Compression ratio??

[bennydodge]

I'm running a 416 stroker SB with zero-deck Ross flat-tops(w/ deep valve reliefs). Anyone know what the comp. ratio would be with open chamber j-heads? Currently with E-heads it's about 11.5 to 1. How much drop with the iron j(587) heads? 

[Chryco Psycho]

to myt knowlegde the Eddy heads are close to the same volume as the J heads so no actual change BUT Alum disapates more heat than iron so the effective compression ratio @ 11.5 : 1 would be the same as 10.5 with iron heads so using the iron heads will increase the heat by approx 1 point of compression to a real 11.5 :1 

[bennydodge]

I didn't realize they were so close. The E-head has a 63cc closed combustion chamber and the J-head is open. I thought the open chamber would be closer to 70cc. Am I overlooking something, Chryco? 

[Chryco Psycho]

the J heads is listed at 68 cc , I thought the E heads was larger with the 340 piston clearance ?

[bennydodge]

My Eddy heads are not clearanced for the early 340 pistons. So how would 68-70cc affect compression vs. a 63cc chamber?

[Chryco Psycho]

you would drop into the 11:1 range but the alum head acts as if it was 10.5:1 with the heat disapation so you would still end up with more effective compression

[moper]

Most J heads if they have been checekd fall in the 71-74cc range from that factory. Any valve jobs will increase that, and milling deacreases it. Edelbrocks are generally 63-64cc. Using the KB calculator, I get 12:! static with auminum, anad this will drop to 11.5 or so using 68ccs on the Js. I have two questions... RPMs are way better in terms of chamber design, engine setup, and ports, plus lighter... Why the "step backward" to a set of Js? You also lose any quench. What cam do you run now? Are you experiencing issues?

[bennydodge]

Looking to go back to a stock look. I know the iron heads will support 500hp when ported properly. The whole "lookin' stock and cleanin' clocks" deal appeals to me now. I don't think I can bring myself to put the stock exhaust manifolds back on, though(soooo restrictive). The cam that's in it for now is a Crane Retro-fit hyd. roller. My oil pressure issues have been well documented on here. I might just go back to solid flat tappet......