Vilhuer

I would change simply because there is no point in running engine anywhere near 3k rpm on highway speeds. Solution is either US '87-88 S4 tranny or keep 1-5 same as in GT and add sixth gear on top.

dprantl

Yes, that's another big reason to swap. And notice my sig... the '86 is not gone yet

Dan
'91 928GT S/C
'86 928S 5-spd w/LSD *sale pending*

Z

No, it's based on injector operation. Put in bigger injectors and less open time will be required to supply the same amount of fuel. The system is set up for stock injector size and stock fuel pressure. It has no idea how much fuel is really going into the engine, and it comes up with the gas mileage based on the assumption of the fuel flow with the stock configuration and the injector open time or duty cycle that the LH is supplying.

Ispeed

One thing I like about my car is the relaxing cruising at 80-90mph. A quick blast of 4th gear is great, but the ability to just relax at speed in 5th is cool on a long highway trip.
I would love a 2.73 rear end....with a 6th gear like my 5th.

I'm not in the 400+ club...

mark kibort

correct, however, the atomization might be better with air and fuel with the higher pressure, same flow injector, as you are right, it has to do with manifold pressure. (ie mass flow) it takes a certain amount of HP to keep the car rolling at 70mph.

the extra rpms can be unneeded friction that will drive down gas mileage as well.

you want gas mileage, follow a truck real close! (gains of near 5-7mph!)

mk

Barry Johnson

Why not the 2.54? I'd rather go that route personally...

mark kibort

The 2.2 is the best of both worlds. it has the same gear spacing as the 2.72 gear boxes (gear for gear , except 5th) this means, you have your chug along , "6th gear" with the 2.2. and down shift in a gear near you , for the punch you want. After all , it boils down to something real simple.
REDLINE IN EACH GEAR SPEED

2.2

1st 55mph
2nd 80mph
3rd 118mph
4th 155mph
5th 225mph.

pretty nice selection of gears and the spacing, again, is the main tool the gear box used to keep you in the most effective hp range for greatest acceleration over any particular speed range. So, you use a different gear box, like a 2.72 or 2.75, (and the gears themselves are different too, so the change is not as drastic as a 2.2 to 2.75:1 change, but more like effectively, a 2.2 to a 2.5. Anyway, it just shifts your operational range down, and you loose the "overdrive, 6th-gear-like" final gear.

mk

hacker-pschorr

5-speeds - the 2.54 only came in an auto (I have one available.....)

Barry Johnson

Selling a 2.54 gear set? PM me...

John Veninger

2.64 is

Sab

rev limiter is bumped on that one...

display will show about 30% better MPG then actual...

dprantl

There's the 87-88 reference again. Was there some sort of strength upgrade with those transmissions compared to the 85-86 ones?

Dan
'91 928GT S/C
'86 928S 5-spd w/LSD *sale pending*

mark kibort

Thats really a guess.

what are the speeds in each gear? (you should know this to make this kind of a statement)

I think they are somewhere in between the 2.2 and the 2.72 gear box, but you still get left with too short of a 5th gear. (both worlds being racing and highway driving, in my book)

mk

Hilton

I was waiting for this to make an appearance 2.64 is what I have (11:29 ring/pinion in G28/12 gearbox)

5th is comfortable on the freeway, and 3rd is an easy drop into the power band for overtaking

Hilton

Here's some more concrete info.. courtesy of FlyingDog's handy excel sheet..

the excel sheet is found here:
https://rennlist.com/forums/showpost...0&postcount=57

Edit: The tyre size is based on my 17" aftermarkets.. YMMV for 18" or whatever you racers use

Disclaimer: I'm not in the 400 rwhp club yet.. still working on getting engine up to stock performance on dyno before I consider mods.