what is typically more desired the m20 or m21?

[Patrick N.]

Re: what is typically more desired the m20 or m21?

[QUOTE=Erik Stroeve (43069);616776]my 1968 L71 still has the original 4.11 and M21.

That thang is a beast!

[Joe R.]

Re: what is typically more desired the m20 or m21?

Hi Joe, Do you know if all 3.36's came with M21?
Best,
Pat

Hi Pat,

I don't think so. I think the 3:36 rear end ratio was available with any of the transmissions. Check out this link for what it's worth:

http://www.corvette.net/tech-2.htm

Joe

[Duke W.]

Re: what is typically more desired the m20 or m21?

Neither the T-10 nor Muncie four-speeds have very good ratio sets for modern driving conditions, but they were developed back before interstate highways allowed cruising speeds in the 70-80 MPH range.

The CR versions require a short axle ratio for good around town performance, but they turn excessive revs at modern freeway cruising speeds. A tall axle yields decent highway cruising revs, but around town it's like a five-speed with no first gear.

The WR versions were created with the expedient of a shorter countershaft ratio, which shortened first second and third, but resulted in a HUGE gap between third and fourth.

Many 300HP/WR owners skip second gear because the engine is so torquey, second is not really necessary. First, third, and fourth are about the same ratios as the three speed.

The later Super T-10 CR (2.43/1.61/1.23/1.00:1)and WR (2.64/1.75/1.34/1.00:1) transmissions have much more sensible gearing for a road car with the biggest gap between first and second where it belongs. A flexible road gearbox should have progressively shorter inter-gear ratios as you go up through the gears and the Super T-10 does so. I believe a Super T-10 can be swapped into earlier T-10 and Muncie applications with a minimum of modifications.

I've worked out some even better ratios, but it would take some tooling investment to make this happen.

.................................................. .................................................. ......Recommended Axle Ratios
.................................................. .............................General Purpose...........Highway Cruise.........Perf./Mountain
WR (base cam engines) 3.11/1.89/1.33/1.00:1.......................3.08:1............... ...........2.73:1...................3.36:1
.......(intergear ratios).......1.65/1.42/1.33
Revs @ 60 w/775 revs/mile tires...........................................23 87............................2116................ .....2604



CR (SHP/FI engines) 2.95/1.80/1.26/1.00:1............................3.36:1.......... ................3.08:1............3.55 or 3.70:1
.......(intergear ratios)..1.65/1.42/1.26
Revs @ 60 w/775 revs/mile tires...........................................26 04.............................2387.............27 51....2868

The two ratio sets, as with the T-10 and Muncie, are achieved with different countershaft ratios. The "Highway Cruise" axles would probably be best for how most of us use vintage Corvettes, today, but even with these tall axles, burnouts would be no problem.

Duke