R6T7

I'm thinking I'd like to change the ratio from my 3:36 non positraction to the lowest numerical ratio rear end I can get on my 67 small block powerglide coupe. I'd like the car to be a better cruiser (lower RPM at 65 to 70) at freeway speeds, and I don't want to go to a 5 speed or change out the powerglide to a new automatic. What are the options, and what's involved in changing the rear end ratio?

Is a 3:08 the lowest ratio available for the C2 rear end?

Thanks,

Bob

tentuna

Yes, 3.36 is available for a C2, if you check the after market gear people you may be able to get a 2.73, but that may make the car real sluggish, being a base engine.

ptwohey

Bob,

With custom aftermarket ring and pinion, I believe that 2:56 is the lowest and with a PG you could get away with it but your acceleration will suffer.
If you want to get a ball park idea of the difference between gas mileage, calculate what you are getting now with the 3:36 and do the math with the other ratios. My '71 454 has the original 3:08's and cruises OK and still has some get up and go...but it is a 365HP BB.

As far as changing them over, if you are mechanically inclined you might be able to be talked through it. If you don't understand what the shims do and how, there are several members who rebuild diff's. If your pockets aren't empty yet, leave your 3:36 intact and buy one already set up. If you look hard enough you might be able to find a reasonably price unit. You might even consider buying a second unit to rebuild so you can swap them back and forth when you get frisky.

Plasticman

What engine?

And what cruise rpm are you looking for?

Plasticman

midyearvette

there is an approx. difference of only 200 rpm in high gear between a 336 and a 308, however your acceleration will suffer in low gear from a stop light...good luck

R6T7

This car has a 300HP engine - it has been rebuilt, mostly to stock specs, but after the rebuild, it feels a lot stronger than a completely stock engine. With the current setup, it will pretty much keep up with my 66 350HP 4 speed car.
I'd like to keep RPM at 65 to 70 under 2500, but it would take a 2.73 ratio to do that. It looks to me like going from a 3.36 to a 3.08 wouldn't make a big enough change in cruising RPM. Hard to figure what driveability would be with 2.73 gears.

Bob

Plasticman

I agree with both you and Midyearvette. Going for a 200 rpm drop is not worth it.

Was the engine rebuilt with the original cam specs?

Plasticman

R6T7

Yes, the cam was supposed to be close to original - can't remember the exact specs.

Bob

JohnZ

3.36 was the standard axle with a 300hp Powerglide, and 3.08 was the "economy axle" option. Hardly worth the time/effort/money to change to a 3.08. 2.73's were available from '68-'70, but they were used with 3-speed automatics with a much lower 1st gear; a 2.73 would be a real turd with a Powerglide.

A better choice for you would be a 700R4 or a 200-4R overdrive automatic transmission with your existing 3.36 axle - you can't get to where you want to go with a Powerglide.

midyearvette

the 200 r4 would be a great tranny , there are many builders out there who have discovered this especially after they installed them in buick gn's and the super fasr 1999?? trans am with the turbo engine......

JDBob62

I have a 62 300HP PG. Would either the 700r4 or 200r4 fit my C1. I might have to rebuild by pg which I do not know if it is a 61 or 62 (see seperate positing on PG identification). My rear axle is the 3.36.

Also, can someone describe the benefits of these 2 transmissions. I am not familiar with all these codes.

Thanks,

Bob

Plasticman

Here is some very basic info:

http://en.wikipedia.org/wiki/Turbo-Hydramatic

Plasticman

midyearvette

the 200 and 700 r4 trannys are overdrive units that have been around since the early eighties. from the mid eighties till now they have proved their reliability. any good tranny shop can hook u up probably just have to change the rear tranny mount and hook up the electronic overdrive and you would be in business.....super up grade to your pg.........im sure others here are more in the know than i and they will chime in...good luck...

R6T7

I guess I was hoping for a miracle cure with my powerglide, but it does not look like it's a very good option.
What is the fourth gear (or overdrive) ratio for a 200R4 tranny - and what would you expect the cost to be (rough estimate) to swap out the powerglide for a 200R4?

Bob

Plasticman

.75 overdrive for the 200-4R in 4th (it would bring your 3.36 down to a 2.52).

Cost to change out is probably a local thing.

Here is a link to some gearing info. Click on "Display Table" in option (2) for trans gearing info:
http://www.stl-vettes.com/65Vette/tr...ns_gearing.htm

Plasticman

silverslashstreak

Bob, the 200 or 700R is about the best solution, the biggest problem you have to address is you will either have to make a removable cross member or lift the engine and move it forward to get the OD transmission in. The good thing is that the 700R will fit the tunnel fine and you can obtain the correct u joint angles with no modifications to pinion angles.

Here is a shot of the 700 R in my 63 roadster project.



Jeff

JDBob62

If I change from a '61/'62 PG to a 200 R4 will I have to either change the radiator or add an external transmission oil cooler?

silverslashstreak

I always run a small cooler in front of the radiator, very simple to install plus most people who build automatics recommend a cooler.

Plasticman

Are you saying that your PG does not use a cooler?
Why not use the original?
Agreed, an extra cooler does not hurt for long life.

Plasticman

JDBob62

Plasticman: Yes I am saying my stock 62 w/PG does not have an oil cooler either as a seperate unit or built into the radiator. Also I have never seen anything in any of the factory manuals. I have the radiator out and there are no provisions for lines other than coolant from the engine.