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Post by john442 on Oct 5, 2020 13:56:02 GMT -5
I had asked this question before and was given a lot of good info. The one response stated that changing my open rear to posi and changing the gears from 3:08 to
3:55 with 26.5" tires with a close ratio M21 might actually slow the car when going stop light to stop light. If that is true then should I use 3:73 gears to increase speed and torque when going through gears. I prefer doing rolling rather than dead starts when going through the gears.
I have a mechanic that will do the change but don't want to make the change if there isn't a good increase in speed and torque by doing so. I had 4:11 gears in a Chevelle and hated them because they were NG for cruising around town or on the highway.
Thanks,
John
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Post by joepadavano on Oct 5, 2020 15:15:36 GMT -5
Why do you want a close ratio trans? The wider the ratio span from first to high, the nicer the car will be to drive on the street. A close ratio trans is suited for the track, where you want to keep the RPMs around the torque and HP peaks. On the street, you want as much flexibility as possible.
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Post by john442 on Oct 5, 2020 15:28:32 GMT -5
When I bought the parts for the 4 speed conversion I mistakenly thought the close ration was a better choice for the street. Now I want to
use what I have and change the rear gears and while at it change the open rear to posi.
I have had some telling me that 3:55 would be a good choice and others saying the 3:73 would be better but it would be noisy. My car now seems to have lots of torque it just doesn't seem to have as much speed going through the gears as I think it should.
John
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Post by jcf85post on Oct 5, 2020 21:18:04 GMT -5
with 3.55 gears the m20 (wide ratio) is better transmission because of the lower first gear,the m21 was meant for 3.73 or higher. 3.55 is a very nice all around gear but will be sluggish with m21 trans taking off.
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Post by joepadavano on Oct 6, 2020 8:31:14 GMT -5
Personally, I'd sell the M21 and get a wide ratio trans. I'd actually look for a Super T10 with the 2.88 first gear. You'll be MUCH happier and the car will be more fun to drive. Alternately, you can get wide ratio gear sets for the Muncie.
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Post by island65cruiser on Oct 6, 2020 9:47:59 GMT -5
John, for what it is worth, my original 65, bought new had an M21, and 3:55 Posi. That combination was fine for racing when I launched at a high RPM, over 3500, and just dumped the clutch and drove through the wheel spin. The car was fast through the gears and was a great road car. I routinely drove between 80 and 90 on road trips. Joe an JC are right, the lower first gear is best for stop and go driving. For this car I bought the M-20 because the wide ratio will allow for lower first gear to get out of the hole, and longer time in gear between shifts, to get RPM and HP up. Love that RPM! With a 4 speed, there just aren't enough gears to give both bottom and top end peak performance. You have to prioritize.
If you have a healthy, stock, short stroke 400, you can run 4,000 RPM all day long (check the oil!) on the highway, if you can stand the noise. With 3:73 and bigger gears, that's what you are going to do at highway speeds.
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Post by john442 on Oct 6, 2020 11:23:18 GMT -5
Thank you to all who gave me great information. The mechanic will check my car out tomorrow to see what
parts are needed and if this is c clip or bolt type. He is suggesting a Detroit locker. Not the one that is a true
lock up type. Price with parts around $1,600 will include posi unit, gears, rebuilt kit and axle bearings and seals.
Because I had this trans rebuilt I think I’ll wait and see what kind of performance I get with 3:55 gears. If
it is not what I want I will change the trans to wide ratio.
Again thank you all for the great suggestions,
John
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Post by joepadavano on Oct 7, 2020 7:38:58 GMT -5
I put a Torsen-type worm gear differential in my truck (Eaton Tru-Trac). It works pretty well but I occasionally get clicking noises on tight turns. The sound is disconcerting but there have been no ill effects in the 5-6 years that it's been in there. I do tow and haul a lot with this truck.
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Post by john442 on Oct 7, 2020 14:02:41 GMT -5
Thanks Joe for your great info. I found out today that the rear is the bolt in axle type not the C clip type. I am unfamiliar with the different types.
I'm still a little puzzled about how responsive my car is in any gear. It feels very much like my 4 speed chevelle with 4:11 gear and a big block. When I had my car on the lift I counted the drive shaft turns to the wheel and it came out to a hair over 3 so I assumed it to be 3:08, Is it possible to get this kind of response with my current setup or is it possible I must have somehow miscalculated the rear end gears I have?
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Post by island65cruiser on Oct 7, 2020 21:40:34 GMT -5
John, if you use the spin the driveshaft method it is best to go ten revolutions on the shaft. Easy to calculate times ten, with less than that if is possible to miscalculate the small difference between ratios. I have miscounted a couple of times. If the cover is off, count the teeth on the ring and pinion, it's more accurate.
If you are running a 455, the extra torque might make the 3:08 move pretty well, even with a high first gear ratio.
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Post by john442 on Oct 8, 2020 10:20:52 GMT -5
I’m gonna recheck the rear ratio using the ten times method.
thanks for the info
john
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Post by jcf85post on Oct 8, 2020 13:20:19 GMT -5
John, if you use the spin the driveshaft method it is best to go ten revolutions on the shaft. Easy to calculate times ten, with less than that if is possible to miscalculate the small difference between ratios. I have miscounted a couple of times. If the cover is off, count the teeth on the ring and pinion, it's more accurate. If you are running a 455, the extra torque might make the 3:08 move pretty well, even with a high first gear ratio. if you have the cover off just look at the numbers stamped on the ring or pinion,no need to count.you could get away with 3.08 with the m20 trans but even 3.55 with a m21 is terrible off the line |
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Post by island65cruiser on Oct 8, 2020 19:15:46 GMT -5
My last comment on this topic I promise: If you want to know if you can tolerate the 3:73, or the 4:10 gears on the road, there is a calculator available on line to compute your RPM at any speed, with any gear ratio, for a given tire size with a 1:1 final drive. If I recall correctly, it may be available from Jegs.
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Post by john442 on Oct 9, 2020 12:31:16 GMT -5
I have used the calculator before to figure out RPM. I'm going to change the rear ratio to 3:55 and add posi at the same time. If I'm unhappy with the
results I will decide to go back to the 3:08 gears or change the trans from M21 close ratio to M21 wide ratio. I don't have money to throw away but enough that I want to be happy with the results.
I feel certain that I counted the driveshaft turns correctly and did it several times. It is just that this car feels just as responsive in any gear as my chevelle did with 4:11 gears.
Thanks for all the great info,
John
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