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 Rear gear for 71 Lemans
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71 Lemans
Tribal Scout

USA
10 Posts

Posted - 14 Feb 2017 :  6:55:26 PM  Show Profile  Reply with Quote
I am changing the 8.2" rear end housing and differential in my 71 lemans putting in a new 12 bolt limited slip unit and am trying to decide between 3.08 or 3.31 to one final gear. The engine is a Butler built 467 all stock cast iron, manifold to manifold, RA IV cam, 800 CFM Q Jet making 440hp at 4900 and 518ft/lbs at 3900. I have an M-22 with M-20 wide ratio gears. The tires are 245x60x15s. Right now the rear end is a single spinning 2.76 and it actually goes like stink, though it doesn't stay hooked up all the time. I am defining the 12 bolt rear end I am going to buy to swap soon, with only the eternal question left, "which gear to go with?" I want to drive the car a lot in in the coming years, cross country and what not, but also I would also like to get a bit more head snap. So, anyone have a similar setup and opinions between the 3.08 and 3.31? Thanks for your time and thoughts.
cheers jimbo

cortcomp
Coyote

USA
5335 Posts

Posted - 14 Feb 2017 :  6:58:51 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
You mentioned driving a lot, and cross country. Either 3.08s, or an OD trans and 3.31-3.42 imho.
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tjs44
Cochise

USA
412 Posts

Posted - 14 Feb 2017 :  7:10:36 PM  Show Profile  Reply with Quote
2X Tom
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AlsCustomS
Tribal Scout

USA
38 Posts

Posted - 15 Feb 2017 :  10:29:22 AM  Show Profile  Reply with Quote
I just pulled the rear end out of my 69 Custom S same as yours, 2.76, bought a 68 BOP gutted it installed all new 3.36 yukon pinion, posi axels everything.... it really puts the power down now.
I still have the th 350 in it, and you can feel the difference in the way the the rpm's are higher.
After some advice on this sight and much comparing the 700r4 and the 2004r the 2004r is the way to go with the rear end I chose.
The 2004r OD will bring my 3.36 to about 2.76 or close in OD for cruising. I was still pulling at 125 MPH with the original 2.76 with minimal RPM's
Hope this helps
AL

What is the S, in Custom S ?
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Phil
The Great White Buffalo

USA
7215 Posts

Posted - 15 Feb 2017 :  11:13:42 AM  Show Profile  Reply with Quote
Al, double check your math. With that rear gear, the 200-4R will put you at about 2.25:1 final drive in OD.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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NOTROD
Buffalo

USA
64 Posts

Posted - 15 Feb 2017 :  12:09:50 PM  Show Profile  Reply with Quote
My LeMans is a four speed with 3.31 rear end. I have 255/60/15 on the rear and the engine runs at around 2800 rpm at 65 mph. It's not too bad, but I would really like to be able to shift one more time. If you're sticking with the four speed, I would definitely go with the 3.08 to get the highway rpm down a bit. Besides, with that much torque at that rpm you don't really want a lot of gear.
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cortcomp
Coyote

USA
5335 Posts

Posted - 15 Feb 2017 :  1:23:46 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
If you can find one, the NP440 is a great setup here. I ran one in my 71. 4 speed, but with the same ratios as a 700R4. I still run one in my 66 truck, and i have a spare out of a 69 gto I bought just because the guy wanted it gone. So, they're around, and i find them to be a good pontiac trans. The one wide gear ratio spot (i think 2-3 iirc) isn't even a big deal with the torque of these motors. It's not too bad even when towing with my 327 powered 66 c10.
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AlsCustomS
Tribal Scout

USA
38 Posts

Posted - 15 Feb 2017 :  4:17:40 PM  Show Profile  Reply with Quote
Phil you are correct, the article went off a 3.55 rear end. Not a 3.36. I'll take 2.25 all day long !

What is the S, in Custom S ?
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Phil
The Great White Buffalo

USA
7215 Posts

Posted - 15 Feb 2017 :  8:11:42 PM  Show Profile  Reply with Quote
Yes, it's a good thing.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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71 Lemans
Tribal Scout

USA
10 Posts

Posted - 16 Feb 2017 :  10:07:58 AM  Show Profile  Reply with Quote
Thanks for all the input gents. It sounds like the 3.08 should be given a day in court. Will let you know how it turns out. Do any of you have a preference / bad experience with Yukon vs. Auburn vs. Eaton vs. TruTrack differentials? Thanks again for your time,
cheers jimbo

Project 71 T-37
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DanM
Buffalo

80 Posts

Posted - 16 Feb 2017 :  11:54:36 AM  Show Profile  Reply with Quote
About a year ago I installed a 200-4R and rebuilt my rear end for my 71 Lemans Convertible. At one point I had a 2.56 open with my 350 V8 and with my TH-350. Then I upgraded to 400 V8 and 3.23. My fresh 400 busted the 3.23 so I went back to 2.56.

Now I have the 400, 200-4R and 3.55 Posi.
Richmond 3.55 gears.
Eaton Posi for 8.2.

The first Eaton I had made all kinds of noise so I had to send it back and get a new one. Eaton claimed they were incapable of building imperfect hardware but eventually I won and got the replacement. I had to pay my rebuilder for the labor. Now I am completely happy with the product. Everyone makes mistakes once in a while, even Eaton, though they may refuse to accept this fact.

I am told that the Eaton can be rebuilt if necessary but that it will never be necessary because it is made to be so durable.

The Richmond gears are quiet enough for me.

I regularly beat the heck out of my rear end doing burnouts and donuts.

Regarding the rear gear ratio. Even with my OD and the 3.55 on the highway I kind of wish I had a taller gear (e.g. 3.08) so my RPMs would be less. I rarely wish I had more torque below 40 mph. It's hard enough to hookup in first gear with the 3.55s. I can only imagine with a crazy 440 HP engine how much more difficult it would be to hookup in Low gear. (BTW: did you put your engine or car on a dyno to determine those power numbers?)

A final note on the "wide ratio" M-20. The wide ratio has nothing to do with the ratio between 1st and 4th. Both the wide and the narrow have the same 1st gear and 4th gear. It's just 2nd and 3rd that change. So, for street and highway driveability you care most about the difference between your lowest and highest gears. The four speed manual essentially has the same 1st to 4th ratio as a TH350. e.g. for drivability the 4sp manual is no better than the 3 speed auto. Yup. It's true. You just have more shifting in between. This is very useful for engines with narrow torque bands.

A final final note. My 2nd car is a 2012 Dodge Charger. Holy cow that top gear is tall. It's barely off idle on highway speeds. Great for mpg.

If you want help getting your speedo correct once you're all done, feel free to ask. Mine is spot on. Error less than 1%. That's nice.

71 LeMans Sport Convertible with 310,000 miles driven year round since 1994.
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71 Lemans
Tribal Scout

USA
10 Posts

Posted - 21 Feb 2017 :  10:48:10 AM  Show Profile  Reply with Quote
@ DanM
The numbers were the engine only on a dyno at butler. Thanks for your input, I will let you all know how it turns out,
cheers jimbo
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