firefighter

Here we go I'm building a 383ci fuelie. I got the short block from the machine shop and it's got the wrong pistons. I ordered a 3.75" stroke with 6" rods and "inverted dome" SRP pistons well I got flat tops instead. The heads are "supposed" to be 64cc buut they are more like 66cc-67cc then I CC'd the cylinder with the piston at TDC and got 14cc (the piston has 5cc valve reliefs) so here is the question whats my compression gonna be? I figure I'm gonna run a .035 to .040 compressed height head gasket.

The problem/question is do I go by the cc'd volume of the cylinder or do I go by the advertised -5cc's for the valve reliefs? The numbers are substantially different the JE chart figures around 10.5 to 1 but the calculator says about the same figuring on -5cc but figure on the -14cc cylinder volume it says about 9.5 to 1.

So what is it?

angel71rs

Go by measured values. Reason you got 14cc is that the piston is not flush with the block deck unless it has been zero decked. So piston is slightly down in the bore (deck height) and that adds volume to measurement. Check out this calculator:

http://www.turbofast.com.au/Tfcomp.html

firefighter

I need to check the deck height in the morning but it looks pretty damn close to zero I think most of that volume is coming from the ring land. But really I wonder if the piston mfg. counts that when they claim -5cc's.

Brandon Boomhauer

iTrader: (10)

RSR Static Compression Ratio Calculator
thats what i like to use.

skippydn

iTrader: (33)

My dad has flat tops with 64 cc heads. His cr is about 11 to 1.

anLS1x2

What kind of heads are they? I'm building a 383 stroker as well with flat top pistons (two valve reliefs). I just have stock heads for now and they are 64cc's. I've been told that is doesn't take much of an increase in gasket thickness to get a dramatic change in CR. I'm counting on this to be able to run 93 octane. I think 10.5:1 is pushing it a bit.

firefighter

Well what has happened in the last few days....

1) Had the heads gone through the valve job was adequate but sloppy (mass production) Edelbrock RPM E-tecs

2) Before the valve job the "64cc" chambers were actually 67cc after the valves sunk a little bit and every thing was at equal height the chambers are now 69cc.

3) Talked to JE pistons and told the guy what I have and he said I would have 10.5-1 with 68cc so now that I have 69cc I'm golden. Also after the shop was done with the heads we figured it out and they came to the same conclusion.

From every thing I've ever read or heard 10.5-1 is about the max with ALUMINUM heads to run premium on the street. I should be fine.


Also I'll prolly make jus shy of 500hp at the flywheel

Lurch84

Actually u can go 11 to 11.5 to 1 with aluminum heads, on 93 thou! 10.5 is about the max with steel heads! Iam glad u got everything sorted out! Yes u should be runnin around 10.5 to 1! Depending on what head gaskets u run!

PortPros1

They don't count the top ring volume, just the valve reliefs and dish. There is a lot of argument on whether the volume around the top ring does anything for C/R. Most all of the C/R calculators don't have a provision for it.

firefighter

Yea thats pretty much what I've figured out the last few days. The end result id I'll prolly be between 10-10.5-1 depending on head gasket.


Thanks for all the input guys.