is there anyway i can get a streetable compression on a 408 with 5cc dome pistons, and still keep the quench tight? (by streetable i mean pump gas)

 

nobody knows?

 

A HUGE combustion chamber. I think the biggest you can get is around 76cc's. That's not gonna cut it! If the pistons have a solid dome you could have the dome machined off. Not a great choice, but a possibility. Of course, then you'd have to re-balance the rotating assembly. Bottom line, new pistons are in order here.

 

damnit. anyone else got any input?

 

if i were to run a 72cc combustion chamber, would it not be the same ratio if i were to run a flat top piston and a 67cc chamber?

 

Ok, I will stand corrected here. Based on a 4.060" bore and 4.0" stroke with your 5cc domed pistons at zero deck with .052" gaskets and a 76cc combustion chamber you'll be at 11.34:1. With a 72cc chamber you'll be at 11.87. With a good tune you could get away with pump gas. I think the LS7's around 11.5:1. Sorry to get you all fired up without doing the calculations before I posted!

Oh yeah, one more thing, that's static compression. You DCR (dynamic compression ratio) is going to play a big role in how well it'll run on pump gas. Without your cam specs I can't help ya with that one!

 

lol whew! thanks man, i knew about the dynamic / static roles. i haven't picked a cam yet so i wanted to see just how high it actually was. but thanks for the info man.

 

Anytime man, glad i could help....even though it caused a little anxiety! Lol!

 

If you need to drop it a little you can easily turn a little off those domes.

 

2618 alloy is 2.76 grams per cc. Knowing the before and after weights of the pistons after you do dome work will tell you where you are. -Brian Nutter

 

yea, thats what i heard. but i was asking if i ran a 72cc combustion chamber with the 5cc dome, would it not be the same as running a true flat top piston with a 67cc chamber?

 

Sorry for the delayed response, but yes, a true flat top with no valve reliefs and a 67cc chamber will be that same as your 5cc dome and 72cc chambers. 11.87:1 based on what I posted above. That's on the ragged edge in my opinion. If you're going that route I'd try to get a 76cc chamber to bring the scr down a little more. Bear in mind that a flat top piston will produce a more efficient combustion chamber than the dome and have more detonation resistance. You're certainly going about dropping your compression the right way, increasing chamber size as opposed to using thicker gaskets, but I'm still leaning towards swapping pistons in this case. Of course, that's just my opinion. The work involved is certainly greater, but it'll be the cheaper route in the end. A set of heads will run ya over 2 grand while a set of pistons, related components and a good balance shouldn't run more than $1500.