Hello. I am building a 350 for my 68 rs/ss camaro and would like a little input to help me get the compression ratio I desire. I will be using Speed Pro H631P pistons .040 over. Heads are 68cc Dart torquers with 2.02 valves. According to the spec sheet for the pistons, this combination will compute to 9.3 compression. I was hoping for something between 9.5-10.0 to one. Does anyone know roughly how much I can gain by getting the heads milled? My machine shop said they could take off .030 but didn t know how many CCs that would reduce the chamber volume or how much the compression would be raised. Also how much could I gain by using a steel shim head gasket instead of a thicker composite type?
Just for fun, any estimates on the h.P. this engine will make. I am using a Extreme engery XE268H Comp.Cam, RPM intake, roller rockers and a good free flowing exhaust. I am thinking around 350HP.
Thanks in advance for any replies.
Phil In Moncton

Please go over to ROSS Racing Piston site and use their CR. calculator! It's fun to play with.

I use .025" down in the hole and -4 to -6 for four valve notch stock type flat-top pistons, then vary cc's of the heads and headgasket thicknesses from .015", .022", .028". 038" or .039. and .040"+ or so to come up with needed CR.

But it's best to stack down in the hole and the headgasket to get a quench of between .035" to probably max. of like .045"?? Even though some do run tighter and looser.. .030" and .050"...

Have fun.

pdq67

Dart should be able to tell you exactly how much milling your heads .030" will reduce the chamber volume. Usually they have a guide that tells you that flat milling every .010" reduces chamber volume by __cc's. It is not the same as reducing your head gasket thickness because the chamber opening is smaller.

To pick the right head gasket just measure your deck height once the pistons are installed then pick a head gasket like PDQ suggested to get your total quench height between .035 and .045". I wouldn't use head gasket selection to change your compression ratio -- quench height is much more important when you consider that changing head gasket thickness will only get you a couple tenths of a CR point anyway.

And BTW with iron heads and a smallish-medium cam 9.3-9.4:1 is perfectly fine IMO. Another .5 point of compression won't get you any significant power anyway -- unless you are going with a medium-large size cam, then you could probably use up to another full point of compression.

I'd be a little conservative wit hthe compression ratio -- hyperutectic pistons don't handle detonation as well as forged pistons do.

Just my opinion.

Milling .030 is only good for about 5cc.

if your pistons are .020 down in the hole like stock, then a .015 shim gasket will give you the .035 quench that everyone claims is the low end of the spectrum, and will bump you up a little bit in compression.