Muncie M21 & Rea rGear Ratio Advice Needed !!!!!!

Hi…I was hoping to get some opinions from you all when it comes to Muncie transmissions and rear end gear ratios. I am building a 67 RS Camaro to take to cruise nights and drive around town on the weekends.
I am putting in a big block with a Muncie M21. I was told that the trans would not be a good street trans unless I went with steep gears in the rear. I know the M21 and M22 have the same gear ratios and the M20 has steeper gears. I was told that the M20 was better for the street cause of the steeper gears, but others told me it wasn't as strong a trans as the M21. My question is this..Can someone tell me if I use 3.90 gears in the posi rear with my M21, and a big block, is that a good combo? I will need to ride the car down to the Jersey shore which is about an hour away. Will the RPM's be killing me with 3.90's driving on the highway for about an hour (60mph)? Am I better off with 3.73's or are they going to be to whimpy for takeoff. I don't plan to race this car but want to be proud of its performance and kick some import with a big mufflers ass if needed :) Would I be better off with an M20 and 3.73's or what? Is the M20 just as strong as the M21? I know the M22 is the strongest of all. I'm just looking for the best combo for street racing and some highway crusin to get to a show. I am putting in a 454 that's going to be about 450hp and have good torque in the lower rpm range. Any advice from anyone who has experience with this setup would be deeply appreciated.!!!
Thanks Gary
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I don't have the answer, but someone on this NG will....I'm wondering the same thing (I have a 383 stroker), what's the best muncie 4 speed/rear end combination?
Terry

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Hi Terry, Its funny but a guy with a 72 Camaro with an M22 and 3.90 called my house tonight about a trailer I have for sale in front of my house. Hes a local guy and has just about the same setup Muncie M22 still 2.20 gear and 3.90 rear like I am thinking of. He has a 383 stroker in the car but I dont think the motor makes a difference. I'm probably wrong. He told me he tachs around 2800-2900 at 65 going down the highway. I can live with that so can get a better takeoff with the lower gears. I forgot what sise tires he has on the rear but that makes a difference. If I recall he said they were a bit tall. I am so interested in what others have to say on this NG. If he wants to buy my trailer I'll cut him a better deal so long as I can get a highway ride in his car to see for myself ...:)
Thanks Gary

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Gary,
My engine has just been rebuilt, but I don't have it back from my builder yet. I told him I'd go with his recommendation for a trans, he told me the best would be a 'wide ratio' muncie 4 speed. I've looked up some info on the difference between a wide ratio and close ratio, like you said the choice depends on the other gear ratios in the drivetrain.-- Terry

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I have a wide ratio Muncie I used to run in a 72 Nova. I can tell you what I didn't like about it. 1st thru 3rd the ratios are nice and close, then there is a big jump between 3rd and 4th. On the wide ratio Muncie's 1, 3, & 4 are roughly the same as 1, 2, & 3 on an old 3 speed. They just added another gear half way between 1st and 2nd. I personaly prefer an even spread all the way through. Have you concidered using a Super T10? They have a slightly lower 1st than a Muncie, and I thing they are evenly spaced all the way through.
Gary
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Hi and thanks for answering my post. Iwas looking into the super T10 but I was told by a trans shop that it wouldnt fit with my console and the shifter needed to be modified to work within the console. He did those fabrications and said it could get costly. How difficult could it be?
I spoke to a few guys who have close ratio muncies and 3.90s and they said its not bad and others told me to go with a 5 speed overdrive to get the best of both worlds. I may start looking into that but righn now I have to work with what I have. No matter what I end up with I'm still happy I have a piece of American muscle car history again. I always had regrets selling my other cars when I was younger.
Thanks again Gary

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The only difference between the wide and close Muncie is the main drive and cluster gears. That's where the ratios are different. The M21 uses different gears, but there is no wide version that I know of.
Al
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Gary,
When GM sold cars "back in the day" the choice of transmission was dictated by the engine size and rear axle ratio. Axle ratios of 3.73 and lower (numerically higher) came with close ratio transmissions (M21 & M22), while axle ratios of 3.55 and higher (numerically lower) used wide ratio transmissions (M20). The Transmission Ratios were as follows:
M20 Wide Ratio M21 &M22 Close Ratio 1st 2.52 or 2.56 to 1 2.20 to 1 2nd 1.88 to 1 1.64 to 1 3rd 1.46 to 1 1.28 to 1 4th Direct Direct Reverse 2.59 to 1 2.27 to 1
It's going to come down to personal preference, but if you plan on making 1 hour drives down the highway, believe me rear axle ratios lower (numerically higher) than 3.73 are going to drive you crazy. I have a 77 Z28 350 (modified) 15 inch wheels with a Borg Warner Super T-10 (M21) and a 3.73 rear. The Z28 has plenty of pick-up and the Tranny/Rear combo seems just right for street cruising and the occasional short highway trip. More than 30 minutes of highway driving at 60+ mph has me begging for a 5th gear.
Paul.

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Paul...Thanks. Yeah I was told the same and others have been very helpful with their experience said that anything over the 3.90 will be screaming at 60 -65 and noisey as hell. I think that I will look for a good m20 that has the 10 slpine input like my M21 so I can swap them out and see which one I like better. The shifters and eveything else are the same so the swaps are easy. I want to drive the car enough to enjoy it not look at it in my garage and never take it out. Besided there isnt much going on on the streets as far as street racing from light to light anymore. Maybe an an occasional rice burner import wit a big muffler trying to act cool. As long as I can shred some rubber and have it stick to his winshield as I blow by him, that wouls keep me happy. Thanks Gary

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I would think that with the big block and all that torque 3.73's would be a good choice if you drive it on the highway. It also depends on how tall your tires are and what you plan on doing with the car... Remember 4th gear is 1 to 1 so just calculate what RPM you would be running with the different gear combo's and go with what you want to turn at 60...
63360(inches per mile) divided by tire circumference(tire dia. times pi) multiplied by gear ratio should give you a close RPM calculation for 60 MPH.... Example... 30" tire with 4.11 gears... 63360 / (30 X 3.142) X 4.11 = 2763 RPM's approx.....
Wish I could help more, but I had 5.43 gears in my '72 Camaro RS with a 454 LS7 big block with a little extra done to it and a M22. Stop light to stop light was awesome shifting at 7,500 rpm's, but highway was murder... even going a couple of miles at 60 mph was enough to...
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snipped-for-privacy@att.net (TraxUnderground) wrote in message

Hi Gary, I have a 70 Camaro with the wide ratio in it, 3.55 gears and 26 in or so tall tires, and it spins around 3k @ 70 on the highway. The M20 has an annoying, steep rpm drop between 1st (2.52) and 2nd (1.92). When racing, I never rev it up very far in 1st, since I really lose time doing it that way. The other gears are fine.
I used to own a car with an M21, and first was a bit tall for good launching (that car had 3.31 rear gears), but the split from 1st to second was much nicer. By comparison, I actually like the 3-4 split in the M20 better than the M21. The M21's spread for 3-4 seemed a bit too close.
Frankly, I know the new aftermarket 5 speeds are expensive, but knowing what I know now, my advice would be to avoid spending big money on an old 4-speed, and save for a 5-gear. Keisler Engineering ( they are in the new Car Craft) has shifter kits for most old iron which makes an install mostly painless. I once drove a later Firebird with a 5-speed... ah, it was so relaxing. Anyway, the 5-speed is what I'm shooting for, since my old 4-speed is starting to show its age. Hope this helps. Good luck.
Ken
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Interesting. The one/two split in a wide and close Muncie are exactly the same. The ONLY difference is third to forth. If you look at a parts list the only difference is the main drive (input shaft) and cluster gear.
For a close ratio box I would not go lower than a 3.73 gear. Chevy sold lots of cars with close boxes and 3.31's, not a good choice for a street rod. I had a Pontiac with 3.42 gears and a close box. It must have weighed 4,000 pounds. It was decent from a roll but slow out of the gate.
Al
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Actually, the ratios are different. The M20 features a steeper first AND second gear, but the split between them is larger. It isn't a huge difference, but multiply it by 3-4 thousand rpms, and it becomes noticeable. Here is just one website which gives ratios for the different Muncies:
http://www.5speeds.com/muncie2.htm
The Muncie M21 is listed with 2.20 first and 1.64 second; the M20 is listed with a 2.52 or 2.56 first and a 1.91 second. The spread is also different for third and fourth.
Also, bear in mind that a large number of the M20s were Saginaws. I don't remember now, but second in a Saginaw may have been taller than that in a Muncie.
Ken
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