81 Corvette
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81 Corvette
Hey guys i have a stock 81 corvette that i am going to redo the engine. what is the combustion camber size on these 350s? I am trying to get atleast 300 hp out of it by uping the compressiona nd a cam change. i dont want to fool with the heads other than a valve job.
also any suggestions on cams?
also any suggestions on cams?
Last edited by ScottMH; Jan 26, 2010 at 11:31 AM.
Burning Brakes
Joined: May 2008
Posts: 1,116
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From: Middleburg Florida
I don't think you'll get 300hp out of that engine without fooling with the heads, like replacing them. They're known for cracking and don't flow especially well for performance applications.
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Race Director
Joined: Mar 2008
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From: coffee
I think that the heads are basically 993 castings with the 76cc chamber, hopefully someone will correct/confirm. The GMPP 290 HP crate motor gets by with 76cc heads and dished pistons. With flat tops and the right cam, 300 HP should be done easily enough.
Drifting
Joined: Apr 2008
Posts: 1,992
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From: Antioch IL
the heads have to go. they are smog heads and are the weak link for that engine. I feel your pain i also have an 81. i think the compression ratio is 9.2 : 1 but do not quote me on that. i can double check when i get home.
Race Director
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From: Kansas City, MO ...I'd like to go fishing and catch a fishstick. That'd be convenient. - Mitch Hedberg
I'm curious why you would rather upgrade your engine with pistons and cam vs. heads and possibly cam? Letting your engine breathe easier would be far more productive than the restrictive stock heads and a beefed up lower end.
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Melting Slicks
Joined: Aug 2003
Posts: 3,415
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From: dayton oh
stock avg is 8.2:1 with 13cc dished pistons IIRC
I really dont see how you'll make 110+ Hp over stock #'s by keeping the heads, but good luck
The ONLY way to clear 300+ hp while keeping the stock heads is a blower or NOS
Last edited by Fast81; Jan 26, 2010 at 02:34 PM.
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replacing the heads is simply not going to happen for me. i am not willing to put that much coin in the motor. im running numbers off of desktop dyno and i know it has a margin of error but using the stock heads and bumping compression up to 9.5:1 and a good cam according to the program i should be close to my 300HP goal. i may port and polish the heads and the intake some to open them up. im sure that would help with the flow. the motor currently runs fine but its not making hardly and power and its starting to smoke. it has a carb from a 75 vette and a msd dizzy so all the computer controlled crap is gone. im on a pretty small budget and not only am i rebuilding the engine im rebuilding the transmission. so if i fall short of my 300hp goal so be it.
Burning Brakes
Joined: Jul 2009
Posts: 1,132
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From: A town SC
Run stock type dished pistons,and find some good used vortec heads. Those heads with a small cam will exceed your 300HP goal,and will be cheaper than buying flat tops and polishing on your stock heads.
Drifting
Joined: Aug 2009
Posts: 1,635
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From: Enid Oklahoma
replacing the heads is simply not going to happen for me. i am not willing to put that much coin in the motor. im running numbers off of desktop dyno and i know it has a margin of error but using the stock heads and bumping compression up to 9.5:1 and a good cam according to the program i should be close to my 300HP goal. i may port and polish the heads and the intake some to open them up. im sure that would help with the flow. the motor currently runs fine but its not making hardly and power and its starting to smoke. it has a carb from a 75 vette and a msd dizzy so all the computer controlled crap is gone. im on a pretty small budget and not only am i rebuilding the engine im rebuilding the transmission. so if i fall short of my 300hp goal so be it.
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i wonder if the torque converter doesnt lock up without the computer hooked up? could that be why it seems so sluggish all of a sudden? what would the effects of a torque converter not locking up be?
Drifting
Joined: Aug 2009
Posts: 1,635
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From: Enid Oklahoma
Yes the Comp locks it up. You can buy a module from B&M that will lock it up or add a switch. It should not make it feel sluggish at all without it. And some say you can damage the converter clutch if it never locks up, but I dont see how that could happen myself.
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thanks for the info. when i rebuild the trans i will do away with the lock up stuff too. got another question. does anyone make a performance exhaust system for the 81s? i have to keep cats on it and im looking to do header back. any kits or will i have to fab something?
Drifting
Joined: Aug 2009
Posts: 1,635
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From: Enid Oklahoma
I don't know if it is that simple, but the lockup converter is a good thing as it drops the Rpm's and gains MPG. The controller is about 150 bucks and a setup self built is dirt cheap. I don't need any smog stuff on mine and had a full length chambered system installed, if I were you I would look into a high flow cat and some high flow mufflers. The various suppliers sell kits, but a good exhaust shop can build a nice setup as well.
