Close Ratio Transmission Definition? Performance?

[Rich C.]

I've always wondered as to what advantage/disadvantages there were between the normal 4/speed and the close ratio, (M-21 and M-22) transmissions offered on the early Corvettes. I take it the close ratio was more performance orientated.




1973 LS-4 454 coupe owned 24 years
1996 LT-4 Collector coupe owned 15 months
Sierra Nevada Celebration Ale currently on tap!

[Joe L.]

Re: Close Ratio Transmission Definition? Performance?

I've always wondered as to what advantage/disadvantages there were between the normal 4/speed and the close ratio, (M-21 and M-22) transmissions offered on the early Corvettes. I take it the close ratio was more performance orientated.



1973 LS-4 454 coupe owned 24 years
1996 LT-4 Collector coupe owned 15 months
Sierra Nevada Celebration Ale currently on tap!

Rich-----


Yes, the close ratio is more "performance oriented". However, it's been my long-held opinion that the performance advantages are primarily applicable to off-road, racing applications. For STREET operation, I have long-considered the wide ratio transmission to be preferable. The VAST majority of Corvettes, old and now, are used for street operation only and never see any sort of off-road racing.

[Glenn S.]

Re: Close Ratio Transmission Definition? Performance?

Actually depends on ones definition of performance. First gear in the close ratio is 2:20 while 2:56 in wide ratio. Don't recall what second is. The latter will make a car with a 3:36 rear accelerate like it has something closer to a 3:90.

Some engineer will post here the actual math. I just know last year I installed an m-22 wide ratio and the performance off the line is remarkable.

[Joe L.]

Re: Close Ratio Transmission Definition? Performance?

Actually depends on ones definition of performance. First gear in the close ratio is 2:20 while 2:56 in wide ratio. Don't recall what second is. The latter will make a car with a 3:36 rear accelerate like it has something closer to a 3:90.

Some engineer will post here the actual math. I just know last year I installed an m-22 wide ratio and the performance off the line is remarkable.

Glenn------


As you may know but others might not, the "M-22 wide ratio" is an aftermarket-only concoction, never available from GM. That doesn't make it bad, though. It's something that GM should have made but never did.

[Leif A.]

Re: Close Ratio Transmission Definition? Performance?

I've always wondered as to what advantage/disadvantages there were between the normal 4/speed and the close ratio, (M-21 and M-22) transmissions offered on the early Corvettes. I take it the close ratio was more performance orientated.




1973 LS-4 454 coupe owned 24 years
1996 LT-4 Collector coupe owned 15 months
Sierra Nevada Celebration Ale currently on tap!

Rich,

Don't you mean the difference between the M20 and M21 (wide ratio vs close ratio)? The M22 had straight cut gears and was basically a racing transmission.

[Joe L.]

Re: Close Ratio Transmission Definition? Performance?

Rich,

Don't you mean the difference between the M20 and M21 (wide ration vs close ratio)? The M22 had straight cut gears and was basically a racing transmission.

Leif and Rich-----


Yes, I should have caught that. The M-20 is wide ratio, the M-21 is close ratio, and the M-22 is HD close ratio. The M-21 and M-22 have exactly the same gear ratios.

[Steve G.]

Re: Close Ratio Transmission Definition? Performance?

An interesting aside (I thought anyways), the AIM for my 67 Olds 442 lists what they refer to as "standard" combinations of axle and transmission. I presume this meant if you ordered other than the base trans ( 3 speed manual) and didn't specify rear axle, these would be the defaults. The standard rear axle with the m-20 was the 3.55 while the standard axle when ordering an m-21 was 3.91. This provided the compensation for the taller first gear in the m-21 and illustrates that the belief was that the best use of the close ratio was with a deeper rear axle and intended for the drag race scenario.

[Bill M.]

Re: Close Ratio Transmission Definition? Performance?

For those who believe driving is both a sport and an art, the close-ratio, with its equal-spaced ratios from 4th to 1st, allows for a heel-and-toe, double-clutched downshift to be performed exactly the same for all downshifts from 4th to 1st.

But I prefer my wide-ratio 3.55 to a close-ratio 4.11 for street use; same overall ratios for 1st thru 3rd, better cruise in 4th.

[Glenn S.]

Re: Close Ratio Transmission Definition? Performance?

Yes I know. I did because in one previous car I had an m-22 and in another a wide ratio. I thought the combo would be fun. I I have a 3:36 rear and since I mostly cruise around town at lower speeds, wanted the benefit of the better acceleration of the wide ratio.

As for the m-22 part, unfortunately, my side pipes are too loud to hear the gear whine from inside the car.

[Peter S.]

Re: Close Ratio Transmission Definition? Performance?

Hi Rich,

Here is a Super Chevy article from "back in the day" that may give you a better understanding.

http://www.superchevy.com/how-to/148-0208-gear-ratio-calculating/

Peter

[James B.]

Re: Close Ratio Transmission Definition? Performance?

I've got to comment on this post and pose a question. Gearing has always left me with a big question mark. I have always preferred a 3.70/4.11 w/ M21 close ratio as it feels like on the 1-2 and 2-3 shift you don't lose a lot of RPMs and are still solidly in the power band. Conversely, a 3.36/3.55 with an M20 (wide ratio) feels like I drop into a hole & lose a lot of RPMs and road speed on those shifts.
Currently I have a '64 fuelie- 3.70 with M20 (close ratio) and a 67 L79- 3.55 with M20 (wide ratio). I will typically make the 1-2 shift at 3000-3500 RPM and the 2-3 shift anywhere from 2500-3500. The 64 just feels great and the 67 feels like I've fallen into a hole (feels like I've lost a lot of RPMs & I'm way down the torque curve). It isn't just the fuelie to carbureted difference. I've had 4 other mid-years 300 HP, 350, 365 and 425. They have all had 3.70/4.11 w/ M21 or I have converted them. They all felt like my 64.
What I am NOT talking about is clutch engagement (the 3.55 w/ wide ratio feels like a stump puller) or the 3-4 shift where the split is much higher in the wide ratio.
The odd thing is, after studying the gearing splits I don't think they support my sense/feel regarding the 1-2 and 2-3 shifts. I've attached a gearing chart I did a few years ago and the percentage split is essentially the same for a wide and close ratio transmission (until you hit the 3-4 shift). Also, the speed drops are not all that different. Does anyone have the same feeling? If so can you explain why that is with the same gear splits on both close and wide ratio transmissions.
At this point I am restoring my '67. It is a very original car and I want to keep it that way so I am "stuck" with the wide ratio so it would help if someone could convince me I'm wrong.preview-web.jpg(Oops sorry spreadsheet so small, click on PDF>)CorvetteGearing.pdf

[Stuart F.]

Re: Close Ratio Transmission Definition? Performance?

I have found that with my 63 L-76, with close ratio and 3.36 final, matched up well with most any small block with lower (higher numerically) gearing for acceleration from 30 mph on up. In fact, that was my greatest source of fun and match money. When challenged, i would claim my handicap of tall gearing and easily talk them into a 30 on up run. It would catch most of the performance geared in between gears or off their torque curve. I easily stayed with them while they shifted away until I wound away from them in second and third, then they would quickly run out of gear at their top end. Goodbye, I'm out of here!

Personally now, in my old age, I much prefer driving in the countryside away from all the stop light commandoes. I have an irreplaceable original engine and a big vented gas tank barely 4 feet behind my butt! Guess I'm just glad to still be here when I think back to the old days and the crazy things we did on U.S. Royal 6.70 x 15 bias ply tires. Those darn things would blow out by just hitting a stone on the road, and we still ran them to 130+ !! You haven't lived (or come so close to dying) until you've run a midyear to 130 on bias ply tires!!

Stu Fox

[Ed S.]

Re: Close Ratio Transmission Definition? Performance?

James,
I believe you may have your 20s and 21s mixed up. Would encourage anyone correct me if I am wrong (or verify if correct) but.. the M20 is the wide (2.56:1 first gear) and the M21 is the close ratio (2.20:1 first gear). The M21 and M22 had the same ratios in all gears, the difference being gears in the M22 were cut at less of an angle to reduce heat.

Regarding what you are / have experienced in performance between an M20 and M21 equipped car - that is exactly what GM intended. The M20 will give much better around town and day to day performance because 1st gear is overall much lower. The close ratio boxes were designed for the driver that needed to keep the revs high and stay at the beginning of or right in the sweet spot of the power curve - i.e., road racers. You are correct in your observation, with the wide ratio there is a tendency to lose RPM between 1st & 2nd. Not so much so with the close ratio M21 (or M22).

By the way - your '64 fulie - you stated that it has an M20, I don't believe fulies were delivered with M20s, only M21s (close ratio) boxes.

[Mike E.]

Re: Close Ratio Transmission Definition? Performance?

I think what makes this confusing to a lot of folks is prior to 1966 Chevrolet used the RPO code M20 for a 4 speed gearbox then install either a M20 or M21 depending on the engine/axle ratio. A wide or close ratio could not be specified by the order. Even after that there might have been restrictions on what gearbox could be chosen for a given engine application.

Mike

[Jim L.]

Re: Close Ratio Transmission Definition? Performance?

As for the m-22 part, unfortunately, my side pipes are too loud to hear the gear whine from inside the car.

Hmmmmm...... you sure you have an M22? My bride's track car (see avatar) has an M22 which, in 1st gear, is unmistakeably "competitive" with the side pipes.