…removed from the combustion chamber. This lowers the compression ratio from 9.5:1 to 9:1.

I think 9:1 will be passible for my modest power goals, but it a still a little high. I ended up going with 1.3 mm base shims on the cylinders. My stock base shim is 0.5 mm because the case halves have been machined. The additional 0.8 mm increases the combustion chamber volume by 5.7 cc. Adding it up, the machine pistons plus the base gaskets resulted in a compression ratio of 8.4:1. Perfect.

I have not seen anyone else cut these pistons down for this purpose but there’s a lot of aluminum up top. I think they will be fine.

I'm curious about the shape left after machining. IDK if the flat top left impacts "swirl" and charge mixing, but the sharp edges worry me a little. I guess time will tell.