781`s or 290`s...? (BBC heads) - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-20-2004, 07:39 AM
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781`s or 290`s...? (BBC heads)

I have a good pair of 781`s (open chamber oval port) and a good pair of 290`s which are closed chamber large oval port... I am building a 427 with closed chamber pistons (ZL1 type) and will be using the engine for strip use mainly but some cruise`s...? I know the 781`s are the desired head but are the 290`s a good casting as well...? The fuel is no concern as it will be running 100 octane or better all the time... So what are the pro`s and con`s...? Also this will have a nice Crane hyd roller cam 610/587 if that helps with the solution...?

Or if all else fails I do also have a very nice pair of 188`s (retangle port HO 454-502 heads)

thanks in advance...

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Old 03-20-2004, 08:33 AM
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For street use i prefer oval port heads this will give you more TQ at the bottom end to move the car also you said you would be using 100 octane gas so i would use the closed chamber head if it were me.




mike
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Old 03-20-2004, 10:00 AM
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Well for starters,I think the pistons are going to need to be fly cut to clear that cam especially on the exhaust side.With the 12.06:1 L88 pistons you can run about .575" lift with a duration of 260 at .050" with a flat tappet.I dont think a roller is going to make that much differance in the ramp rates that it will cure this problem,I think the exhaust side will 90% chance need to be fly cut.With that done,you need to figure out how much compression you have.With the L88 pistons in at zero deck and a .040" hgead gasket,you could fly cut the pistons about .080",and then mill the 781 heads about .030' and have a nice 11.75:1 combo.If you did it with the same combo with the 290 heads and opened the 290's up to 106cc's with the same fly cut the compression works out to be the same.If you milled the 290's down to 102cc's you would get about 12:1.as far as perfromance goes,if both heads were fitted with 2.19/1.84" valves,and both had the chambers opened up to clear both the valves,they would both flow withing about 10cfm of each other.I would run the closed chambers because you can get more compression with less milling and therefore less piston flycutting.Good luck.
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Old 03-20-2004, 10:36 AM
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I would use the 781's. They will have hardened ex. seats and will breathe well with the semi-open chamber. These heads respond well to putting 2.19/1.88 valves into them. You have to be careful with the valve spring installed height, as the factory valve installed height is 1.800", not the usual 1.880". That's not a problem on the exhaust side because you will eliminate the rotator, but the intake spring seat will need to be cut deeper.

These heads will out torque your rectangular port heads and out breathe your closed chamber heads.
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