drnick

i have the opportunity to purchase either a GT or a GTS transmission for my conversion/repower project and im wondering how significant the differences are? i know the GTS has a mechanism to enhance cooling internaly and slightly different ratios, do these give improved acceleration? any other differences?

the engine going in is a GTS unit and the car is my 86 with street mods.

Lagavulin

The GTS unit is supposed to be the best; consider also it's what the racers always want. Likewise, the GT trans is supposed to be better than the S4's. I would get the GTS one just because of the built in cooling pump.

How much are each tranny?

drnick

lag, the difference in price is aprox $800.

Vilhuer

I would think it would cost much more than $800 to add oil pump to GT box. In that sense it's money well spend. Completely other thing is do you ever really need it.

Lagavulin

Nice pics!

hacker-pschorr

If you buy the GTS trannie - will you be sharing the souce for the GT trannie?

heinrich

Agreed, thanks for sharing the pics.

heinrich

btw aren't '89 GT boxes race-cut gears?

Garth S

Erkka, great shots - first I've seen of the constant mesh drive for the gear oil pump: no rubber drive bands off the half-shafts for our designers .

Nick, to add an unqualified opinion - assuming both boxes are of equal condition, I would choose the one with the ratios to match your rear tire selection that will set speed/shift points best suited to your planned application. Both boxes should do superbly: the gear pump is neat, but will you be endurance racing, whatever long enough to overcook 4.5l of gear oil ( where I suspect it would be of greater utility) ? If the better ratios turn out to be the GTS box, that's a double solution minus $800.

drnick

nice pics indeed! so what are the differences in ratios between the two boxes? im not sure if the GTS is higher or lower than the GT? as for the oil pump, i think that is a nice inovation. im intrigued as to why porsche put it in there at all if its only needed with endurance racing?

Jim bailey - 928 International

the gt BOX HAS A 2.7272 FINAL DRIVE RATIO RING AND PINION GEAR (11/30) . first gear is 3.7645, second 2.5122, 3rd 1.79,4th 1.3541 ...5th is 1 to 1 . The GTS box the layshaft ratio is different 35/24 1.4583 instead of 32/22 which is 1.4545 otherwise ALL the ratios are the SAME. The difference is so slight 1 st gear at 3.7745 instead of 3.7645....... The GTS has the same final drive at 2.7272 So really no difference in performance at all. The cooling is only needed after extended very hard ,very high speed driving when the transmission can get a bit notchy and not shift as smoothly quickly. Street car it makes little difference since you usually can not drive like that with out being arrested

drnick

yeah jim, maybe i do need the oil cooling feature after all...

heinrich

I'm starting to learn that the hard way. I need to move.

Vilhuer

Thanks guys. Those are some of the better ones. Usually can't take anything worth mentioning.

Other than pump there are other differences like reinforced drive shaft/drive pinion bearings, improved countershaft needle-roller and cage assembly, ground drive pinion/ring gear teeth and oil deflector/drilled drive pinion. I sound like salesman, don't I. Few more pics:











Two small bolts at center of last picture hold plate that directs oil to pinion screw which is of course directly below.

There is great software called CarTest http://www.cartestsoftware.com where it's possible to play with gearing and see how it affects performance. Older version 4.5 works fine for most purposes. Hämeenlinna race circuit isn't exactly correct and real lap times are slower in real life (probably on many other tracks also) but software is very good anyway. Had all different 928 variations in database including some tuning options like strokers but lost the file in hard drive crash.