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Canadian Poncho Superstar!

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What axle ratio would you go with?


So I'm contemplating building up the bare 12 bolt housing I have sitting. To go into the 65 post project. Need a winter project I guess.

Reason being, I'd like to be rid of the 3 arm 10 bolt. I'd probably not even notice, but I'd still know it was there. Plus the 10 bolt is leaking like a sieve. The current gearing is 3.36 in the 10 bolt, which would probably be pretty nice.

So it'll be paired with the fresh 327 / M20. I'd like it to be more of a open road driver, not so much a street car. I have a 3.55 in my 67, and it's probably too much for distance travel.

So what ratio would you choose?



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The 4 speed cars would have come with 3.31 (or 3.36, I can never remember the proper ratio) but you'll find yourself revving kind of high on the open road.


My 4 speed car has a 12 bolt 3.07 with taller tires (235 x 70R15), and I still find myself revving 3000 rpm at 80 mph. 





-- Edited by seventy2plus2 on Saturday 14th of December 2019 10:32:09 PM

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I like how the 6 in 65 came with a 3.36. Trying hard to make that engine pull that big car I guess.



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3:73 and a 28'' high 15" tire, my combo in my '65-- 2800 rpm @ 60mph

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A friend has a 68 sd396. I was over at his place and noticed a Muncie under the bench. I asked if it was a spare for his car. He said that he put a Saginaw in the car and swapped 2:73 gears in the rear. If I recall he said the Saginaw had 3:50 first gear and with the 2:73 rear it made the car more driveable on the highway.

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327, what characteristics? Peaky power or broad torque range? Car laden with accessories or stripped down? It makes a difference as to a recommendation.

3.07s for all round use today, or 3.31s for spirited compromise. I'm thinking 3.55 for around town cruising while listening to the exhaust note. 3.73s are what manual Z28s came with through '79, even with garden variety LM1 power. Think of how they ran. Or 4.10s with sticky street / drag radials (run a safety blanket around your clutch for this & higher numerical ratios).

You have the Muncie already? 2.52:1 low (or 2.56 if a '65 trans). If you aren't married to a Muncie, consider the flexibility of a 5-speed.

 

I would personally run 3.31s with the 327 / M20. With free-flowing exhaust & open-element air cleaner.



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It should have a pretty broad torque curve Cameron. It's a 67 327 275 HP. Rebuilt, stock double humps. Stock compression with an aftermarket dual pattern 214/224 112 LSA cam. Rams horns, Stock QJet with an open element. Will have duals all the way back with free flowing mufflers.

2.52 1st gear. It's a 69 M20.

I admit, it's kind of between a 3.07 or a 3.31. Doesn't look like a 3.23 (which might be a good compromise) is available for the 8.875" Passenger car 12 bolt 3 series.

 



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A Poncho Legend!

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I've experienced the open road driving in your area. Find a set of the rare-but-they-were-made 2.56's for your 12 bolt!

All kidding aside, I have 3.07's with my 4 speed, 225/70/14 tire and I wish I had 2.73. I find it effortless to leave a stop sign/light and it's too much rpm on the highway, I just drive about 60 mph because of that.

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I once put a 454  and an M21 Muncie in a 68 Impala I had. I also put in a 3.31 12 bolt posi I had laying around. That combo did not work very well. The axle gear combined with 2.20 first gear in the trans was not good for cruising. I assume an M20 with that rear gear would have improved the experience somewhat but I'm guessing that  a taller rear gear would be even better with a Saginaw or Muncie M20. Our 73 Camaro small block has a Saginaw 4 speed with I believe 3.07 rear gears and that cruises better than my 68 Impala did. 



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I had a 12 bolt diff with 2.73's in it that I debated putting in my 4 speed car. It would have dropped the highway rpm's (see my previous post in this thread, 3000 rpm @ 80 mph), and I don't think it would have been too bad around town as I'm not normally in stop & go traffic. I abandoned the idea, and sold the diff to a member here.

Maybe I just need to drop to 65 mph highway speed, but that'll never happen.

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Realistically it'll never be driven for any amount of time at anything beyond 70 mph. Most of its driving will be at 50 or so on the backroads and secondaries. 2100 seems reasonable at 55.

Just want a decent get off, with a reasonable cruise rpm. Seems 3.07 is a good compromise.

The tires I'm looking at are 26.5" tall.

 

http://www.csgnetwork.com/multirpmcalc.html

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Interesting topic as I'm also close to having to make a decision for the rear ratio for 2 B body projects . Both Big Blocks , but one will be a M20 and the other a M21 . Since I'm rural , I'd like to stay below 2500 rpm @60mph. I was leaning towards 3.31 as I already have a few sets , but I do have a 3.07 too. Which would best to mate up with a M20 , and which is more suited to a M21 . The rear tires  would be 15" and either 65's or 70's



-- Edited by Howmac on Sunday 15th of December 2019 03:21:44 PM

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15" Howard? If so you might get away with 3.31 depending on the profile tire you choose but if you are running stock 14" wheels you have no choice but to go to 3.07. Personally am totally satisfied with 3.07 and an M20. With an M21 I don't think you'd like the 3.07 rear.

As I'm sure you know, GM never offered an M20/3.07 combination. Maybe next summer I'll stop by and let you drive mine to see what you think.

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Thanks Carl,  I'll probably be using 15" rims on both projects , as I like how they fill up the wheel opening .  



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4SPEED427 wrote:

15" Howard? If so you might get away with 3.31 depending on the profile tire you choose but if you are running stock 14" wheels you have no choice but to go to 3.07. Personally am totally satisfied with 3.07 and an M20. With an M21 I don't think you'd like the 3.07 rear.

As I'm sure you know, GM never offered an M20/3.07 combination. Maybe next summer I'll stop by and let you drive mine to see what you think.


 They did offer a 3.07 with an M21 close-ratio 4-speed. I know of a car that was built that way, along with a solid lifter square port big block (66 Chevelle SS396 w L78). It would be a clutch-burner just to slip it to get a launch, but specialized drivetrains were possible back in the days before CAFE. If you were not launching but running the car at high speed, the closer ratios of the M21 would allow a smaller RPM drop between gears, but in normal traffic it would be a stone.

On a street car with 3.07s I would definitely recommend the wide ratio M20.



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The calculator that Mark used confirmed my revs at 80 mph with 235 x 70R15 tire.

27.95" tire diameter with 3.07 gears @ 80 mph = 2952 rpm.

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CdnGMfan wrote:
4SPEED427 wrote:

15" Howard? If so you might get away with 3.31 depending on the profile tire you choose but if you are running stock 14" wheels you have no choice but to go to 3.07. Personally am totally satisfied with 3.07 and an M20. With an M21 I don't think you'd like the 3.07 rear.

As I'm sure you know, GM never offered an M20/3.07 combination. Maybe next summer I'll stop by and let you drive mine to see what you think.


 They did offer a 3.07 with an M21 close-ratio 4-speed. I know of a car that was built that way, along with a solid lifter square port big block (66 Chevelle SS396 w L78). 


 Any documentation for that? I have never yet been able to find any Muncie M20 or M21 offered with a lower gear than 3.31. As always, never say never and I'm not too old to learn!

 



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How can you tell what 1st. gear ratio is??? My impala [409] 4 spd. is hard to get rolling even after switching to to 3:70's turns around 3500 at 60 mph. I know it's a Muncie but don't know which one???

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All Muncie 4-speeds1:1 final drive ratio. The three first gear ratio choices are to 2:20 2:52 or 2:54. One way to figure out first year would be on the hoist. Count engine revolutions for one drive shaft revolution in first gear. 



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If you knew your speedo calibration was fairly accurate, and you know your rear end ratio and tire diameter, this could help you find your first gear ratio.

At least it would let you recognize a difference between  2.20 and 2.56. 

https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

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4SPEED427 wrote:
CdnGMfan wrote:
4SPEED427 wrote:

15" Howard? If so you might get away with 3.31 depending on the profile tire you choose but if you are running stock 14" wheels you have no choice but to go to 3.07. Personally am totally satisfied with 3.07 and an M20. With an M21 I don't think you'd like the 3.07 rear.

As I'm sure you know, GM never offered an M20/3.07 combination. Maybe next summer I'll stop by and let you drive mine to see what you think.


 They did offer a 3.07 with an M21 close-ratio 4-speed. I know of a car that was built that way, along with a solid lifter square port big block (66 Chevelle SS396 w L78). 


 Any documentation for that? I have never yet been able to find any Muncie M20 or M21 offered with a lower gear than 3.31. As always, never say never and I'm not too old to learn!

 


 Ah, Jeff Helms bought such a car from the elderly original owner about 2 or 3 years ago. The car was a white '66 Chevelle SS396 coupe with a 396-375, and M21 & very tall 3.07 gears (original & documented). Still that isn't as bad as the '67 Camaro SS396-RS convertible that was built with an L78, M20 & factory 2.73 rear gears.



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I will openly admit I'm skeptical!



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4SPEED427 wrote:

I will openly admit I'm skeptical!


But I'm also admitting I was wrong. I found something that says 3.07 gears with an M21 was available in a Chevelle.  I guess I better stick with B bodies!

I'm still not believing 2.73 with a 4 speed though...



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all scoffing aside, your 327 will not be a troque monster, so frankly, I'd go with as high a numerically gear set you are willing to do;
if it's between 3.07 & 3.31 - I'd go 3.31 - but if you say that your current 3.55 are too high, you might not be happy with 3.31 either... so we have a conundrum;
a higher numeric gear will reult in better stop and go driving, but will rev higher on the highway.

Ideally you want your combined first gear ratio north of 9.00, and assuming a latter M20, your looking at:

3.07 = 7.74
3.31 = 8.34

My car as I bought it had an M22 with 3.55's;
it wasn't the best off the line, but the 430 ft/lbs torque sure helped;
even so, I had an effective first gear ratio of 7.81.

My brother has my old M22 in his car, and until he gets another rear setup for his car, he's got 3.08 - which is an effective 6.78 - it is doable, but far from ideal.

In closing, I would again encourage you to go with 3.31 so that stop and go driving is better...
or(!)
if you hadn't thought of it, you could leave the 3.55's in your car (which gives you an effective first gear ratio of 8.95), and go with a gear vendors overdrive which would convert your 3.55's to 2.77's on the highway.

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