hey guys im looking into getting some Speed Pro. Part Number: H624P30 pistons which have a dome volume of 3.5cc

I did all the math and the Compression Ratio came around 11.35:1 (4.040 bore, 3.75 stroke, 68cc head, .055 gasket, .0001 deck height & 8 Cylinder)

I was wondering if the CR is too high even with running the highest octane pump gas. Here is a list of mods that are on the engine:

350 4 bolt block bored .040 and stroked with 3.75" crank (so a 383)
Edelbrock RPM Air Gap Intake, Edelbrock 750 CFM Carb
Lunati 108° Sep 104° Int C/L .534/.534" lift 295deg/245deg
1.6 Roller Tip Rockers
AFR 195cc Street Heads
Heddman 1 3/4 headers with 3" collector to 4" dynomax
Im sure their is some other little stuff but cant think of it.

Anyways i need to know if the CR would be ok so i can order the set of pistons. Oh yeah the car would mainly be my weekend warrior and couple times here and there to work. Thanks

 

Its close, you may have to run octane booster. It all depends on how good your tune is. Personally i would get flat tops and call it a day. What are the cam specs at .050" lift?

 

That strikes me as being too high.

I'd suggest their H860 (flat-top) instead.

I'd also suggest a .039" head gasket (FelPro 1014) instead of the 1044 you list.

And, it's going to be pretty tough to get somebody to machine your block to one ten-thousandth inch of deck clearance; I'd call that "zero-deck".

 

From what I've heard, the highest you want to go with the aluminum heads is about 10.6:1, unless you run a really radical cam to bleed off some of the compression.

I have heard of guys doing this with up to 14:1 compression on 93 octane without issues, but I don't know if thats fact or fiction.

And if the valves are open that long, wouldn't that defeat the purpose of a high compression motor in the first place?

 

the cam @ .050" In/Exh is 245*.

So my best bet would be to go no higher then 10.5:1 to not run in any problems even though im sure there might still be some..

Anyways ill check into those flat top pistons that Speed Pro has

 

Oh you can run 14:1 on street gas, but your not going to ignite the a/f until it's on the down stroke – i.e. retard the timing to nothing, and kiss any performance good bye.

BTW: I like the Felpro 1010 (.039) because it uses copper instead of steel for the compression seal. A lot better on aluminum heads.

Ron

 

Ok well i did the math and im getting the Speed Pro H860 (flat-top) which when i did math came close to 10.3:1. And that was with the .039 gasket which i will going with also.