C3-71 LT1 - 4spd and read gear combo

[Peter M.]

To all,

Was looking at Dealers manual on 71 Corvette and noticed that the standard setup when the LT1 was ordered was the M21(Close ratio) and 3:70 rear. If you opted for economy rear, you were given 3:55's or if you opted for the performance rear option you were given 4:11's.

But I noticed you could also get the LT1 with the M20(Wide ratio) which then came with the 3:55 rear as standard unless you asked for economy option which resulted in a 3:36 with the M20 or if you opted for the performance option with the M20 you were given the 3:70 rear.

When I multiply the trans ratios by the rear end, it appears as if the M20 and 3:70 rear(performance option with the M20) would provide the strongest performance as it multiplies the torque higher. When I mentioned this to a performance person and that I was shocked as I thought the M21 and 4:11 combo was the setup for hot, straight line performance, I was told it is. When I asked how could it be, they said that another factor has to be included: how quickly the rear end ratio winds the rpms into the powerband of the LT1. Thus the 4:11 will put the car in the upper rpm ranges(where most of the LT1 is making it's power) more quickly than the M20 and 3:70 combo.

Not being very mechanical, I ask this board for their thoughts - so , all things being equal, is the LT1 quicker with the M20 and 3:70 or the M21 and 4:11?

Thanks

Peter

[Joe L.]

Re: C3-71 LT1 - 4spd and read gear combo

Peter-----

Rear end ratios being equal, an LT-1 (or, just about any other engine) will be quicker off the line and through 1st gear with an M-20 as opposed to the same engine with an M-21/M-22. However, it will likely be quicker through the 1/4 mile with an M-21 or M-22. Since most folks don't usually run 1/4 miles on the street or do any road racing requiring close spaced gearing, the M-20 provides more of what you need and use. There are all sorts of tradeoffs when perfomance considerations are concerened. You must decide what's most important to you. For me, the M-20 is the better street transmission, especially for small block torque. When you get up to a 500 ft/lb torque big block, the lower 1st gear probably isn't as much of an advantage, but even the high torque big block will be faster off the line with an M-20.

As far as the M-20 with 3.70 and M-21 with 4:11 goes, the M-20 with 3.70 should be SLIGHTLY quicker off the line.

As far as cruising goes, regardless of rear gear ratio, the M-20 and M-21 will be the same. Both use a 1:1 4th gear ratio.

[Duke W.]

Re: C3-71 LT1 - 4spd and read gear combo

Multiply each transmission ratio times the axle ratio. The combination with the shortest (highest numberical result) will be the quickest to accelerate, up to the top of third gear.

The difference between the two tranmissions will be apparent when shifting into fourth gear. Both have the same ratio spacing between 1/2 and 2/3, but the WR has a big gap to fourth gear, which will cause the revs to drop much further down with the shift into fourth.

If the two cars were side by side at this point the CR trans car will pull ahead after shifting into fourth gear because the revs will pick up at a higher point, and it will be delivering more power at the bottom of top gear, but with a 3.70 or 4.11 will hit the rev limit at 130-140 MPH. The car with the WR and 3.36 will eventually catch it and probably pull to close to the redline and 150 MPH.

The WR with a 3.36 is a good compromise because it provides three fairly short gears for good acceleration, (the first three overall ratios are about the same as the CR with a 3.70.) and a decent cruise gear. It just doesn't feel good to go through the gears and feel the car "fall on it's face" when you shift to top gear at about 110 MPH, but that doesn't happen too often.

Duke

[Harmon C.]

Re: C3-71 LT1 - 4spd and read gear combo

Joe It seams to me that the close ratio trans makes them a dog off the line with the same rear gear as the wide ratio. Is this the way it works? My 73-454 with the close ratio and a 3.36 is a real dog off the line to about 30mph. What would be my best choice for stock 73-454 engine, close ratio trans, for a rear gear to have fun close to home with no high speed trips? Lyle

[Dave McDufford]

Re: C3-71 LT1 - 4spd and read gear combo

Peter,

I had a similar question and decided the M20 and 3.36 I had in the car was good. In first gear this combination is effectly a M21 with about a 3.90 rear end, with a 4th gear like an overdrive.

The math for the combination you are taking about is

M20/3.70-ttl gearing-% drop
1-9.32
2-6.96-22%
3-5.40-22%
4-3.70-32%

M21/4.11-ttl gearing-% drop
1-9.04
2-6.74-25%
3-5.26-22%
4-4.11-22%

Up until about 87mph when the M20/3.70 with 205R7015's hit 6000 rpm in 3rd, the M20/3.70 should be faster. Since most street driving is below 87mph, I think the M20/3.70 is better for the "overdrive" effect. At 70mph the 3.70 is turning 3,307 rpm and the 4.11 is turning 3,673.

My 3.36 at 70mph is turning 2,994 in 4th.

Dave

[Dave McDufford]

Re: C3-71 LT1 - 4spd and read gear combo

I wish you could correct a post...

The drop from 1st to second for the M20/3.70 should be 25% not 22%. The drop is the same except for 3rd to 4th.

Dave

[Chuck S.]

You Can Correct A Post...

But, it's not as easy as it used to be. You have to use the "brute force" method now":

First, highlight ONLY YOUR TEXT in the incorrect post and copy contents to your "clipboard" (under "File"). Then, enter your password and delete the incorrect post. Click on the post you want to respond to, and click "Post Response". Paste the contents of the incorrect post in the white field, MAKE THE NECESSARY CHANGES, and then click "Post Message".

Unfortunately, if the individual has requested to be advised by email when he receives a post, he will get an email for every change you make. Some folks have gotten three or more from me...Sorry, guys.

[Larry E. Howard]

Re: C3-71 LT1 - 4spd and read gear combo

I would like to point out that both the M-20 and M-21 (the M-22 is identical to the M-21 except for the angle of the gear teeth) use the same gears except for the input and cluster. The only difference in the cluster is the number of teeth on the gear that rides with the input. The number of teeth for the gears that ride with 1st, 2nd, and 3rd, are the same for both transmissions. Thus the cluster turns faster in the M-21 than the M-20. Now this means that the RPM drop for each gear shift with the M-20 is more than the M-21. But the percentage change is the same with both transmissions! Those folks who show a larger percent change in the shift from 3 to 4 between the two trannys need to recalculate. The ratio change is different not the percentage. With all due respect, the idea that there is a difference between 3/4 and not 1/2 and 2/3 is incorrect.
Larry

[Duke W.]

Re: C3-71 LT1 - 4spd and read gear combo

Before you change the gear consider remapping the ignition advance. Backdating the vacuum can to '66 with a full manifold vacuum signal, and increasing the rate of centrifugal will considerably improve low end torque. The timing maps for emission controlled engines reduce timing from optimum for torque. You should also be sure that your total WOT timing is 38-40 degrees. If you need more than about 12 initial to achieve this, grind out the slot in the autocam to increase total centrifugal.

Even with regular fuel you should be able to run a much more aggressive cengrifugal curve, and if you are willing to use mid-grade or premium you should be able to run a very aggressive curve.

Duke

[Duke W.]

Re: C3-71 LT1 - 4spd and read gear combo

The gear spacing and rev drop for 1/2 and 2/3 on both WR and CR are the same, whether you express it as a ratio, percent, or rev drop, but 3/4 is different.

Consider the term "inter-gear ratio" that I use of analyse gear spacing. You are correct that the difference between the CR and WR is the countershaft drive gear ratio, which means the 1/2 and 2/3 intergear ratio is the same, because the countershaft and mainshaft gear for each forward speed have the same tooth count. The total reduction is the product of two gear sets - the countergear drive gearset and the countergear/mainshaft gear for each forward speed. The only difference between the two boxes is the countergear drive ratio. which yields a different 3/4 spread. Using the '63 gearing as an example, since I know it (later gearing was slightly different) one can compute the intergear ratio.

'63 WR ........2.54, 1.89, 1.51, 1.00:1
intergear ratios..1.34, 1.25, 1.51

'63 WR.........2,20, 1.64, 1,31, 1.00:1
intergear ratios..1.34, 1.25, 1.31

For a given shift point, the point where the revs pick up in the next gear will be the shift point in revs divided by the intergear ratio, and the higher the intergear ratio the larger the rev drop, however, you can see by inspection from the above data that the gear spacing and rev drop for the WR and CR is the same for 1/2 and 2/3, but then there is a big difference with 3/4, the WR having a much greater gap. With a wide ratio a 3/4 shift at 5500 will drop the revs to 5500/1.51 = 3666, with the CR 5500/1.31 = 4198, but the rev drop for 1/2 and 2/3 for each box will be the same because they have the same 1/2 and 2/3 intergear ratio.

Alternately you can divide the lower ratio by the higher ratio for two sequential gears, subract from one, multiply by 100 and compute the percent rev drop. Same conclusion, but I like working with my computed inter-gear ratio concept.

Duke

[Peter M.]

Re: C3-71 LT1 vs

Duke,Joe,etc,

Thanks for the feedback. I always learn here.