781 bbc heads from auto?
#31
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mine are large oval port.. If you ground out too much meat in the exhaust port eventually it starts to leak. The original heads that came with the boat both went bad, both were leaking from the exhaust ports after pressure testing. So this time we went not so large on the valve sizes and no deep porting...The problem with new heads is that most if not all of them like world products, dart have raised the exhaust ports 1"....SO after you install your exhaust manifolds and risers nothing lines up again. 1" at the bottom changes a lot on top of the engine. Years ago I had Gil exhaust on a single big block with new heads. Gil made wedge plates for this type of situation. Try and get a wedge plate for stainless marine exhaust...or many others. lots of luck.
Last edited by 35fountain; 07-10-2015 at 08:22 AM.
#32
mine are large oval port.. If you ground out too much meat in the exhaust port eventually it starts to leak. The original heads that came with the boat both went bad, both were leaking from the exhaust ports after pressure testing. So this time we went not so large on the valve sizes and no deep porting...The problem with new heads is that most if not all of them like world products, dart have raised the exhaust ports 1"....SO after you install your exhaust manifolds and risers nothing lines up again. 1" at the bottom changes a lot on top of the engine. Years ago I had Gil exhaust on a single big block with new heads. Gil made wedge plates for this type of situation. Try and get a wedge plate for stainless marine exhaust...or many others. lots of luck.
#33
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#34
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Think i am going to look at a set of 781's this weekend already upgraded to 2.25 / 1.88 stainless valves. Used take-offs (car) but pics look to be good condition. I am taking some barsol and check for leaky valves. If all looks ok i will probably buy. Found on craigslist for asking $600 will try for $500. Take them to machinest for final checking and surfacing. Need to decide how much material to remove.
Like i said have stock 454/330 strock cams & 8.5:1 compression flat top pistons and would like to remove enough to get to about 9.75:1. Any ideas what the chamber cc's required for this?
Not trying to make race engine just adding some spunk and freshness to engine.
Like i said have stock 454/330 strock cams & 8.5:1 compression flat top pistons and would like to remove enough to get to about 9.75:1. Any ideas what the chamber cc's required for this?
Not trying to make race engine just adding some spunk and freshness to engine.
#36
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With that combo, you're about 8.4/8.5 ish with a typical .040 head gasket. You could get that up a bit with a thinner cometic head gasket, or if the block is going to be machined, have the decks milled a bit. Nothing wrong with a 8.75:1 engine. Mercury did it on many engines. I'd enjoy the 87 octane fuel and run it.
#38
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Merlin made the majic aftermarket boat heads . Under a grand . Cc your oval ports , use the drugstore kiddo syringes if you cant get a moroso acrylic or comp big glass, 9cc can be the difference between yea and nay
Big john got over 500 out of peanut ports at under 5000 rpm , yea he could make hummers do wheelies in street trim , but the peanuts its mostly in intake port volume and cam cause your exhaust is still going to suck anyway
Your never going to be king with a bbc and flat tops but with some science you can get within spitting distance of the mag on wet exhaust boat app without pulling the motor
Big john got over 500 out of peanut ports at under 5000 rpm , yea he could make hummers do wheelies in street trim , but the peanuts its mostly in intake port volume and cam cause your exhaust is still going to suck anyway
Your never going to be king with a bbc and flat tops but with some science you can get within spitting distance of the mag on wet exhaust boat app without pulling the motor
#39
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Did you cc those? I've cc'ed probably 7-8 781 heads (exhaust issue mentioned above got me), and I've never seen one below122 cc chamber - and that one had been shaved previously. Starting with that, you can't get to 108, not enough deck thickness. Online, they list 781s at lower cc's, but those are inaccurate when considering everybody's actual cc readings.