My motor right now has 10:1 static cr...i have edelbrock rpm heads and was wondering how much compression would I drop by having the combustion chambers professionally reworked to 80-84cc instead of the 70cc chambers the heads came with. I'm thinking about bolting on a weiand 177 roots blower to it and would like at least 9:1 compression.

 

Don't do it that way. 9:1 is still too much for a roots blower on pump gas. Trust me, you'll blow it up like I did..... twice. You want to be down around 8:1 if you're going to throw any meaningful boost at it on pump gas. Get dished pistons and do the motor right.

No, the low compression won't cost you any real power. Mine still makes the same power even with almost a full point less compression. It's just roots blowers ain't like nitrous or a centrifugal- detonation is always right around the corner and high compression is not the thing you want to be on the ragged edge with.

 

i agree on dish pistons as well. also why ruin a good set of heads.

 

If you were to change the combustion chambr volume from 70 to 84 cc on a 10:1 cr motor, your new cr would be 8.75:1.

Taking 14cc's out of the chamber is a little much. How about
6 to 8cc's. chambers would be 78cc finished.

You want to end up with 7.5:1 for maximum performance on pump gas with a roots blower. (allows the most blower boost with the least retard) =makes the most power. About 10-12 lbs boost. This is how the pro's like Jim Oddy set up a roots blown motor for max street/ marine performance all day on pump gas.

At a 8:1 cr you can/could make some serious reliable power with slightly less blower boost and some more spark retard.

Changing the pistons is the "right way".


But you *could consider* stacking a few .060" thick copper head gaskets , combined with adding 8 cc's to your heads ( your self). Your cr would be 7.94:1 , assuming your cr is actually 10:1 now. About 1/2 the cost of new pistons.
Should be ok at 8lbs boost with a retorque after install.
You may need new longer pushrods.

-22cc D cup forged pistons are common. Yo could swap to a set of these combined with 8cc larger chambers and a standard gasket and get 8:1 also.
Optimum build for max street performance and power using pump gas would be -30cc dished pistons and 78cc heads cr would be 7.54:1.

What are the exact specs of your engine? bore, stroke deck clearance, Valve relief volume and gasket thickness, now?