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Old 05-12-2012, 08:11 PM
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1970 Pontiac Grand Prix

Any of you guys know what rearend gears came stock in my GP? I want something that will smoke the tires. I have a 1971 H.O 455 engine bored 30 over with 278/288 Camshaft. BM Holeshot 2500 stall, with a th400 trans. I installed a shift kit in the rebuilttranny. I had roller rockers installed and Edelbrock 750 carb with edelbrock performer intake. Headman headers, March pullies. The Engines getting installed now and I want to make sure all the power gets to the tires
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Old 05-12-2012, 11:50 PM
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There were several different ratios available, so your best bet is to put the rear on jack stands and count revolutions of the drive shaft as you turn a tire one revolution, if it has positraction. If it has an open rear, leave one tire on the ground and spin the other tire two revolutions while counting the number of drive line revolutions. Bill

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Old 05-12-2012, 11:54 PM
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Quote:
Originally Posted by billonwheels
Any of you guys know what rearend gears came stock in my GP? I want something that will smoke the tires. I have a 1971 H.O 455 engine bored 30 over with 278/288 Camshaft. BM Holeshot 2500 stall, with a th400 trans. I installed a shift kit in the rebuilttranny. I had roller rockers installed and Edelbrock 750 carb with edelbrock performer intake. Headman headers, March pullies. The Engines getting installed now and I want to make sure all the power gets to the tires
ANY ratio you have in it will be fine if you want to annihilate the tires. I would caution you against going to a too low (high numerically) rear ratio. IMO the 55 engine does not want nor need any more gear than a 3.5:1 range rear gear- and if it was ME
(and it has been in the past ) I would use a 3.08-3.23 rear ratio in it, unless you are drag racing it or acceleration is way more important than a good all around gear. If that is the case, use up to a 3.73 but no lower than that.
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Old 05-15-2012, 08:21 PM
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ok Thanks,
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Old 05-16-2012, 12:22 PM
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Its a heavy car. You will want 3.73's. Don;t waste your time with anything less.
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Old 05-16-2012, 12:40 PM
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Quote:
Originally Posted by F-BIRD'88
Its a heavy car. You will want 3.73's. Don;t waste your time with anything less.
BULL!!! A Pontiac 455, having a 4.21" stroke doesn't need anything more than about 3.23s. 3.08s would be fine, especially if it was used on the highway, much. If used on the highway a lot, I would lean more toward a 2.70-2.90 gear.

Bill

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Old 05-16-2012, 09:57 PM
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I'm putting the car on the rack tomorrow and I'll try the turning of the wheels. Everyones telling me its Posi....
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Old 05-16-2012, 10:04 PM
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A couple pages that may help:

Telling an open diff from a LSD

Checking the gear ratio

I always tell guys to pull the cover or drop the center section- who knows when the last time (if ever) the diff lube was changed! Plus the condition of the gears can be seen along w/whether it's a posi or not...
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Old 08-19-2012, 06:12 PM
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The Grand Prix is running, and it's burning the rubber off my tires, I'm going with a new Posi unit, 3.08 gears should be fine....
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