1965 wide ratio vs close ratio

[Timothy B.]

In 1965 if L-79 option was ordered could M-20 wide ratio transmission be ordered. I read in the archives but conflicting info so I wanted to post now for comments.

I realize the standard rear axle with L-79 is 3.70 open and optional is 3.70-3.55-4.11posi.

[Stephen L.]

Re: 1965 wide ratio vs close ratio

The 1965 CORVETTE ORDER FORM does not show an M21 transmission available for any engine.

The 1966 CORVETTE ORDER FORM shows M20 and M21 available with L79

The 1967 CORVETTE ORDER FORM shows M20 and M21 available with L79

[Wayne M.]

Re: 1965 wide ratio vs close ratio

In 1965 if L-79 option was ordered could M-20 wide ratio transmission be ordered. I read in the archives but conflicting info so I wanted to post now for comments.

I realize the standard rear axle with L-79 is 3.70 open and optional is 3.70-3.55-4.11posi.

You are reading the power team charts correctly. As Stephen has indicated, no wide ratio with the '65 350hp engine. But in terms of differential ratio availability, here's an interesting note in the March 1965 issue of Chevrolet Service News. For Corvette,
".... ratios 3.36:1 and 4.56:1 in Positraction Axle option (RPO G81) are no longer available with the 350hp Corvette V8 engine."

Implied in that notice is that these ratios WERE available prior to then, and this agrees with the dealer showroom brochure, which shows the standard L79 as 3.70, with optional over the full spectrum, from 3.08 thru 4.56. My first car had L79 with 3.08 posi (see pic below of PoP imprint -- AL_12_09_64 code [3.08 posi]).

[Joe L.]

Re: 1965 wide ratio vs close ratio

In 1965 if L-79 option was ordered could M-20 wide ratio transmission be ordered. I read in the archives but conflicting info so I wanted to post now for comments.

I realize the standard rear axle with L-79 is 3.70 open and optional is 3.70-3.55-4.11posi.

Tim-----


Prior to 1966 all Muncies were M-20; there was no M-21 or M-22. However, there was a wide ratio M-20 and a close ratio M-20. Only one was available with any particular engine option. For 250 hp and 300 hp, only the wide ratio Muncie was available if a 4 speed was ordered.

For all other engines, only the close ratio version of the M-20 was available.

[Timothy B.]

Re: 1965 wide ratio vs close ratio

Thanks for the reply's everyone, I bet that wide ratio would wake up a 350 hp car with 370 axle for street use..

[Todd L.]

Re: 1965 wide ratio vs close ratio

Sorry to hijack the thread, I just want to understand. How does a close ratio, and a wide ratio differ, and how does that give or take away from a cars, torque/hp if any?

Todd

[Joe C.]

Re: 1965 wide ratio vs close ratio

Sorry to hijack the thread, I just want to understand. How does a close ratio, and a wide ratio differ, and how does that give or take away from a cars, torque/hp if any?

Todd

Here's the answer to the first part of your question:

http://www.dandltransmission.com/muncieratio.html

* note: there is an error in D & L's chart of RPO designators for "M-21 close ratio 63-65". This should read "M20", not "M21"

Quick answer to second part: the higher the numerical gear ratio, the more torque multiplication..............
Corvettes ordered with higher horsepower engines (intended for "off road" use) were generally equipped with higher numerical ratio differentials along with CR 4 speed. The reason for this was to get the engine up to its power (not torque) peak quickly, and keep the RPM range within a tighter band around that POWER (not torque) peak while shifting. If theoretical shift points were a small amount higher than the power peak, then RPM after shift would drop to only a small amount below the power peak.............

[Duke W.]

Re: 1965 wide ratio vs close ratio

Sorry to hijack the thread, I just want to understand. How does a close ratio, and a wide ratio differ, and how does that give or take away from a cars, torque/hp if any?

Todd

The four-speed transmission "gear ratios" are made up of two gearsets - clutch gear to countershaft and countershaft gear to mainshaft gear of which there are three sets for the forward gears. Fourth is always "direct" with torque being transmitted directly through the mainshaft.

Below top gear (and reverse) torque is transmitted through the countershaft. The countershaft gears that mesh with the mainshaft gears are exactly the same on both the WR and CR versions of the T-10 and Muncie.

The difference is the clutchgear-countershaft gear tooth count, and, in fact, a WR can be changed to a CR and vice versa by swapping these two parts.

At a given engine input speed the WR countershaft turns slower than the CR countershaft, which yields "lower" ratios in the first three gears of the WR with the same 1-2 and 2-3 intergear ratios as the CR, but the tradoff is a big 3-4 gap compared to the CR.

The intergear ratio is the "ratio of the ratios" that determines the PERCENT RPM drop when you shift to the next higher gear.

For example the CR 1-2 intergear ratio is 2.20/1.64 = 1.34. So if you shift at 6000 the revs drop to 6000/1.34 = 4478. Likewise for the WR 2.56/1.91 = 1.34.

T-10 CR: 2.20, 1.64, 1.31, 1.00:1
T-10 WR: 2.56, 1.91, 1.51, 1.00:1

(Muncies have slightly different ratios in some gears compared to the T-10.)

Using the above gear ratio data for T-10s you can compute the intergear ratios, and 1-2, 2-3 are the same for both versions. The big difference is 3-4, which represents a trivial case since the intergear ratio is the same as the third gear ratio, and for the WR it is 1.51 and 1.31 for the CR, so the WR sees a BIG RPM drop on the 3-4 shift relative to the CR.

For typical street use and drag racing the WR is a good choice because you are probably done accelerating in third gear, but for road racing the big 3-4 gap kills acceleration due to the big RPM drop shifting into fourth gear. You can also feel it in normal road driving.

The best ratio scheme for a road car is increasingly closer ratios as you go up through the gears. A big 1-2 gap is much less noticeable than a big 3-4 gap. This is the philosophy used for the later Super T-10:

Super T-10 CR: 2.43, 1.61, 1.23, 1.00:1
Super T-10 WR: 2.64, 1.75, 1.34, 1.00:1

As is the case with the T-10 and Muncie, the difference in the Super T-10 ratio sets is just the clutch gear-countershaft ratio. The intergear ratios are the same exept for 3-4, but the biggest gap is 1-2.

Very high specific output/revving engines like F1 typically have very narrow power bands as a percent of maximum revs, and for this reason the seven-speed transmissions used with F1 engines have an average intergear ratio of about 1.1, so if you shift at 18,000 (which I believe is the current F1 rev limit imposed by the rules), revs only drop off to a little over 16,000. So delivered power to the wheels is nearly constant and averages very close to the engine's peak power.

A 1.1 intergear ratio shifting at 6500 would only see a rev drop to about 5900, but because vintage Corvette engines have much broader power bands as a percent of peak revs, intergear ratios of about 1.25 are ideal on SHP type engines. Base and medium performance engines can handle slightly wider intergear ratios without a loss of acceleration performance because their power bandwidths (say the 90 percent bandwidth) are wider as a percent of peak revs than SHP engines.

Duke

[John H.]

Re: 1965 wide ratio vs close ratio

In 1965 if L-79 option was ordered could M-20 wide ratio transmission be ordered. I read in the archives but conflicting info so I wanted to post now for comments.

Tim -

In a nutshell, there was no customer choice of wide-vs.-close transmission gear ratios in 1965, which is why M21 doesn't show on the 1965 order blank. Engineering made that decision based on which engine you ordered. 250hp and 300hp engines got the wide-ratio, and everything else got the close-ratio.

[Timothy B.]

Re: 1965 wide ratio vs close ratio

Thanks everyone, I understand now.. So in 1966 you could specify M20-M21 but before that the engine option dictated the transmission type.

At the same time you could order optional axle ratio in 63-4 and into 65 until GM narrowed the selection.

[Jack P.]

Re: 1965 wide ratio vs close ratio

Thanks for the reply's everyone, I bet that wide ratio would wake up a 350 hp car with 370 axle for street use..

You bet, I have a m-20 wide L79 66 roadster, with a 3.70 rear, and light flywheel. The car really ......... takes off. Rubber everywhere.

I put in Audiovox Cruise Control CCS-100 and it now is a pleasure to drive on highways. Just set forget. With the 3.70 your gas foot would get tired holding the pedal down. Now I can move my legs around,

I was temped to go back to the 3.36 but the 3.70 is just so much fun and it would take 30,000 miles driving(3.36) to get back the money spent to change the rear out.

Jack

[Joe L.]

Re: 1965 wide ratio vs close ratio

Thanks everyone, I understand now.. So in 1966 you could specify M20-M21 but before that the engine option dictated the transmission type.

At the same time you could order optional axle ratio in 63-4 and into 65 until GM narrowed the selection.

Tim------


There was not "infinite" flexibility with respect to type of 4 speed ordered even from 1966 onward. 300 hp engine with 4 speed could only be equipped with M-20. L-79, L-36 and L-68 had a choice of M-20 or M-21 but that choice then dictated available rear axle ratios. L-71, L-72, and L-88 with 4 speed were only available with M-21 or M-22.