915 gears

John914somers · 03-03-2015, 10:30 PM

I have some literature that says a R:Q, 27:24 gearing can be either a 4th or 5th gear. My understanding is that 4th and 5th are not interchangeable so does that mean that they make a R:Q in both 4th and 5th gears? Thanks for your input!

Matt Monson · 03-04-2015, 05:04 AM

I've never seen a 4th in that ratio but it might have been something for the 4spd gearbox. I've got a 29:21 4th that was designed for the 4spd.

Walt Fricke · 03-04-2015, 02:59 PM

John - are you sure the literature is for the 915? I think 4 and 5 could interchange on the 901 box (and the 911 box?), as they are in the same position there as 3 and 4 are in the 915. A W gear for a 901 is a 27/24 on someone's listing I have, though I don't find it listed for the 901.
T
he '74 5 speed had a 27/25 4th. And I find a 27/24 5th (ratio 0.89) listed as 915.302.904.30 on a chart I once made of all the 915 gears I could find, factory and aftermarket (which, of course, moves on).

A point of obvious confusion with gears is the fact that the tooth count is not always and consistently given as the drive gear followed by the driven gear (or vice versa). In fact, checking with the catalog for the 911 transmission years, it appears that Porsche always gives the tooth count for the loose gear first, and then for the fixed gear, so a guy needs to remember which is which for which gear and transmission type. It also seems that the convention for the ratio is to divide the driven gear teeth by the drive gear teeth in order to get the ratio. Alas, 4th is the only gear I think where the ratio could be either above or below one, so more ambiguity there.

While 3 and 4 on the 915 can be swapped, you can't swap the drive gear for the driven gear - one is splined, and the other is on needle bearings.

Unlike Matt, I don't have this stuff memorized (maybe he doesn't have quite all of it memorized either), but have created or saved quite a bit.

Matt Monson · 03-04-2015, 03:47 PM

Walt,
I've got the memory of a goldfish. If I blink, it is gone. I think I remember something, and then I look it up.

The Carrera RS also had the RP 27:25 gear. Historically it is a gear with high demand though I have never really understood why other than the RS cache. It's taller than a stock 915 4th for most other gearboxes. Performance minded folks usually want to go the other way around as Walt mentions.

Unfortunately, Porsche has not been 100% consistent. When they moved over to the G50 family of gearboxes they flipped how they list ratios. Porsche calls a 1987 G50 3rd a 31:22 1.409. I call that gear a 22:31 by the same decimal count. Leads me to ask my customers about what decimal they want for 4th gear when they try to order by toothcount on any gearbox. Clearly a 24:27 4th will be nothing like a 27:24.

Regards,

Matt