BIG BLOCK GURU'S i need your advice

[into69z]

Hey bbc guys-I have a chance to score a set of 1967 3904390 98cc closed chamber heads. I think it's a decent price.($300) I just rebuilt a 74 454 for my 69 truck. The problem is ,I went too low on the compression (8.25:1) so the motor does not have the extra horsepower I thought it should (about 250 @3800 rpm) the heads on the 454 are stock casting #336781 with about 113cc chambers. The engine was bored 40 over and has a comp cam with .2980 lift. My question is, how big of an improvemnt can I expect with these different heads? Would it increase the compression ratio too much? I don't want to run 93 octane...the pistons are flat top so I don't think there would be clearance problems but I just don't know-anybody got any thoughts?

[clay68c10]

I've got 101cc heads and flat tops on my 454 and I'm at about 9:1. I can run crap gas thru it if I keep the timing low, but it's WAY more fun to run the good stuff.
I'd guess you would be at about the same compression as me with those heads, maybe 9.2 or so. There's no way to be totally sure without measuring how far down in the hole your pistons are at TDC and doing a CC measurement on the heads.
If they did a good valve job with hardened exhaust seats that is a pretty good price. You may have to swap your valve springs if you've got anything beyond stock valve springs on your heads now. Stock BBC springs are pretty low-lift and don't like aftermarket cams.

[knsmodels]

would the new heads be oval port or square....that could make a difference as well.......

[1968SWBBigBlock]

Both the answers above nail it on the head - those should be oval port heads and would be a great addition to your engine.
What intake and carb are you running?
Plus you can sell the 781 heads for pretty good money since that is a head everyone is looking for.
FYI - I cc'ed my 781's before I decked them - with flat top (which I did not use) the CR was 7.9-1 - if you used an on line calculator and the cc's listed out on the net your CR is on the high side as most 781 heads CC out and considerably more CC's than listed on the net at 118

[into69z]

i'm running an Edlebrook 750 with an edelbrook performer manifold-any ideas on head gasket thickness-also-is yhere any method for checking cc installed in the truck?...thanks

[clay68c10]

Pick which ever head gasket gives you the compression you want, someone probably has an online calculator for that.
You have to have the heads off to cc them.

[into69z]

does anyone have an online calculater so I can figure out the correct gasket thickness?

[70rs/ss]

http://www.rbracing-rsr.com/compstaticcalc.html


Here ya go!

BTW the 98 cc (closed chamber heads will be a nice addition to the 454, I used the Vortec 100cc heads on my 468 and flat tops, they were near 9-9.5:1 and i used cheap stuff in it, and a standard head gasket, if you do that you should be OK, BTW did the machine shop Zero deck the block?? If so you might have too much compression, so you should ask the machine shop what they did to the block first to be sure?!