I have a 454 bored .040 over with flat top pistons.I also have a holley 294-R solid lift cam and a strip dominator intake.My heads are #3917215, my brother helped me on a search and we found out they are possibly 396 heads with the smaller valves and closed chambers.My question is what kind of compression is this gonna result in? This motor is for my street truck and I am worried that i will have detonatoin and have to run my timing slightly retarded to avoid tearing it up.I would really like to be around 9.5 and am considering trying to open the chambers up to lower the compression ratio.What do you guys think???? If they are not perfectly matched will it affect my performance???? I am open to suggestions and opinions on if this is a mistake or how best to do it.I have a little experience with a grinder(and dremil) and I already have plans to port match and bowl port the heads but i don't have the funds to put the larger valves.Are these heads rare enough i might swap them off for a set of open chambers with larger valves? I am at the point of finishing it up and now I am worried what to do, so all you guys that know anything about this PLEASE help me make the right decision and I will forever be thankful.Please remember I am on a budget.

Those 215 heads were used on a number of engines. One example is the Corvette 427/400 and 390. You can make good power with them for sure. I will try to dig up some info on them, my dad runs a set on his 427.

You should be in good shape. Actually....most likely on the low side! Flat tops dont make much compression in a big block. Youve got to figure, even though they are closed chambered heads....the big chevy uses rather large combustion chabmers.

Your particular heads are 1967-68 oval ports for a 396/427 and have 101cc chambers(well....thats what they say but it wouldnt be surprising if they werent spot on). Anyways, even if the quench distance is nailed at .040 and the pistons have no releif at all...your gonna be no better than 9.6:1 compression. Depending on the piston tops and the gasket bore size and such....you could easily be looking at 9:1 compression with flat tops. Going to open chambers is not a good idea.

Have you got any measurements on anything? If you could, post info on how far the pistons actually are in the bore(deck height), gasket thickness, gasket bore, head chamber size(if you could CC the chambers to find out how they actually check out), overall cam specs(or even just a part number so we are sure to look at the right one), and anything about the pistons(valve releifs?....and if so, what size in CC are they total). Even though I said theoretically you can be as high as 9.6:1, Im guessing right now that your probably not even going to see 9.5:1 unless those heads have been cut before.

http://www.race-cars.net/calculators/compression_calculator.html

There's a simple compression calculator. I would do all the measuring and then calculate it out, just to be safe. Sounds like you are OK, but it would suck to get everything together and then have lots of pinging or detonation.

Sounds like you got a big cam, which would lower chances of detonation. I was looking for the cam on Holley site, but couldn't find info. You sure that's a valid cam part number?

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Well I really appreciate everyones response.Man that sounds awesome Greg!! The pistons have two valve reliefs (top an bottom) and I do not have the stuff to cc anything or the know how.The pistons do come almost all the way flush with the deck and the heads have machine marks from being milled but that is all I know.Anyway, I would be happy with around 9 to 9.8 to 1 on my motor.I was thinking that it was much higher.I cut my teeth on small blocks and flat tops with close chambers = high compression ya know so i was a lil bit worried.I am glad that I asked and am very happy I will be in the ballpark as far as compression goes.My cam was bought second hand but never run so I don't have the spec card.I would be happy if anyone has knowledge on the exact lift.I was told that it was around 550 lift which was right where I was wanting it.I don't plan on turning more than 5500 and I love a toque monster.The cam is Holley Part# C2M-294-R,on the end of the cam is #965703.Any information on this cam or my heads would be greatly appreciated.What kind of power with a mild port job are yalls air dynos reading?I was hoping for a 500 500 build on this motor and if anyone wants to take a shot it,it will be dynoed and a buddys shop (rear wheel)and I will let you know how close you got.Yall are great thanks for everything.

If you're looking for lots of low end performance, I think I would look for a little smaller cam. Im not personally familiar with that one, but the number listed is generally the advertised duration, and that one is on the bigger side...not terrible, but might make it a little soggy on the bottom end. So will that intake. I would go with a dual plane intake, and a cam w/ duration at .050 of maybe 230 to 242. Heads and pistons I agree on, good combo.