545svk

Did all 928s4 come with the same final drive ratio in the diff?
I ask as my '87 auto is very long legged for the Australian speed limits, and changing the final drive ratio might be a "easy" way to have more fun at low speeds.
Any thoughts and suggestions will be appreciated.

dr bob

In the US, the S4 autobox changed in MY 1989 from 2.20 to 2.54 final drive ratio, and also had internal ratios shortened in lower gears to improve launch performance a little more.

545svk

Thanks Dr Bob - any idea what revs the later cars pull at 60 mph?

VaporiZe

My owners manual says autos did 60mph at about 2400rpm in 4th, 3400rpm in 3rd, 5500rpm in 2nd with 245/45 R16 tires (Eyeballing the values off a graph)

Schocki

Here is a GTS with Rogerbox, converted to a S4 2.2 final drive.

From 2009: At 2000 RPM and exactly 64 MPH the instantaneous fuel consumption displayed is 9.7 l/100 or 24.24 MPG.

AFAIK 4th (or 3rd) gear in Rogerboxes is always 1:1 in any 928, just like 5th in the "do it yourself" version.

dr bob

Calculator math says 2,018 engine revs/mile with 2.54 final drive and 25.4" tire diameter, no converter slip allowance. This corresponds with my admittedly-feeble memory of about 2200 revs @ 65 MPH indicated cruise in my '89 automatic car. GPS speedo shows 69 MPH when dash indicators show 65 due to slight tire size increase with 255/35-17 rears, but that's not really relevant to this discussion.

Same calculator says 1,748 engine revs/mile with 2.20 final drive, no converter slip allowance. Shocki's reported 2000 RPM @ 64 MPH shows about 8% (135 RPM) converter loss/slip/conversion, making the huge assumption that he's running a 24.5" diameter tire in that "test" condition.




I have a good spare '88 gearbox with 2.20 R&P, not LSD though, that I was thinking of putting in the car to extend its touring legs a little. Other option is to put the 2.20 R&P into the existing '89 box, and not lose quite as much launch in gear 1 (shorter 3.87 v 3.676), then gear 2 (slightly longer 2.25 v 2.41), gear 3 the same (1.44), gear 4 the same (1.00).

Bigfoot928

A guy in canada years ago did a 3.09 custom gear set in a 928.

Schocki

My test back then was just done to see the difference between the two different final drive ratios. As soon as the weather is better again, I will check out the real speed with GPS this time and at exactly 96 km/h.
Car was and is still running on standard tires front and rear with Cup 1 rims. Torque converter efficiency is rated at 98 % officially. Might be another two weeks from now but I'll report back.

Gary Knox

The '90 and '91 GT models for the US market had a 2.73 differential ratio. If I recall correctly, all the US GTS's had a 2.54, same as the '89 GT..I've had 2.20 and 2.54, and ridden in several with the 2.73.

I like the 2.54 best for all around driving in the USA.. However, if I were driving in Germany with no speed limits on the Autobahn, I might like the 2.20 better. A good bit lower engine rpm's at 120-140 mph.

Gary

Mongo

This needs to happen again.

PorKen

Consider a "shift kit" for the transmission. A firmer 2-3 shift raises the shift point which helps the overly large 2-3 ratio change.

Also try +3 degrees cam advance over the stock settings. This will noticeably help low rpm torque. (And is hard to discern the hp trade at max rpm in an auto.)

GregBBRD

There's three different sets of shims to control the pinion depth and the end play in the transmission.
And because the Workshop manual regarding this procedure is like reading ancient Sand Script, this is not a simple task.

hacker-pschorr

Plans for my 87 include a brand new 89 automatic with LSD and a modified torque converter with a slightly higher stall speed.

dr bob

Andy -- Drive a later ('89+) S4 with 2.54 and decide then if you really want to invest in a custom R&P set plus setup. The later S4 R&P set is 15.4% quicker in any given gear at the same RPM. Or it's 15.4% slower (MPH) in any gear at the same RPM with no other changes. I may need to send you to the MK School of Torque and Horsepower Calculations to get the colloquial opinion on this change, but it's easier to just book a ride and decide for yourself. I don't know if or how many of the factory-new '89 boxes Greg is still hoarding. Else, find a used one with LSD and have it refreshed to slide into your car. The change does seriously liven up the throttle response, and makes the car more fun in most "sport" driving situations below about 120 MPH.

Speedtoys

I have a GB rebuilt 2.54:1 Rogerbox if you wanna try it out next time I have the car up that way at Sonoma..