Factory race set up for 915 gearbox

[stevepatfred]

Purely out of interest, does anyone know how the factory and period race teams set up the 915 gearbox? Short shifter? 20%, 30%? And did they use some sort of gating like the seine mod?

[Geneman]

interesting question stevefred... lets see what the braintrust says

[RSTarga]

All they added was a cooler with a pump driven off the box. Maybe the factory short shifter later. I am referring to the pre74 models.

[Superman]

I'm guessing they use custom gear ratios. Various combination, depending on racing application. First gear in these street cars.....is WAY too low for racing. For racing, first gear is used to get to second. Second is commonly also too low, unless the track has sharp turns.

These are guesses. By someone who actually knows very little.

[stevepatfred]

Originally Posted by Superman

These are guesses. By someone who actually knows very little.

That’s how I get through life!

I read somewhere that one of the theory’s about why they changed from the 901 dogleg layout was so that 5th was easier to change out depending on what ratio was needed for the particular track.

Was it the 2.7RS that introduced the 915 to road cars?

[manbridge 74]

I recall reading the engine mounts became solid because in turns with maximum loading the chassis would twist so much that mis-shifts into the wrong gear or hard shifting were common with rubber mounts.

[Walt Fricke]

Agree - pump and cooler (note all the 915 gear boxes are cast and otherwise set up to have a pump and spray bar, though the casting isn't machined in the regular boxes), and gears. The '70-71 were offered with maybe five different gear sets? Airport, hill climb, etc. But that was a 901 inside. There was a 6/31 R&P available (for the 901 box only?). Don't know what special gears were also available.

I've got, somewhere, a factory catalog of race parts from the late '60s. But that wouldn't cover the 915.

The street turbo showed up at the 1973 Frankfurt auto show with the 930 gear box. I assume that the subsequent turbo race cars - 934 and 935, had that box. It wouldn't have been needed on the 3.0RS, or the IROC cars, would it?

Per a book on the 934/5, there was a short shifter and a tranny oil pump, with a radiator under the rear deck lid opening. 80% lock LSD, and later a spool. Three R&Ps available. Solid motor and tranny mounts. Stock plus 2 alternates per gear allowed, which the author suggests was certainly adequate. But these were the 930 box.

Maybe someone more like a racing historian knows?

[Bill Verburg]

For 1973 the 2.8 RSR used the same 915 transmission as the 2.7RS
Stock gears as delivered were
CWP 7:31 4.429
1 11:35 3.182
2 18:38 1.833
3 23:29 1.261
4 27:25 .926
5 29:21 .724
Teams did use different stacks at their discretion

LSD was 80% ZF, a 1 kg lighter than stock flywheel and a special steel clutch
oil pump and serpentine cooler in the right front fender w/ an internal spray bar
solid engine mounts
Here's an original RSR 915/08




here are some of the stacks used in the 901s