M-21 with a 3:55 posi - Hot Rod Forum : Hotrodders Bulletin Board
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Old 02-08-2006, 03:15 PM
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M-21 with a 3:55 posi

I am building a 65 GTO Convertible. The engine is a built 455, tri-power, M-21 close ratio tranny and a 3:55 posi rear end. A friend told me I would not like that tranny with that engine and 3:55 rear end. I have a Doug Nash 5-speed but to use it I would have to change out my new Centerforce clutch (10 spline) and put a new end on my driveline. Anyone have a combo like this and if so do you like it? Or, any comments about this combo. Thanks, DaleB

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Old 02-08-2006, 04:01 PM
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This setup will work fine especially with the 455.
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Old 02-08-2006, 04:52 PM
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Can't understand what you would not like about it. I agree, it will work fine. That is what Pontiac had in them

Vince
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Old 02-08-2006, 05:12 PM
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Also with a Nash 5-speed you would need to go to a 3.23 gear as the 3.55 is to steep for the first gear in a Nash. By all means stay with what you have as the 455 has plenty of torque and that will mor than make up for the gearing of the 21.
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Old 02-09-2006, 05:32 AM
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combo

I also agree you will love that set-up,, just stick with what you have..

Keith
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Old 02-09-2006, 08:42 AM
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Thanks guys for the input. I feel better about the Muncie setup now.
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Old 02-09-2006, 09:05 AM
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Tires

How tall are the rear tires you plan on using?
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1935 chevy,masters
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Old 02-09-2006, 11:40 AM
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hay Ive tried a400 with a m-21 to411 posi and felt that tork off the line but it wasnt practical for street at all and fuel use was crazy , Ive gone to 323s and Im really happy with that combo. on the street and at the drags.
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Old 02-09-2006, 09:31 PM
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I have 255/60/15's for the rear, on an 8 inch rim.
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Old 02-10-2006, 03:56 AM
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Dude, they used Muncie's for years in cars like yours! lol I've got a M21 in my 67 Camaro with 3.42 gears and 255 50R15 wheels on the back. I'm running 3000 RPM at 70 mph but I get up and go! I think you will be very happy with your 4-speed set up.
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