.030 OVER COMPRESSION

5150

Does a .030 over amc 360 pick up a noticeable compression increase over a stock bore, and if so how much?

tufcj

Overbore has nothing to do with compression. Compression is all in the piston and/or head design. If you use a stock style replacement piston, you will have stock compression. Some replacement pistons actually lose a bit of compression if you don't deck the block or mill the heads because they move the piston about .020" LOWER in the bore to allow for decking. A .030" overbore will gain you about 6 cubic inches of displacement. Bob tufcj

ironman_gq

doesnt compression increase slightly because you are pushing more air and fuel into the same amount of volume in the head? provided you deck the block for 0 deck height.

Wrench Man

tufcj wrote
they move the piston about .020" LOWER in the bore.

You'll probably have LESS than you had before!

tufcj

ironman_gq wrote
doesnt compression increase slightly because you are pushing more air and fuel into the same amount of volume in the head? provided you deck the block for 0 deck height.

The volume of the cylinder increases by less than 1 cubic inch, probably negligible in terms of compression increase.

Bob
tufcj

jeepsr4ever

You get a small bump if you are using the same dish the same compression height and the same valve relief but on a larger piston the dish diameter is wider to make up for this.