[1978malibu]

I have a .30 400 that has 5.7 rods and trw flat top pistons with 2 valve reliefs and was thinking about a set of 225 promaxx heads but not sure whether to get 67 or 73 cc heads and haven't decided on a cam yet but this build will be for a street car that will see some track time occasionally

 

[ExcellenceAuto]

Can you get your stats put into an online compression calculator? Or tell us how far below the deck does the piston sit? How big are the valve relies? Head gasket compressed?
You can get away with 11:1 or less on good pump gas, if everything goes right.
Get you a nice cam matched to those heads and you will be in business.
Someone with more experience will come in soon!

 

[1978malibu]

I'm need to measure where the pistons are but I'm thinking. 20 in the hole and haven't got as far as head gaskets yet

 

[ExcellenceAuto]

You will be right at 11:1 with the 67cc chamber. If I remember correctly gasket choices are pretty [email protected] .035 So that is gonna make it tough if you are that far down in the hole. Others know more, and can explain much better but you will be making some good power that's for sure!

 

[1978malibu]

Yeah should run decent for a street car

 

[ericnova72]

I calculated with a 67cc head chamber, 5cc valve relief volume, .041" thick x 4.190" bore head gasket, and piston .020" down bore @ TDC and that results in 10.72:1 compression ratio with a rather wide .061" quench clearance at TDC(quench is actual assembly piston-to-head clearance).
With a 73cc head and everything else the same compression ratio drops to 10.09:1

Zero decking the block to tighten quench up to a more optimum level results in 11.25:1 with the 67cc head and 10.55:1 with the 73cc head. Optimum quench is in the .035-.045" range and helps reduce octane sensitivity even as compression ratio is slightly higher. Tight quench improves power also, most effective at part throttle.

Without more guidelines on the rest of your combination, can't recommend a cam....but I can say this, any cam except a hydraulic flat tappet.

The ProMaxx 225cc is a pretty good head for the price.

 

[johnsongrass1]

I have a .30 400 that has 5.7 rods and trw flat top pistons with 2 valve reliefs and was thinking about a set of 225 promaxx heads but not sure whether to get 67 or 73 cc heads and haven't decided on a cam yet but this build will be for a street car that will see some track time occasionally

Are you limited to pump gas or can you go E85?

 

[1978malibu]

Are you limited to pump gas or can you go E85?

I'd rather stay pump gas because not to many places close to me has e85

 

[johnsongrass1]

Id go 68-75cc chambers on a 0.000 deck.

 

[ericnova72]

Yeah, the zero decked block, .039" to .041" thick gasket and 73cc chamber would get you right there around 10.5:1, and with aluminum heads pump gas is not a problem.

Nice safe place to be if you aren't looking to push the limit on pump gas use.