[limited]

Originally Posted by dcook_14 I think around 9-9.5 is good for centri and 8.5 is good for roots/turbo. What is the best way to lower a compression ratio without changing rods/pistons? Say if I wanted to lower the CR on an engine from 10.5 to 9.5. The two headgasket thing isn't going to make that much of a difference, so forget about that. Can material be safely removed from the combustion chamber on our PI heads? Is there an easy way to check the compression ratio of an engine? Or do you have to do the thing where you use a graduated burret to check volume at bottom dead center, at top dead center, and the volume of the combustion chamber of the head and then do BDC+CC divided by TDC+CC?

the more compression you can get away with(without taking a hit on timing) the more power you are going to make, for example I have ran 12to1 with a 12-71 spun 60 over(30+) on methanol. I believe on pump gas, more power is to be had on a lower compression and larger amount of boost than high comp. and less boost

taking material off the cc will lower cr but not a extreme amount(you won't get a point out of it) you probably can gain a .5 or so from a polish on cc and thicker gasket,anymore you need to change pistons. there are several calculators avail. online to play with, just google them

[dcook_14]

Well I did what limited said and did a few online calculators. I put in stock bore, stroke, deck height, head gasket thickness and bore, and the dish of my piston and got an average of 10.67:1 compression ratio.

Then I added 2cc, a reasonable amount of material to be removed during polishing, and doubled up the head gasket thickness and it brought it down to an average of 9.51:1 on all the calculations I tried. I didn't think it could make that much of a difference but apparently it does!