Close Ratio Transmission Definition? Performance?

[James B.]

Re: Close Ratio Transmission Definition? Performance?

Ed, thanks for the comment but I am pretty sure you are wrong. In 1964 all manual transmissions were labelled "M20". If the car was high HP- 365 and 375 your M20 was close ratio. If lower HP it was wide ratio. I am not exactly sure when GM went to the M20 vs M21 distinction, it was certainly done by 1966.
Another oddity, in 1964 if you ordered the 365 or fuelie you got the close ratio M20 but you could order any axle ratio with that. My 64 was speced with the 3.08. That must have been a real dog off the line.

[Ed S.]

Re: Close Ratio Transmission Definition? Performance?

James,
Have to agree with you, in 64 all 4 speeds were RPO M20 - as indicated by Mike above. Thanks for the correction.

[Rich C.]

Re: Close Ratio Transmission Definition? Performance?

Thanks for all the responses...enlightening!! In my original inquiry, yes, I meant the difference between wide (M-20) and the close (M-21, M22). Thanks again!!



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

[Duke W.]

Re: Close Ratio Transmission Definition? Performance?

To detemine an appropriate ratio set, one has to consider vehicle use and the shape of the torque/power curves. A key factor is what I call "inter gear" ratio, which determines the RPM drop when shifting at any given speed, and, frankly, the early four speed ratio selection was driven by manufacturing simplicity rather than engineering analysis to determine optimum ratios for best road performance across the entire speed range.

The 1957 three-speed has ratios (see AMA specs) of 2.21, 1.32, and 1.0:1 (direct). The 1-2 intergear ratio is 2.21/1.31 = 1.69. When shifting Duntov cammed engine from first to second at 6500, revs dropped to 6500/1.69 = 3800, which is below the torque peak and WAY below the power peak and acceleration suffered.

And because the Duntov cam engines biased power to the top end at a sacrifice to low end power, a short axle ratio was required to achieve good off the line performance with only a 2.2:1 total ratio spread. Back in those days there were few freeways and most state and federal highways had 55 MPH speed limits, so cruising a 80/4000 revs with a 4.11 axle was not an issue for most owners, nor was top speed.

The four-speed was made by the simple expediant of moving reverse to a new tail housing and substituting another forward gear with a ratio of 1.66. Now the 1/2 intergear ratio is only 2.21/1.66 = 1.33, so shifting to second at 6500 only dropped revs to 4900, and the engine remained in the power "sweet spot" that I'll roughly define as the range were the engine produces 90 percent of peak power, or the 90 percent power bandwidth.

When the decision was made to migrate the four-speed to heavier cars with lower revving engines and taller axle ratios, a lower first gear was required, and GM chose the simple expedient of merely increasing the counter gear ratio to shorten first, second, and third. (The overall ratio is determined by two gear sets - clutch gear to counter gear and counter gear to main shaft gear.) This preserved the same ratio spacing between 1-2, and 2-3, but produced a HUGE 3/4 gap.

The best ratio selection for road use are "progressive ratio", which means the intergear ratio starts out at a fairly high value and becomes smaller as you go through the gears. A high 1-2 intergear ratio is not that noticeable because total drag is low at the shift point, and plenty of power is available to accelerate the car. But at 100 MPH when you shift into top gear, most of the power is consumed by drag and little is available for acceleration, so you want to keep the engine in that 90 percent power bandwidth.

The later B-W Super T-10s had much better ratios, 2.43, 1.61, 1.23, 1.00:1 for the CR, and 2.64, 1.72, 1.33, and 1.00:1 for the WR. Try computing the intergear ratios. They make a lot more sense for a road car.

Many owners find that second gear is somewhat useless on base engines because the engines are so torquey, so some shift 1 - 3 - 4 and rarely use second. The Super T-10 ratio set would make a lot more sense. A slightly shorter first, taller, second and third, and no huge 3/4 gap.

Modern Corvettes have a total ratio spread of 5-8 to 1, so there is a gear for everything, from a great starting gear to a super overdrive top gear for easy high speed freeway cruising at very low revs with excellent fuel economy.

About a year ago I posted a simple Excel program that allows you to play games with gearing and tire revs per mile. Just search "gear chart" under my handle.

It's especially useful if you have a chassis dyno curve for your engine, which will allow you to select optimum shift points for each gear for maximum accleration performance.

Duke

[Wayne M.]

Re: Close Ratio Transmission Definition? Performance?

..... In 1964 all manual transmissions were labelled "M20". If the car was high HP- 365 and 375 your M20 was close ratio. If lower HP it was wide ratio. I am not exactly sure when GM went to the M20 vs M21 distinction, it was certainly done by 1966.

Another oddity, in 1964 if you ordered the 365 or fuelie you got the close ratio M20 but you could order any axle ratio with that. My 64 was speced with the 3.08. That must have been a real dog off the line.

Yes; for 1966 they used the M21 and M20 (and M22) designations.

Even in 1965, with the new L79 350hp, seems you could get any posi ratio you wanted, according to the dealer brochure. Here's an imprint of the P-O-P from my first car, VIN 04835; [close ratio trans obligatory] with 3.08 gears selected.




c

[Duke W.]

Re: Close Ratio Transmission Definition? Performance?

M-20, M-21... it can be very confusing for the above reasons. So why not just say WR or CR. According to the NCRS Spec Guide the '65 L-79 was only available with the CR. The standard axle was 3.70 and 4.11 and 3.55 were the only options.

The first year one could choose either WR or CR was 1966 for L-79 and L-36.

L-79 was never available with a 3.08 axle and a 3.36 could only be had with the WR four-speed.

I ordered my SWC with L-76 and the 3.08 axle, and yes, it's no drag racer. A 30-30 cam engine would be even worse since it has poorer low end torque than the Duntov-cammed engines, but I know there are a few around.

Duke

[William C.]

Re: Close Ratio Transmission Definition? Performance?

Actually the ratio drop In am M-20 and M-21 are exactly the same for the 1-2 and 3-3 shifts. The disparity is made up on the 3-4 shift with the close ration having approximately the same drop between gears on 3-4 as the 1-2 and 2-3. The 3-4 shift has a wider drop to match the difference in low gear between the M-20 and M-21.

[John H.]

Re: Close Ratio Transmission Definition? Performance?

Here's the story on M-20 vs. M-21 description and availability vs. customer ordering paperwork.


M20RatioTech.pdf

[John D.]

Re: Close Ratio Transmission Definition? Performance?

In terms of overall drivability my 67 with L79/M20/3.36 is just about perfect. Good performance in 1st and 2nd and decent rpm's at highway speeds.

[Stuart F.]

Re: Close Ratio Transmission Definition? Performance?

May I pose a question of the old timers who experienced BW T-10's: I recall that "back in the day", The T-10's used with CI cases on tall gear installations, I.e. 3.36 or 3.08 finals and CR gear sets had a problem caused by long winding in that they would weld the counter shaft thrust washers (those with little needle bearings) to the case from excess heat. Does anyone remember a similar problem, not so much with Vettes, but full size sedans with SHP engines such as Chevies with 335hp "W" engines or Ponchos??

Stu Fox

[Wayne M.]

Re: Close Ratio Transmission Definition? Performance?

posi

M-20, M-21... it can be very confusing for the above reasons. So why not just say WR or CR. According to the NCRS Spec Guide the '65 L-79 was only available with the CR. The standard axle was 3.70 and 4.11 and 3.55 were the only options. ......
Duke

Duke --- I beg to differ, based on a few data items from back in 1965. With L79, L76 and L84 engines you could order the same differential G81 posi axle ratios, IMHO (see dealer brochure pic below). The 1965 order guide says to "see finger tip facts (power teams) for availability" (of G81, G91), which, IMO sends us back to the power train chart in the dealer brochure. The only problem I see with this table is that it says that the "standard" ratio for L79, L76 and L84 is 3.70 non-posi (ie. stamped code "AS'), while not including the 3.70 posi in the list of available ratios. I'm sure this an error. I see no reason why the L79 should not benefit from the same selection as its close ratio "brothers" (L76 and L84). What we need is evidence of L79 P-o-P's with AM, AO, and even AQ (4.56 posi).

Noland Adams pg 271 shows a list of ECL's (ie. shipping/dealer copy RPO suffixes, not stampings). L79 (along with L76 and L84) all share codes EA, FA and GA for 3.55, 3.70 and 4.11 small block posi ratios, respectively. This seems to check with what you are saying, although in contradiction with the dealer powertrain chart.

Check the imprint of the P-o-P that I attach to post #20, above. This was in a package of maintenance paperwork I got when I bought my '65 in the spring of 1968 (was 3rd owner at 24 yrs old -- my first car ). Clearly shows an HT engine (350hp L79) with an AL stamp code for the posi 3.08 differential.

[Duke W.]

Re: Close Ratio Transmission Definition? Performance?

I went by the power team chart in the NCRS Spec Guide, but the power teams data from mulitple documents in the vehicle information package from the GM Heritage center conflicts.

It's possible that axle ratio availability changed during the year. The dealer technical data power team page is dated March, 1965, which would indicate a change from earlier in the year. The AMA specs show no update during the model year.

Duke