I have a balanced 350 bored .060 over with GM #3991492 angle plug heads. I have TRW 11.7:1 pistons with 1.5 aluminum roller rockers. The head gaskets are Felpro .042. If it matters the cam is .483/.507 lift. Edelbrock air gap with a Holley 750. When I had the motor dynoed, the shop told me it would not run good enough on pump gas with this setup. If i change the heads to a aftermarket pair with a larger cc., would that change my compression enough to run pump gas? Thanks!

Well....what cc heads are you running right now? I know that if you go with higher cc heads would certainly drop your c/r...maybe 76cc heads? However, heads are expensive. I could recommend you putting a copper head gasket on that is a little thicker. This should help reduce your c/r the tiniest bit. However, I would possible recommend going with different pistons. I know you already have a set of TRW's but honestly...pistons are the easiest way to lower your c/r and stay a little cheaper than $1200 bucks for a nice set of assembled heads.

Hey! One more thing...your 350 is bored out to 4.060? You ever have any over heating problems with that block?

What's the dome volume of the pistons? What's the chamber volume of the new heads?

Another thing my pistons said "10.8:1 speed pro" in summit when I actually cc'd them and cc'd the head I ended up with 11.32:1 with .037 quench. Id check the pistons before assuming they are "11.7". Im assuming you havent really chcked them and your just going by what was published.

Hey guys, I don't know the cc's of a set of stock angle plug heads. I will try to research what they are. I see Edelbrock and other heads and they list the cc's and it appears that a person has a few choices when purchasing them. As far as overheating, I've had no problems. I went to Car Craft Nationals in Saint Paul last year and it was about 95 degrees outside. Cars were blowing hoses all over. I cruised the fair grounds and mine hit around 200 degrees. I'm just going off the part number on the pistons for the 11.7:1 compression. I have not checked anything. I bought the motor this way and pulled the intake, heads, and pan off to see if the guy I bought it from was correct. I wrote down part numbers, etc. and put it back together and had it dynoed. It does run great but the race gas is getting a little old.

Fwiw, -492's are made w/ two different chamber sizes. 64 and 75 cc's!

Anything from 1971 on newer should be 75 cc's.

Try here..

http://www.chevytech.net/

So post back the casting date info off your heads..

You just may be able to change from 64 cc or so to 75 cc or so heads and drop your CR. down to pump gas level fine..

I would use either the .015". .017" or .020" shim headgaskets to keep the quench as tight as possible as long as it doesn't go below about .035" or thereabouts..

And check your CR here too....

http://www.campbellenterprises.com/Race%20math%20calculators.htm

pdq67

My casting number shows it to be a 64cc. head. I tried to use the calculator on the website but I don't know the dish or valve pocket number. Also, my block was decked so that number would be different also. As you can tell, I'm not real up on this part of the motor. Thanks!

Surely somebody out there has an old TRW cat. that list's an 11.5 to 1 CR. piston using a 64 cc head so that he can get the dome cc's to figure it more accurately.

Anybody?????

pdq67

.200 dome 10.2CCs 11.87 to 1 with 64cc heads.

If going from 64cc heads to 76cc heads , you're dropping estimated 1 full point.

That said DougG, then I would be at a 10.7 to 1 instead of 11.7 to 1? Then, would pump gas work? Thanks!

What's the advertised duration numbers for your cam??

It ain't the E-brock RPM is it?

pdq67

10.7 with Aluminum heads is doable depending on cam, if steel heads it's borderline.

.200 dome 10.2CCs 11.87 to 1 with 64cc heads.

remember to subtract for the valve relief mine are .125 domed and after the valve relief Im only at 1.5cc

My cam numbers are .483/.507. That what it measured out to be. Does this help?