[Jack Blum]

Finally getting my 68 GTO road racing/rally car back on the road after the garage fire. Looking for ideas on a switchable/swappable rear diff. High gears for rally and low for road racing. Would like to be able to swap without dropping gear oil, but if that's what it takes I will. I currently have the T56 trans dual overdrive with the 0.62 6th gear. Motor is a stroked 472 with EFI and Butler Edlebrok heads. With the old 3.55 posi it did 467 at the rear wheels. May go with a supercharger at a later date. Running Silver State Classic we put an open 2.78 in to run 140+.

Any ideas or suggestions? Thanks all, Jack.

[Sirrotica]

Quick change, you can have almost any ratio you want by dropping the cover with 6-8 bolts. Designed originally for oval track race cars that run different tracks and changing conditions during the evening of racing.

http://www.halibrand.com/rear_choice.htm

[Jack Gifford]

Even a quick-change needs the lube drained/replaced when changing spur gears. [Other than the top-loading "front" Q/C box I designed/built for my mini-puller. But that's a whole different ballgame.]

How about a Gear Vendors under/over-drive box?

[Zimtok]

Didn't Ford have a 2 speed rear end in the 70"s as an option on the Torino...


.

[BruceWilkie]

Install a Gear vendors overdrive and call it a day... it will take more abuse/power than your tranny will. Can be shifted on the fly and allow you 12 speeds instead of your current 6.

[Skip Fix]

The guy that owns my machine shop has had a quick change in his 68 Camaro that runs deep in the 9s.

[Sirrotica]

GV only have 2 options for ratios for every gear in your transmission, a quick change you have about any ratio you want or need by switching 2 gears manually and refilling the rear chamber. Anyone familiar with working on one can change ratios in under 10 minutes. They are extremely fool proof unless you forget to refill the rear gear section with lube.

Reliability would go to the quick change developed over 65 years ago and has been a trouble free simple low maintenance item used in thousands of race and street cars. Available used for substantially less than GV units. There are multiple manufacturers for new parts for quick changes, where GV has the market all to themselves and charge a premium for parts.

Ratios available are from a 2.38-7.14 with a 4.11 ring and pinion, 2.80-8.42 with a 4.86 ring and pinion. 116 choices for each axle ratio, 4.11 or 4.86.

Link to available ratio table:
http://www.sardesonracing.com/page%20156.htm

One drawback to a quick change is gear whine, if you have no aversion to the whine then it may be what you need for your application. Thicker viscosity gear oil will minimize the noise and they are designed to be abused by 800 + HP engines constantly and live.



Reliability for GV products is less than stellar from reports of some owners I have read about. Used ones are difficult to find, and new is fairly high cost. They however can change your final drive ratio just by flipping a switch with only 2 choices of ratios in each gear same as an OTR truck uses 2 ranges for each gear. Your car, your choice.

[Jack Blum]

Thanks for the links I will check those out. We thought about a gear vender's. Only problem with that is once we get this bad boy pushed up to the 200 MPH range we are going to exceed our 10,000 RPM drive shaft limit.

Thanks for all the input.

[BruceWilkie]

Just some data taking a WAG on on tire and rpm but...
(TD*RPM) / (GR*336) = MPH
(26*5000) / (2.78*.62*336) = 224.4747 mph exceeds goal
(26*5000) / (3.55*.62*336) = 175.7858 mph
(26*5000) / (3.08*.62*336) = 202.6103 mph meets your goal
(26*5000) / (3.23*.62*336) = 193.2012 mph
(27*5000) / (3.23*.62*336) = 200.6320 mph meets your goal. drive shaft rpm 8064
(26*5500) / (3.23*.62*336) = 212.5213 mph exceeds goal. driveshaft rpm = 8870
(26*5500/.62) / (3.23*336) = 212.5213 mph (5500 engine rpm /.62 overdrive= 8870.9677)

... do you need multiple gear sets??? 3.23 aint bad gearing... not terribly different feel than 3.55 IMO... a tid more tire diameter and/or rpm... you got your 200 mph goal.