Performance for 71 350/270hp
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From: MAYWOOD MO
Performance for 71 350/270hp
My 71 coupe is very original with its original engine. The only things that have been lost over the years are the factory carb, air cleaner, and ignition shielding.
The engine needs rebuilding, and I'm wondering just how much power I could gain with internal mods so that cosmetically the engine continues to look original and all the numbers still match. My thoughts are
1. New rotating assembly to bring it up to 383ci and set the compression wherever I want it.
2. Modern hydraulic roller cam.
3. How much of a restriction would the heads be? should they be machined for the bigger valves like the Lt1?
4. Could I make any kind of power with the factory intake and exhaust manifolds, or would I be better served to set them aside and run headers and an aftermarket intake? I would prefer to keep the exhaust manifolds.
Who else has gone this route? I would love to hear others experiences. I'm not trying to build a drag car, just something that has some guts to it. I'd like to be able to dump the clutch and do burnouts at will. How mature.
The engine needs rebuilding, and I'm wondering just how much power I could gain with internal mods so that cosmetically the engine continues to look original and all the numbers still match. My thoughts are
1. New rotating assembly to bring it up to 383ci and set the compression wherever I want it.
2. Modern hydraulic roller cam.
3. How much of a restriction would the heads be? should they be machined for the bigger valves like the Lt1?
4. Could I make any kind of power with the factory intake and exhaust manifolds, or would I be better served to set them aside and run headers and an aftermarket intake? I would prefer to keep the exhaust manifolds.
Who else has gone this route? I would love to hear others experiences. I'm not trying to build a drag car, just something that has some guts to it. I'd like to be able to dump the clutch and do burnouts at will. How mature.
Le Mans Master
Joined: Sep 2001
Posts: 7,739
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From: Unreconstructed, South Carolina
You say yours need a rebuild.
Others will say don't waste your $ on your heads ... I say they're OK for rebuild for mild performance. If you're looking to save a few $ you don't need a roller or a 383. You need compression and better breathing.
Simple but complete rebuild using Sealed Power-SpeedPro pistons p/n H618CP with small dome ... at thirty-overbore & w/ FP1094 gasket will yield a true static compression ratio about 9.8:1 with a great 0.040 quench height. Inexpensive cam-lifter kit like summit's K1103 is good match & under $100. This w/ headers will easily make 340-365 flywheelHP on pump gas and retain vacuum for C3 headlights, brakes etc & retain street manners ... & it'll spin tires. Your intake is OK w/ above. Headers better but your iron exhaust OK too but loses about 20 hp. Both your carb & your ignition are AOK so long as they're in good condition & tune.
Yes the heads can be rebuilt ... instead of using 1.94/1.50 valves ... use 2.02/1.60. You can probably buy a set of 16 stainless valves & Z28 springs off internet for under $125. The bigger valves really won't do so much.
A rebuild w/ cheap cast 383 crank & flattops will also work for small $.
Others will say don't waste your $ on your heads ... I say they're OK for rebuild for mild performance. If you're looking to save a few $ you don't need a roller or a 383. You need compression and better breathing.
Simple but complete rebuild using Sealed Power-SpeedPro pistons p/n H618CP with small dome ... at thirty-overbore & w/ FP1094 gasket will yield a true static compression ratio about 9.8:1 with a great 0.040 quench height. Inexpensive cam-lifter kit like summit's K1103 is good match & under $100. This w/ headers will easily make 340-365 flywheelHP on pump gas and retain vacuum for C3 headlights, brakes etc & retain street manners ... & it'll spin tires. Your intake is OK w/ above. Headers better but your iron exhaust OK too but loses about 20 hp. Both your carb & your ignition are AOK so long as they're in good condition & tune.
Yes the heads can be rebuilt ... instead of using 1.94/1.50 valves ... use 2.02/1.60. You can probably buy a set of 16 stainless valves & Z28 springs off internet for under $125. The bigger valves really won't do so much.
A rebuild w/ cheap cast 383 crank & flattops will also work for small $.
Last edited by jackson; Jul 28, 2008 at 02:34 PM.
Le Mans Master
Joined: Dec 2003
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From: Fremont CA
I would go with a flat-top piston and smaller chamber heads along with the 383 crank. World, Comp Cams have iron heads although with the Brodix IK 180 and Dart aluminum heads in the $600 range I would go with these in my rebuild. The original heads have a large open chamber that isn't the best for power or fuel economy but, make great Emission numbers buy lowering the NOX count (due to lower combustion tempratures, not a good thing!) I would estimate that part of the 40HP loss a LT-1 took from 70 to 71-72 was due to the poor combustion chamber design, same problem Ford has with it's "modular" OHC motor in current trucks/cars-BAD combustion chamber design.
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From: MAYWOOD MO
Thanks for the responses. I really want to retain the matching #s engine, so a head swap is out. If I replace the pistons, can't I increase the compression that way rather than needing a smaller combustion chamber?
Le Mans Master
Joined: Sep 2001
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From: Unreconstructed, South Carolina
... hence the H618CP with your heads & crank ... or use flattops w/ your heads & a 383 crank. It's quite likely '71 L48 has OE heads w/ casting number ending in 487 ... they're really not bad heads ... & they are thick castings that can be machined a lot if need be.
