EB338

I've been digging through the archives and haven't really found the answer to these questions so if this is a redundant post I apologize.

What really are the differences in the 85 and later 5 speed gearboxes? Can any of them truly handle more HP and Torque than others? What all if anything can be done to strengthen them for higher HP applications? Can the cooling and oiling modifications of the later GTS boxes be adapted into earlier units? What are the full race guys doing to keep these boxes alive under race conditions? Surely the race cars have upgrades and modifications that would make our street cars boxes live a long time.

I have read through the thread about use of the Corvette box and all gear ratio issues aside, dimensionally how close is the corvette box to fitting? The Corvette race and aftermarket guys do offer a wider array of gear choices than just what the factory has put out so if we can make them physically fit the car I would think the ratio issues could be overcome. Not to mention the much wider selection of final drive ratios and LSD units. Maybe we could finally get the Quaife unit in our 928s!

Fabio421

I know that the synchro's were upgraded to a longer lasting and more trouble free design in 85. I'm sure others here can tell you alot more.

GlenL

A few quick thoughts:

Later boxes have different synchros (Borg-Warner) that are more durable.

Cryogenic treatments have been done to the gears with perceived strength improvement.

People have made fluid coolers that use the fill/drain plugs. With a pump, radiator, hoses and fitting one can be fabbed.

What's your goal or just curious?

John Veninger

85 and newer have borg warner syncros.
The GT and GTS boxes are stonger.
Racers use a GTS box with the stock cooler. Others add external coolers.
Racers with strokers are known to break the GTS boxes.
Ratios are limited unless you have a bunch of $$ for custom stuff.

sublimate

I don't think the newer syncros make the box handle more horsepower, they just don't wear out as quickly as the old syncros. So I'm not sure if an 85-89 box can handle more hp than an early box, and you're possibly giving up the great 2.7 gear ratio.

The GT and GTS boxes have a bigger input shaft, IIRC, and should be able to handle more hp (but how much?). Plus the GTS boxes have a cooler which should extend high hp life.

fst951

Hi there.

Actually you can add GT2 syncros to the transmissions for faster and easier shifting. The 1985 and 1986 boxes shift nicer because of their bronze 1st-5th. The 1987 and later use sintered iron syncros that shift a little harder, but last longer. The GTS appears to have the strongest gears due to a different heat treat. The real problem is shaft flex more than anything else.

Good luck.

EB338

I am well aware of the syncro change from the pre 85 to the later 85 gearboxes. That basically makes them shift easier and last longer but not really a strength improvement. I was curious about the differences between all the 85-95 boxes. Im building a high HP stroker and my biggest concern is keeping the gearbox alive behind it.

John Veninger

Add a cooler. Use street tires and it should be ok. They will act like a fuse. Most of the boxes break due to very sticky track tires w/ high HP

EB338

Which box should be ok? Any of them? I was planning on wider tires but still street tires.

sublimate

To which transmissions? The early ones or the later ones?

Is there anything that can be done to the early trannys to make the syncros last longer?

heinrich

Garrity what is the cost implication of the GT2 gears?

Vilhuer

If GT2 syncros fit they should fit to all '85-95 boxes as syncro design is same. Material was chaged to iron as fst951 says. Input shaft bearing is different diameter in '87 and later, 92mm compared to 100mm IIRR. Counter shaft is drilled hollow in a way that it directs oil downhill inside itself to roller bearings and several roller bearings are larger than on previous types on GTS box.

GTS cooler pump takes it drive from diffrential by means of large tooth ring. It might be possible to install this ring on earlier diff assembly. Especially if speedo aluminium wheel is not needed. Almost all pump parts are in transmission rear cover. Delivery back into box from cooler is just about only part that is not in rear cover. Mounting it to front of the box paraller to gears would require some additional material on gearbox casing. GTS box has maybe 1" x ½" x ½" extra material between ribs where long needle like thing goes in.

Kaz

I remember hearing one of the racers tell me that 89 manual transmissions are the strongest for whatever reason.

BC

I'll find out if the early 2.75 transmissions are weaker. I'll pit over 525 through it hopefully soon, and do some light DE and autocross.
...Soon...

Jim_H

What size tires will you be using?