What is the stock:

R&P ratios,
gear ratios,
weight differences including clutch systems.

Of the folowing transmissions:

901 w mag case (69-71)
915 w mag case (73-77)
915 w alum case (78-86)
G50 Carrera (87-89)

Which would be the best for a track car w a stock 3.2 motor weighing near 2200 lbs. I have to stay w a stock transmission.

Thanks in advance.

Keith

 

If I were limited to a stock box, I'd go with the 73 or 74 7:31, for the gearing, and plumb in a spraybar and a cooler to improve longevity as much as possible. The 8:31 boxes are more durable, but you're wasting all of the gearing for speeds over 140.

 

Just to be ornery,....

The 7:31 is too fragile for a 3.2 track car and we always use a 8:31 tranny (Mg or Al case) and install custom 2nd through 5th gear ratios.

Race boxes get a pump & cooler and high powered cars get a spray bar system to cool the R&P and gearsets. Sometimes they get "full-active" gearing where 1st gear is raised and made a part of the overall gearstack.

Remember,...the rear issue with all Porsche tranny's is that the gear ratios are simply too far apart and the engine falls far off its power curves at each shift. Using a lower F&P does nothing to address this issue and one must reduce the RPM drops at each shift to improve acceleration.

A good close-ratio set of gears makes a "50 HP" improvement in the overall performance of these cars.

 

Thanks Steve & Jack.

I would love a custom trans w close gears but the class I want to run requires "any stock 911 trans". No gears can be run.

Might try to back date to a 901 if it is like 50 lbs lighter and has better gears as I hear it shifts better than the 915's/

So my real choice is likely a Carrera 915 v a G-50.

Did find a Euro car w an extrnal oil cooler but does not have a ltd slip.

Don't know I need a ltd slip/ And don.t know if a Qualif can be fited to a euro box becaus the trans lube system is different.

Car will be striped to 2200 lbs, torsion bar race suspention, plex windows, cage, small spoilers, Carrera body, stock 3.2 w headers and stock injection.

Keith

 

Keith:

A few thoughts about what you are proposing to do;

1) The 901 will not hold up at all. If you use it, budget appropriately for a very short life. BTDT.

2) Use a '77 mag-cased 915. Its plenty strong and lighter than the later Al cased ones.

3) You need an LSD. You do NOT want a Quaife for track use.

4) Spend your money on a real good suspension! Thats the biggest bang-for-the-buck and yields the biggest change in lap times.

 

Steve,

Thanks! Good advice as alwas!

What would you ext the weight diff between the mag & alu 915??

Is the Carerra G-50 a lot heavyer? Might it be worth the penalty (better shifting strohger?)

Keith

 

As I understand it, the G50 has a lot of downsides, including weight, the need to alter your torsion bar tubes, or go to coil-overs, to install it, and (importantly) the need to move your engine an inch further back in order to fit it (if you don't go to coil-overs), which makes your weight distribution even worse.

It's a very expensive job, with really tall gearing once you're done.

Then again, it is very durable.

 

Jack,

Thanks for the input.

I was refering to a 87-89 Carrera 3.2 that came stock with a G-50.

Hear the G/50 trans shift better. Have a G-50 based trans in my C2 Turbo and it seems to have better syncros than my 72S 915 did, even after a rebuild.

Do any 915's shift fast/clean???

Keith

 

"I was refering to a 87-89 Carrera 3.2 that came stock with a G-50." As I understand it, they changed to the different torsion tube design in 87. But if you're talking about racing an 87-79 tub, then I'd stick with the G50, probably. If you've already got the car for it, the tall gearing is its only drawback.

The 915 is more of a race box; there are more gear sets available for it, for example. The G50 is more of a consumer product -- it indicated the direction Porsche was moving in. Although don't get me wrong: it is a very durable and reliable transaxle.

Especially if you already have a G50 in the car, then I agree with Steve's advice: see what the rules will allow for suspension modifications. Short of close-ratio gears, a transaxle isn't going to gain you a whole lot of performance. Suspension components and suspension tuning can make a huge difference.

 

Hi Keith:

LOL,...As Jack said,.....

The mag-cased 915 is about 15-20 lbs less than the aluminum cased ones.

The G-50 is about 40 lbs heavier and if you are not permitted to change gear ratios, I would not use one. The stock gears are far too tall.

 

I did the G50 conversion in my car. The Alu. 915 weighed @122 lbs and the G50 weighs @ 144 lbs

 

Keith, Here are the various gear ratios for 84-89 based Carrera. You can take the ratios, plug them into the formula and calculate speed/RPM for the various transmissions in Excel. You might also look into the 964 trans, especially the Cup car transmission which has shorter gears than the standarad G50. In the class rules, can you mix and match stock ratios in a 911 trans between all of the different models available?

915/67, 69, 72
Final Drive 8:31
1 - 3.1818
2 - 1.8333
3 - 1.2608
4 - .9655
5 - .7631

915/68, 70, 73 (My choice between the two 915 trans)
Final Drive 8:31
1 - 3.1818
2 - 1.7777
3 - 1.2608
4 - 1.000
5 - .7895

G50/00
Final Drive 9:31
1 - 3.500
2 - 2.059
3 - 1.409
4 - 1.074
5 - .861

G50/01 (My choice of the three G50 trans)
Final Drive 9:31
1 - 3.500
2 - 2.059
3 - 1.409
4 - 1.125
5 - .889

G50/02
Final Drive 9:31
1 - 3.154
2 - 1.895
3 - 1.333
4 - 1.036
5 - .861

 

Thanks all on the info. Very good & usefull.

Have not chose a tub yet. Starting now looking at the rules and varaibles to get it ready for next year.

Easeyest car to build from rules is a stock Carrera. Has bigest Class legal brakes, no motor mods alowed except free exaust & driled pre airfilter section. Already has Carrera tensioners for safety.

However, also alowed to build a 2.7 w max 9.5 CR & G60 cams any intake/single plut ignition.
OR-
a 3.0 w Euro CIS P&C's, shaved case for more compression (10.2/1??), intake ports, Only SC cams, and any intake & ignition.

Then add a mag case 901 for less weight and lower/closer ratios(Steve, will it hold at 240hp/200 ft lbs on track, you have me concerned?))

Suspention, Cage to stiften car, and the most effecent brakes with in the rules is where most of my effort will go.

However if a car is to fast they will slow it down so it stays competitive.
--

What I have learned so far about transmissions:

901 v 915 mag: 20 lbs,
915 mag v 915 alu, est 15-20 lbs dif
915 to G50, 22 lbs
((thus 60 lb variable.))

US Carrera 915 & G50 have almost the same final ratios
ROW Carrara 915 has external oil cooler and a bit shorter final ratios.

Assunm, G50 shifts best folowed by 901 w 915 last.

Porsche spec book shows a lot of different ratio choices from factory for ratios as delivered from factory as a "stock box".

Rules put the motor at around 200-240 hp at 200 ft lbs torque.

Coments welcome.

Keith

PS How much lighter is a mag 2.7 case than a 3.o alu case???