1986 944 Turbo gearbox

[Mike Cornelius]

It's time to start thinking about replacing the gearbox on my '86 951. Any ideas on possible upgrades, what to look for or think about?

I currently run in JP and would like to stay in that class.

[Steve Grosekemper]

The trick set-up is a 944S gearbox w/ LSD.
Stronger final drive and 14% shorter R&P (Or close)

I have a 944S gearbox w/ open diff... just cause you never know when you'll want one.

But that's 6-points.

[Mike Cornelius]

I think that I am only at 14 pts. right now, so that might work!

[PShipman]

The trick set-up is a 944S gearbox w/ LSD.
Stronger final drive and 14% shorter R&P.

Steve,
Couple of questions on this:

1) How close ratio is the gearset compared to the 951 or a S2?

2) Are all 944 boxes (non-951 and pre-S2) shorting in the drive ratio?

3) Would you want the 944S box in a 951 IF it were going to be used as a TT car (since there is no oil cooler?

An inquiring mind wants to know

[Jad]

I am not as knowledgeable as Steve, but ONLY the 944S has the 'good' ratio. I put it in my turbo S and it was much better. You do need to shift a lot more, but you are always in the power band.

The actual ratios are the same, just the final drive is lower I believe.

The short box limits top speed to ~140mph depending on tire size which is more than enough for most cars and just perfect for my turbo S at Fontana, 6000 rpm in 5th was the maximum reached on my best run.

[Steve Grosekemper]

The trick set-up is a 944S gearbox w/ LSD.
Stronger final drive and 14% shorter R&P.

Steve,
Couple of questions on this:

1) How close ratio is the gearset compared to the 951 or a S2?

2) Are all 944 boxes (non-951 and pre-S2) shorting in the drive ratio?

3) Would you want the 944S box in a 951 IF it were going to be used as a TT car (since there is no oil cooler?

An inquiring mind wants to know

Jad has it right...
All 944's except very early are very near the same ratios.
The 944S uses the same gear ratio with a lower final drive only.
It will only fit into the 944S box as it is physically larger.
Oil temps are lower than the 951 box as the R&P is larger and has less stress. (all the heat comes from the R&P)
We have run gear box temp test at our local tracks and they are well below the safe operating range.

[PShipman]

I am not as knowledgeable as Steve, but ONLY the 944S has the 'good' ratio. I put it in my turbo S and it was much better. You do need to shift a lot more, but you are always in the power band.

The actual ratios are the same, just the final drive is lower I believe.

The short box limits top speed to ~140mph depending on tire size which is more than enough for most cars and just perfect for my turbo S at Fontana, 6000 rpm in 5th was the maximum reached on my best run.

So Mike (or anyone else ) could rebuild a 951 or ??? trany and use the R&P from the S to get a little more acceleration from their box??? That would leave him with the oil cooler if he rebuilt a 951 box, eh?

[Jad]

Don't think so as the larger R&P take up the room that the cooler pump normally goes in, or something like that. I added an oil temp gauge for the tranny to my turbo after removing the cooler and at TT and AX, the gauge hardly moved enough to read, let alone get hot. The only time it was even a little warm was during a club race at Fontana, which meant 30+ minutes of 100% driving on the worst track for tranny heat. So, if you are going to do enduro's, it may be a factor in a turbo S, but a stock S2 or 951 would probably be fine with the lower HP from my experience.

[PShipman]

Cool ... thanks

[pdy]

Mike:

The shorter R&P is a definite improvement
ONLY at autocross, where you want to be
really quick off the line. For Time Trials,
it depends on the track. If you find you
are upshifting just before the end of a
straight, shorter R&P will put you in the
higher gear sooner, and overall lap time
MIGHT be a bit less. OTOH, there might be
track situations where now with the shorter
R&P you need to upshift, whereas before you
could have stayed in the lower gear. Here
your lap times MIGHT be a bit worse.

My '78 911SC (Butch) hit the rev limiter in
fifth gear at 142 mph - and that's with the
"tall" tires. With a tallel (stock) R&P,
it tops out at 152 with the "short" tires.
At Fontana it would just be bouncing off
the rev limiter in fifth halfway around
the oval. Depending on your tire sizes,
you may want to make sure you won't run
out of gear at Fontana - and I know you
are signed up for that event

Finally, unless you want to the advantage
at autocross events, IMHO there are much
better ways to spend six points.

[pdy]

Mike:

Quick calculation is that with the 944S R&P
you will be at 140 mph at 6000rpm with 25.4"
tire diameter (265/45-16 - don't remember
what size you run). I suspect you are more
than 140 in turn two, but you can use this
as a basis to figure it out.

[Steve Grosekemper]

Paul,
You think would change your mind after driving the car.
Fontana is only one track and you will make up more time on the infield than you would loose on the oval.
I played gear games for Fontana in the 914 and it was slower with 6000 in 5th than when I ran out of gear just after start finish. This is really the case with big aero. Put a wing on it and you will never lift from the time you shift to 5th until you brake for the infield and you will net lower lap times.

Just what I have found, but a stock aero 951 may be different at Fontana.
At all the other tracks it is the perfect box...

[Jad]

This is one of the very few time I disagree with Paul. The shorter gears helped at all times and all tracks with the turbo as I effectively had more power at all times. The only time it would be a problem is if you run out of top speed. I find it hard to believe a SC could hit 152 mph at Fontana or even 142 with a remotely stock engine as the aero-drag it too much. My cleaner turbo with more power could only hit 135-140 depending on tranny, wings, etc. Of course my numbers were GPS data, speedo's with a common 10% error will read 14 mph high, which puts Pauls speed around 130 MPH which seems more appropriate for lap times and other similar cars. I know I can not out drive him on the infield by enough to make up for a 12+ mph deficit on the entire bank.

So basically I am saying a turbo S never got close to the redline of 6500 with the shorter gears, so I really doubt an S2 cab or stock 951 could and you would be getting there faster anyway, so even if you do top out briefly, your overall oval time would still be better I am quite sure.

[pdy]

Aero and gears are different, but I do
understand what you are saying about
tailoring both so you don't have to lift.
In measuring differences and changes,
you need to be careful about combining
changes and trying to measure the results
of each one.

If the gears are more-or-less evenly spaced,
it will be better to be near redline in a
particular gear at the end of the longest
straight as opposed to having just shifted
into that gear shortly before the end of
the straight. Here the shorter gear (R&P)
helps.

On the infield of Fontana, having a shorter
R&P might help by allowing you to keep in
the power band. There are some corners that
are slow enough that it kinda bogs down, yet
you don't wat to downshift.

But other tracks it can hurt. Spring Mountain
in the two big sweepers, you might find that
with the shorter R&P you need fourth instead
of third, now the car doesn't pull out of the
corner as well as it did in third.

Compare my '78 SC (Butch) with the 7:31 R&P
versus the stock 8:31 from a rolling 50mph
start over a 1000 ft and 1500 ft straighaway;

7:31 R&P

1000ft 8.46 sec @ 102.8mph
1500ft 11.62 sec @ 115.6mph


8:31 R&P

1000ft 8.41 sec @ 103.8mph
1500ft 11.50 sec @ 114.7mph


See how close they are? The 8:31 has a very
slight advantage, but this could easily be
offset by having an awkward rpm/gear range
somewhere else on the track, which is most
likely the case for a number of us.

Of course the 7:31 will get you from zero
to 50mph (and beyond) more than 0.3 second
quicker than the 8:31, which is a huge
advantage for autocross.

In real-world driving Butch on tracks, the
8:31 feels and measures just as good as the
7:31, although I do shift at different points.

Tailoring the gearbox for a particular track
will help a bit, but I can't drive consistently
enough to really take advantage of this. I do
know that bumping off the rev limiter for half
the straightaway is not as fast! And I also do
know that there are a number of tracks where
the taller gearing feels awkward in several
corners, so that is definitely a factor.

And, at Fontana in Snickers (the '79 SC),
I see 6600rpm in 5th with 8:31 and 23.0"
rear tires. That's 142mph, and although
the car is still pulling, it is not pulling
by very stinkin much - it would take a lot
longer stretch to get even a few more mph
out of it. With a 7:31, it would be at
rev limit at 135mph before turn one.

I just feel that for Time Trials, six points
might be better spent elsewhere, but then it's
not my six points.

[Mike Cornelius]

Thanks for all of the input guys. I guess that the bottom line is whether it is worth it to spend the 6 points on the tranny or to find other ways to spend the points that would add up to faster lap times. It's all about the lap times!

I don't have GPS but per my speedo my top speed at Fontana seems to be between 135-140 mph with stock 16x8 phone dials, running 245/45/16 all around. Who know how accurate that is though?

Another question: Would an LSD from the 951 tranny work with the 994s tranny? My thinking is that it would not or at least would not be ideal due to the shorter R&P.

also, anyone have any thoughts for and ideal setup for the 951, assunming starting with a stock car, that would use the allowed 20 points for JP to max performance?