transmission differential interchangability

diverdan · diverdan's Garage

Are transmissions and differentials interchangeable? IE: if you have gearbox with a 2.20 differential , can a manual 2.75 differential be swapped?

Diverdan

928sg · 04-08-2008, 07:25 AM

final drive gears cannot be switched betwenn manual and auto trans. on the manual trans the pinion gear is also the shaft that goes thru the trans. The auto pinion bolts on to a flange.

Danglerb · 04-08-2008, 10:34 AM

With the auto trans I think ring and pinion pairs can be switched, so that more favorable internal and differential ratios can be matched up than were available from the factory. If you take a 2.20 auto box the internal 1st gear ratio is numerically higher to compensate for the 2.20 ring and pinion. Swap a higher ratio differential and you end up with even higher ratio for 1st, which should give some more grunt off the line.

I've heard that all the ring and pinions are matched pairs, so I don't think you can just swap the ring and expect it to work.

diverdan · diverdan's Garage

Thank you. Very pertinent information. How much more economical is the 2.20 over the 275 or 2.73?

Diverdan

928sg · 04-10-2008, 10:29 AM

I'm not worried about economy.... I am however trying to make my lower gears more useable.

RyanPerrella · 10:53 AM

Correct me if i am wrong but i believe the rear end isnt the only gear thats different when comparing a US and ROW gearbox. When Porsche spec's a 2.20 I believe they also change the individual gear ratios. So the difference from one box to the other isnt just the rear end, but all the internal gear sets as well. These gearset changes offset the final drive ratio and the net effect is not the same as the rear end ratios would suggest. Correct me if i am wrong, but this is how BMW does it and i would think its pretty common practice because Euro cars and NA cars seem to ALWAYS have different gear box's in the 80's and 90's. Actually, my 90 GT has a 2.73 : 1 rear end , yet it has a higher top speed then an S4 with a 2.20 rear end. The RPM redline in the GT is only 200 rpm higher so where does the extra speed come from? Not from the engine spinning faster, 2.20 to 2.73 is a BIG difference if it was just the rear end which was different. I believe the 1-4 gears are all different and matched to whatever final drive the factory decided to go with.

Is this also the case with the Porsche transaxle's as well?

I am checking gearbox ratios now as we speak.

Here are gearbox specs: I know 5th was always a direct drive so in 5th a 2.20 rear end should run at lower RPM so your fuel economy should go up slightly.

5 speed S4: 2.20:1 I:4.07, II:2.71, III:1.93, IV:1.46, V:1.00
5 speed GT: 2.73:1 I:3.77, II:2.51, III:1.79, IV:1.35, V:1.00

Danglerb · 04-10-2008, 11:07 AM

I'm not sure if the individual gear ratios are in the PET6 or WSM, but I did dig through and post the info in some old rennlist post. I was only looking at 5 spd's and mostly concerned with 1st gear, which varied IIRC between about 7.5 and 9.5, +- 10%. Easy way to look at it would be redline in 1st being something like 27 vs 33 mph. *** 2nd through 4th I believe were about the same, with differences again in 5th.

Top speed is different, final drive ratio is important, but the cars aren't generally running out of RPM, they are running out of HP or RW torque. Neither 2.20 or 2.73 is optimized for top speed, its one of the factors, but more likely cruise RPM, and acceleration at lower speeds played a larger role. 2.36 is the magic ratio for a few more MPH at the top.

*** note, I hate to pull numbers out of the air, but too lazy this morning to look up the actual gear ratios, and somebody may find a link that shows them all, pretty sure I recall seeing it.

RyanPerrella · 04-10-2008, 11:12 AM

yes your right that the engine runs out of puff before the gearing becomes an issue in regards to ultimate top speed.

Someone told me that with enough power (Twinscrew or TT) my car would do 190+ (limited by redline) with the stock 2:73 rear end with my 19" rear wheels.

RyanPerrella · 12:04 PM

yeah but you don't get the sportier gearing that the 2:73 provides. Your reasoning is valid though as to total engine revolutions, but most people when asked if they would take a close ratio gearbox or a tall one in a sports or GT car will likely choose the close ratio version, which is what the GT has. Why have a sports car with a lazy gearbox? I know which i would take.

Unless of course your like Sterling and making so much power that 1st gear is useless, in which case a higher final drive is beneficial.

928sg · 04-10-2008, 01:14 PM

2nd is nearly as useless as 1st......

Danglerb · 04-10-2008, 01:18 PM

What drives me nuts is that most Mustang guys will say first thing to do is swap in some 3.55 gears, and that really wakes up the car. It does, seat of the pants throttle response is great, feels like you just added a bunch of cubic inches. On the street with limited traction though, you just made the car less "streetable" and no faster, more of a wheel spinning ticket magnet.

With the 928 I think as soon as you make even minor power improvements, like an X pipe, what works best on the street may change. Whats fastest may not also be the most fun.

I like personalty in the 928, where it seems to "respond" to aggressive driving. Its a big positive reinforcement to go fast when you run the RPM a bit closer to redline before shifting and drop into a higher point of the torque curve in the next gear. Your brain notices "something has changed" and the car wants to go faster. I've heard the GT personalty is very much like that.

Danglerb · 04-10-2008, 01:33 PM

Quote:

Originally Posted by 928sg

2nd is nearly as useless as 1st......

What about starting in 2nd?

Casually driving my 83 around town I skip 2nd gear most of the time. 1st, 3rd, then depending on final cruising speed 4th or directly to 5th. When I drive it hard my "peril" is that 2nd is so much fun it makes it very tempting to keep my foot to the floor in 3rd, and that is SO illegal.