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Old 01-05-2003, 04:14 AM
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Cool smc 64cc flat top or 76cc popups

hi everyone,this is my first time on this board, i hope someone can help clear the mud.
I recently bought my first big toy its a 1975 stingray, unfortunately it is the slowest vette since 55.a mind numbing 0 to 60 in 8 seconds,no need for o.m.j.handles.lol
anyway i am restoring it from the ground up.
i have a set of headers.weiand accellerator intake,holly 650 carb,accell supercoil,wires etc.
do i use 882 casting 202int 160 exh w/popup pistons or leave the vette flat tops and install 462 castings 202/160 . either will be ported and polished. any insight will be appreciated thanks <img src="graemlins/drunk.gif" border="0" alt="[drunk]" />

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Old 01-05-2003, 05:03 AM
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If you are wanting more performance, then the old, small chamber heads will perform better, but if you use the 462 head, you won't have any accessory bolt holes for your bracketry. Find a set of 186 or 492 castings, if you want original GM stuff.
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Old 01-05-2003, 05:04 AM
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If you are wanting more performance, then the old, small chamber heads will perform better, but if you use the 462 head, you won't have any accessory bolt holes for your bracketry. Find a set of 186 or 492 castings, if you want original GM stuff. The small, compact combustion chamber coupled with a flat-top piston will make better power than the open chamber head with a small dome.
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Old 01-05-2003, 06:16 AM
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[quote]Originally posted by NAIRB:
<strong>The small, compact combustion chamber coupled with a flat-top piston will make better power than the open chamber head with a small dome.</strong><hr></blockquote>

Can you explain the theory behind that. It has always been interesting to me and I am sure others as well.

Chris
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Old 01-05-2003, 06:23 AM
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Mostly it's in the smaller chambers superior burn characteristics. The smaller, more compact combustion chamber propagates the flame front more rapidly compared to the larger, open chamber design head. Now, add a piston dome into the equation, and the flame front has to climb over a small dome, further impeding the flame front.
In back to back tests, with identically prepared engines, one having a 76 CC Open Chamber head with a flat-top piston, and the other, having a 64 CC closed chamber head with a dished piston where both engines had identical compression ratios, the small combustion chamber consistently produced an additional 20 horsepower over the 76 CC, open chamber head.
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Old 01-05-2003, 06:33 AM
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Yup, I used 18 CC dish and closed chamber 461's on my N/A 400.

Chris
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Old 01-05-2003, 07:02 AM
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hey nairb,was there ever a comparison done between 64cc closed chamberc/w flattop pistons and open chamber heads with pop ups??just wondering
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Old 01-05-2003, 07:40 AM
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I performed this test myself on an engine I built for my chevy pickup.I used flat-top pistons with a set of pocket ported 441 Iron heads, then changed to a dished piston, with 64 CC, 186 style heads, which were pocket ported, both heads had 1.94, 1.5 valve size, and each had 8.8:1 compression ratio. All other factors were left constant, and each engine had 5 pulls put on it. The small chambered head, consistently did 18-20 horsepower more than the 441 headed engine, and did it with less spark lead. I think the decrease in spark lead is a good indicator that the flame front speed is better with the small combustion chambers. Not to say open chamber heads are terrible, but just not as good as the closed chamber stuff.
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Old 01-05-2003, 09:33 AM
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thanks again for the info
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Old 01-05-2003, 03:25 PM
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I would use the flattop piston over the dome combo anytime. with the flattop you will have better flame travel and make better power. watch out for the acc.bolt holes YOU need them.

GOOD LUCK
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Old 01-05-2003, 11:14 PM
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I wouldn't use the 882s not just because of the chamber but they are prone to cracking.You won't have bracket holes on the camel humps but I'd work around that,they are superior in everyway except for those d@mn holes.
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Old 01-05-2003, 11:27 PM
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We don't need no stinking holes! :p
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Old 01-05-2003, 11:32 PM
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I have seen many 882's come back from the machine shop with JUNK, cracked written on them. Why not try vortec heads with flat top pistons? 10 to 1 and great street performance right out of the box!
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Old 01-06-2003, 04:32 AM
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I already have the 882 castings they came stock on the car with 202/160 valves ,with what my buddy calls corvette flat top pistons (since they have 4 dishes in them),they're not just flat.should i even mess with this setup?I'm not trying to do wheelies,i just don't want to be embarrased by some kid in a turbo yugo.
the valve train is working good (57000 orig. miles) i recently bought a 650 holley,weiand excellerator intake, a 530 cam ,and headers.
glenn doesn't think i should change the heads as this is a matching numbers car, and the add ons i have will make it run a lot better than 0-60 in 8sec.i just thought a port and polish and pop up
pistons would add well to overall performance.since i have to change the rear crank seal anyway .is it worth the extra effort to change the pistons?? <img src="confused.gif" border="0">
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Old 01-06-2003, 11:20 AM
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With only 57,000 miles on your motor, I would get the heads checked for cracks, have a reputible engine builder do a mild port and polish, mill the heads and/or run the thin shim type head gaskets to bring your comp. up a bit. Your list of add-ons looks like your moving in the right direction. Just remember, at this point , the cam is the heart of the engine! Your selection here will determine, drivability, throttle responce, H.P., and torque curve.
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