7:31 5th gear conundrum

rtu0894 · 01-21-2018, 12:35 PM

Fellow Carrera constituents... my 86' 915 gearbox finally decided enough is enough. 1st and 3rd synchros have been struggling for a year or so and recently completely let go. Engine off at a standstill, first will not engage whatsoever. Either broken ring, dogteeth or who knows.. nevertheless, time for a rebuild. Question is: it is stock 1-5 gearing with a 7:31 R&P. Generally happy with this setup except for the highway cruise portion. I was going to swap back to 8:31, which I don't have any longer. I was pondering the idea of just replacing 5th gear. Does anyone know what gear set for 5th with 7:31R&P would yield the same overall ratio of the stock 5th with 8:31R&P? Lastly, if there is such a scenario, would the 4-5 shift be too much?

uwanna · 01-21-2018, 01:24 PM

Quote:

Originally Posted by rtu0894

Fellow Carrera constituents... my 86' 915 gearbox finally decided enough is enough. 1st and 3rd synchros have been struggling for a year or so and recently completely let go. Engine off at a standstill, first will not engage whatsoever. Either broken ring, dogteeth or who knows.. nevertheless, time for a rebuild. Question is: it is stock 1-5 gearing with a 7:31 R&P. Generally happy with this setup except for the highway cruise portion. I was going to swap back to 8:31, which I don't have any longer. I was pondering the idea of just replacing 5th gear. Does anyone know what gear set for 5th with 7:31R&P would yield the same overall ratio of the stock 5th with 8:31R&P? Lastly, if there is such a scenario, would the 4-5 shift be too much?

A TL 29/21 .72 5th gearset would certainly be a help. They were available in some early 70s 915s and also the '76 912 trans. Sets appear on the for sale forum occasionally, as a matter of fact, a set just sold on the board and it lingered for a couple of weeks before finding a buyer. I have that set for my 915 w/8:31 ring gear behind a 3.6 and it's perfect for cruising.

kevingross · 01-21-2018, 02:56 PM

Quote:

Originally Posted by uwanna

A TL 29/21 .72 5th gearset would certainly be a help.

Agree that it would get you to roughly the same redline speed in fifth with the 7:31. The concern would be that with 1st through 4th shortened, 5th gear would feel very tall with the 0.72 gearset. Throwing out the boat anchor. Good luck!

rtu0894 · 01-21-2018, 03:26 PM

Yeah, thanks. Agreed on the 4-5 shift. May be too big of a stretch. I have some time to think about. Everything breaks at once. This happened to my other toy today.... an easier fix so that's first on the list. always something....

Matt Monson · 01-21-2018, 03:41 PM

Top of 4th gear with a QQ against a 7:31 is between 105 and 110mph depending on tire and redline. Where on US roads are you going to attempt to keep pulling once you shift into 5th? The only place that matters is in a track/race car. It's irrelevant to 90% of the 911 owning population. You're still around 4500rpm on that shift anyway. If you've got a bigger engine, you can still pull it, but very few of us actually need to.

rtu0894 · 01-21-2018, 03:58 PM

Nothing crazy intended here. I just never liked the rpm's while cruising in 5th on I95 in FL. Close to 4K flowing with traffic. I want to make a few long road trips but this weighs on my conscious

john walker's workshop · 01-21-2018, 03:59 PM

Perxactly! 0.72 912E 5th.

uwanna · 01-21-2018, 04:04 PM

I just ran an Excel gear chart using your '86 gears with a 7:31 ring and the TL 29/21 .72 5th gear. It shows 70MPH at 3000RPM in 5th which is precisely what your trans would do with a standard 5th gearset with 8:31 ring gear. I think it would work out just fine. Just my $.02

rtu0894 · 01-21-2018, 04:16 PM

Quote:

Originally Posted by uwanna

I just ran an Excel gear chart using your '86 gears with a 7:31 ring and the TL 29/21 .72 5th gear. It shows 70MPH at 3000RPM in 5th which is precisely what your trans would do with a standard 5th gearset with 8:31 ring gear. I think it would work out just fine. Just my $.02

Thank you for that, that's what I was after

Matt Monson · 01-21-2018, 04:24 PM

There’s no downside. Now the challenge is finding a good used one that’s not so shagged it’s gonna howl. I make them, but my new gears make noise. Has to do with the tooth profiles and we sell more of them to FIA racers than highway cruisers.

rtu0894 · 01-21-2018, 04:29 PM

This may not be an issue. I have the blue pillow wevo mounts on engine and tranny, so it basically wines all the time anyway. Between that and exhaust, it's already difficult to have a normal conversation inside..

Matt Monson · 01-21-2018, 04:46 PM

Ping Tom Oxner. He's got a pile of stuff he's been sorting through. He might have a TL. Even a TM would be better than now.

rtu0894 · 01-21-2018, 06:02 PM

How are these gears indexed? I thought a-1 b-2 c-3 but doesn't jive. Same principle, just different starting point??

Matt Monson · 01-21-2018, 06:06 PM

Add 10.

rtu0894 · 01-21-2018, 06:13 PM

I see said the blind carpenter as he picked up his hammer and saw...

Matt Monson · 01-21-2018, 06:58 PM

Guy who built and maintains this car is blind.
https://m.youtube.com/watch?v=LnvzCUIgXTQ

rtu0894 · 01-21-2018, 08:13 PM

I want that!

And 2 hours later... I'm back from YouTube....

Bill Verburg · Bill Verburg's Garage

Here's a comparison of stock to mod w/ 21/29 5th

rtu0894 · 01-22-2018, 09:09 AM

So.. if I'm reading that correctly, an 8mph difference in 5th?
And thanks, nice spreadsheet

uwanna · 10:05 AM

Quote:

Originally Posted by rtu0894

So.. if I'm reading that correctly, an 8mph difference in 5th?
And thanks, nice spreadsheet

rtu0894,
If you have Excel, you can download the Excel spreadsheet ap from
Gbox and add/change variables for tires, gears, etc. It's a fun ap to play with.
GBOX Transmission-Gearchart
Best,
Grant