how much compression / hp gain?

[philoldsmobile]

from a 20 thou head skim?

just had the head gaskets done on my 96 3.8, and the garage took the heads to a machine shop who skimmed 20 thou off them.

the car is noticably more punchy, especialy mid range, how much hp is it likely to have picked up? how much is compression likely to have increased by?

any ideas?

BTW, we have 95 as standard grade petrol here, so no det problems at all, i think it could take quite a bit more compression....

 

[HDJ]

Assuming:

bore: 3.81
stroke: 3.39
stock CR: 9.0
same head gasket thickness as before

Then:
V stroke = (bore/2)^2*Pi*stroke = 38.65 in^3
V compression = V stroke/8 (derived from CR=(vstroke+Vcompression)/Vcompression) = 4.83 in^3

20/1000 skim = (bore/2)^2*Pi*0.02 = 0.23 in^3

So your new V compression = 4.83-0.23 = 4.6 in^3

New CR = (vstroke+Vcompression)/Vcompression = (38.65+4.6)/4.6 = 9.4:1


But then, according to this site,

http://www.bgsoflex.com/crchange.html

It should give you only about a 1% increase in HP

However, depending on your reason for having the heads skimmed in the first place,
You may still notice a difference if your previous setup had some kind of power loss due to minor leaks.

 

[philoldsmobile]

hmmm.... would be interesting to put it on a dyno