915 to edelbrock performer rpm heads, bad idea?

[lars]

Hi! first post on this forum   I have a question about compression.
I've got a rebuilt (at least what I have been told) -67 440, 030 over, steel crank, forged pistons (stock replacement), 915 heads (1.60 exhaust)  . was told it should be about 9.8:1 compression (shouldn't  it be 10.1:1 if it's built with stock replacement pistions?  )
Cam shaft is Comp extreme energy XE268H
My plan was to replace the 915 heads with aluminum Edelbrock performer rpm 84cc heads
would this work, what about compression will it drop too much, would this be a performance loss or would I gain hp ?

Thanks!
Lars

[firefighter3931]

Most stock replacement pistons will not yield over 10:1 unless the block has been decked and the heads are milled. You need to find out how far your piston is below deck at TDC....then calculate your static compression. Once that is done you can determine what size chamber you need to hit the target compression ratio. The increased flow of an RPM alone will make a huge difference in power output.  


Ron

[Cooter]

IIRC, the Eddy heads are of the closed chamber design and offer MUCH larger valves and ports, so they should out perform the "915" heads hands down...

[lars]

Ok! but lets say the compression is right at 9.8:1 with the 915 heads 78,5cc combustion chamber, how far will compression drop with the Edelbrock performer rpm heads with 84cc combustion chambers?. Will the compression be to low for using the aluminum heads? could you use a thinner head gasket to increase compression, how much?

Thanks!
Lars

[Cooter]

You can always mill the Eddy heads to the desired 78 CC to get back what you've lost...

[lars]

I will not able to mill the eddys down cause they might be used with another engine later on. If my 915 heads has 80++ combustion chambers, and the new eddys got 84cc that just about 4cc in difference, is that a huge difference in compression? how much will I drop?. I have heard that aluminum heads doesn't like to low compression that you should aim for 10-10.5 and likes more heat in the combustion camber??

Thanks!
Lars