2.73 rear end on a 928 S4 (-'82) Trans


Has anyone ever decided to backdate the S4 transaxle into the short rear end ratio'd 1982 transmission? Ratios went to 2.54 in 1989, but going to a 2.73 would probably improve acceleration noticeably on an S4.

What would be involved doing the installation of an '82 auto transmission on an '87 car? Would the torque tube also need to be changed?

 

Can't you just unbolt the transaxle from the transmission and replace it and leave the transmission alone?

 

Dunno. All I wanna know is if I can get a 2.73 ratio on an S4. My acceleration would be killer....so would gas mileage, but who cares. It's only gas.

 

Queue Kibort in 3... 2... 1...

 

From what I know, AT trans has a pinion gear bolted in the front of the diff. case. So ring and pinions are plug-n-play.

Not the case with the 5speed and the pinion goes all the way thru the tranny. Require substantially more disassembly.

 

Really? SO all I have to do is swap the differential out of my '87 with an '83? Or do I have to just swap the ring and pinion?

 

Can't you hunt down a 2.64 S4 manual gearbox from somewhere? That way you wouldn't get the earlier weaker syncros

 

Sorry guys, got my facts wrong, it's actually a 1982 transmission that has the 2.73. Would the differential still work without losing the 4th gear???

 

Andy--

I'll dip into MK territory a bit. The advantage of the higher numerical ratio is great at any specific RPM in any specific gear. It may affect 0-whatever times some if the gears happen to line up with the stars and the whatever the speed is.

I have the higher gears in my '89, and regularly think about how nice it woulde be to have 2.20 final dive and the lower cruising RPM's. If you don't drive the car hard enough to get close to redline in the gears now, your extra performance from shorter gears is not needed yet. Use what you have under your foot, hold the gears eoither withe the pedal, the gear lever, or a PKS. Just a thought.

 

the torque tubes are different lengths....the offset of the ring and pinion is different. The advantage of lower gears (higher numeric) is for drag racing 0-20 mph plus more gears between 0-120 mph. It allows the engine to get to peak HP quicker and stay close to peak hp.

 

Sterling is selling an 85 tranny with low gearing over in the For Sale Forum. From a Euro I think. I have the Euro R&P in my 85, and it is very snappy. OTOH, cruising at 80 is about 3000 RPMs and I often shift in to fourth, looking for another gear. I think the Euro box like Sterlings may have different cogsets than my box, which would make it a little better on the road I think.

 

Anyone with a GT can tell you what a 2.7X'ish final ratio feels like with a 5L. granted they are five speeds, but top gear cruising would be the same. In my opinion, if you do any highway cruising at all, you'll want to stick with the 2.20, the 2.7's have a reputation of a droning exhaust note at highway speed. Also, the '82 tranny is a 3 speed isn't it? any "gain" on the shorter final ratio would probably be lost with the lost of the additional gear in the 4 speed box. Once you shift to second, I think any advantage of the 2.7 box is lost anyway, if you were to overlay the gear ratio's like Kibort does, I think you would find little net advantage, and with third gear in an S4 good to 145mph, you're really only driving the first three gears anyway and never get to the straight 2.20 ratio.

I think if you swap gears you have to swap the diff. Putting an older LSD diff into a newer transaxle requires use of a spacer. Carl sells these at 928motorsports. I don't know if the open ended differentials require the same spacer, but I would think they would.

 

I think I've also heard the point made that the factory accelaration times didn't differ between the early and later S4's, even with the final ratio change.

 

Oh yeah. My 80 US spec five speed was quicker off the line than my 89 S4 five speed, which dynoed 289 at the wheels. My 85 will easily spin the tires in second, and I don't speed shift. There is something to be said for gearing, but as Kibort has been known to say, once you get up to speed, it is all relative.

 

Would'nt that be HP orientated rather than ratio
Dont the S4's have 30 -40 HP more