C2 Rear End ratios

[Jack W.]

Re: C2 Rear End ratios

I would be willing to bet that, if you took your 64 L76 to a 3:36 ratio rear end, we will be seeing a topic from you along the lines of "Help! My L76 Can't Get out of It's Own Way at a Stoplight!"

Like John and Duke said, you won't able to have your cake and eat it too - something is going to suffer, and you'd be best served to compromise rather than serve just one issue (like trying to completely minimize rpms at cruising speed).

I have a 65 L76 with an "AO" (3.70 posi) rear, still okay off the line (dont be a atranger with the go pedal though) and cruise rpms are not too bad, even with the sidepipes . . . .

[Joe R.]

Re: C2 Rear End ratios

Hi Mike:

You have gotten some good responses here, and I suggest that you think carefully about the options before you select your action plan. There are several paths you can take depending on how much time and money you are willing to invest and how important it is to maintain an original appearance for your transmission.

I think that 64s used T10 transmissions and I am not familiar with the gear set options for them, but I suspect that both close ratio and wide ratio gear sets are available. If so, and if you presently have the close ratio gear set, you might consider changing to the wide ratio version.

In the case of Muncie transmissions that I am more familiar with, the wide ratio gear set has a 2.52 first gear instead of the 2.20 in the close ratio version. With the close ratio Muncie and a 4.11 rear, the effective first gear is (2.20)x(4.11)= 9.042.

With the wide ratio gear set and a 3.55 rear end, the effective first gear ratio is (2.52)x(3.55)= 8.946, which is essentially unchanged from the close ratio with a 4.11. However, since both gear sets have a 1:1 fourth gear, the 4th gear cruising RPM will be only (3.55)/(4.11)= 86% of what it is for the close ratio and 4.11. With only a slight decrease in the effective first gear, a 3.36 rear end will drop the cruising RPM even further.

Any solution that uses a 4-speed transmission will be a compromise, but I think a wide ratio transmission and a moderate rear end ratio is the best possible compromise for a street car.

If you don't mind having a non-stock transmission, I think the no-compromises setup is the Tremec/Kiesler 5 speed set up so that 5th is an overdrive gear. This is a bolt in solution that involves no cutting of the body. There are enough gearing options to choose from that you can have your cake and eat it too (low effective first gear and tall top gear for cruising).

[Bill Stephenson]

Re: C2 Rear End ratios

Joe,

------All 64s and actually alot of 63s came with Muncies. Other than that,,,good info!........Bill S

[Mike Iachelli]

Re: C2 Rear End ratios

Thanks to everyone for the comments and suggestions. The best but probably pretty expensive solution would be like Duke said, change out the trans and rear end to match each other and the desired results. On the other hand, if a rear end swap to 3.55 gears is within a decent price range, that seems to give a good comprimise. Maybe I can get lucky and find a complete unit with 3.55's and store the original.

PS: The original post desiring a 2500 cruising rpm was way off base. Have a 6 speed in my 96 LT4 and had that on my mind when first asked for advise.

Mike

[Joe R.]

Re: C2 Rear End ratios

Hi Bill:

Thanks for the clarification. This means that the calculations I cited are directly applicable to Mike's car, rather than just general guidelines.

In thinking about this further, it seems that any solution that uses the Muncie wide ratio gears will involve changes to BOTH the differential and the transmission. That's a lot of money and effort.

I think a bolt-in Keisler 5-speed that has an overdrive 5th gear might allow Mike to leave his existing 4.11 differential unchanged. The Keisler is around $3500, but the swap would be less work than opening up both the differential and the Muncie to change gear sets.

[Mark #28455]

A little e-bay luck

I have bought a few complete differentials on e-bay. I look for an auction that is listing a later 1970's unit (less stock HP, less likely to trash the rear) from an auto car (less pounding from hard shifts) and with a little luck, have ended up with 5 or 6 for $75 plus shipping.

The input yoke will need to be swapped or you will need a conversion u joint.

Good luck,
Mark

[Mike Iachelli]

Re: A little e-bay luck

Mark, thanks for the idea re: a later year. Three questions please:

1. Up to what model year differential (complete) would slip under the 64 coupe?

2. Does it have to be a Corvette differential? Does a 64 posi differential cross reference to other GM postitaction units? Different side yokes should not be an issue since they are easily replaced) other than the input and side yokes? (no matter what, keeping the original either on the car as is or in storage if change the rear end)

3. If able to find a used unit, probably woul have it rebuilt before installation since R&R labor is already covered. Am I correct that finding and adding the correct input yoke should not be an issue except for the price of the yoke and that the input yoke is not an expensive item?

[Mark #28455]

Re: A little e-bay luck

If you swap the entire differential as a unit, it is externally interchangeable from 1963 to 1979. The 1963-64 have a weaker internal posi case and are much less desirable than the later parts. The earlier units also apparently have less splines on the pinion gear, so those gears need to be used with the correct yoke - you probably can't just swap your old yoke to the new unit. The yokes are commonly available on e-bay for about $40 or so (remember that from 1970 or so, the automatics used the bigger yoke). All of these are Corvette only parts.

I would just swap the differential unit as a whole so you can take your original sealed unit with rear cover and store it indefinitely in the garage.

Mark

[mike iachelli]

Re: A little e-bay luck

Thanks for all of the information.

Mike