Question for Experts on 915 Gear Sets

[porschedude996]

I am about to overhaul my 1983 Transmission and building my parts List. My desire is to fix both a whiney 5th gear, as well as the others. I would also like to get a taller 5th gear and my choice at this time is a 32:20 or (.62) in the 5th position. I am looking at different sources and have found the gear sets, but I am confused about some of the ads and titles. One particular gentleman’s Auction Title states “Porsche 911 915 Transmission 2nd /3th /4th Gear set, 20:32, 1.600”, which the reciprocal is (.62). What is confusing is the “2nd /3th /4th” statement in the auction title.

To my question; are the gear sets interchangeable in gear case positions? If I wanted, could I build the transmission backwards or with different ratios changes the are not progressive? Not that I would do something crazy, I’m trying to understand if the shaft splines are the same on all or most of the gear sets.

I contacted the seller and couldn’t get a response.

Ebay auction:

https://www.ebay.com/itm/255418305084?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=ySxZwKM5Sai&sssrc=2349624&ssuid=zn12q52GSG2&var=&widget_ver=artemis&media=COPY

[gled49]

In a 915, only 3rd & 4th gears are interchangeable to a limit. The gear shown is a 20:32 2nd gear, very high and usually found in 4 speeds. The advertiser I guess is selling gears in those categories, but in 901’s or 915’s not all gears are interchangeable.

[GH85Carrera]

I went to a 915 TL gear or the .724 (29:21) gear. I think it is perfect for highway cruising in the 75 MPH speed limit days. If I get down to 60, I pretty much have to downshift to 4th. 55 is the minimum speed for 5th on level ground and I usually just downshift.

I love the TL gear after many long road trips. It is an older gear, and does not have the machining to keep it as quiet as a factory 5th gear, so there is a little bit more whine. At 75 or 80 (our legal speed limits) the wind noise and engine noise is such that there is no noticeable gear noise. The only time I hear it at all is downhill at 60 MPH with engine braking.

[porschedude996]

Thank you sirs.

[Matt Monson]

I make new TL 5th gears for 915. They run $995 and do have a little whine.

powerhaus used to make some 0.65 and 0.62 5th gears for the v8 crowd, but they require a bit of machining of the tail housing to clearance them, or they will hit. I haven't seen one of those gears in half a dozen years or more.

Regards,

Matt Monson
Guard Transmission LLC

ps. that 1.600 2nd gear is for v8 or some guys use them at Daytona.

[kevingross]

The beauty of the 901 and 915 is that there are (or were) so many gearing options available. This created good options for racers who wanted to match their transmission to the particular race track and its corners.

When I get a request for different gearing, I do a gear chart for the customer and lay out the progression, shooting for a nice smooth one especially if it's a street car, taking into consideration for example a target highway cruising speed. There are a few different analytic frameworks for street car gearing with names like geometric progression and increasing root. Most other transmission people I mention these methods to think I am crazy, and they have a point maybe. But there is value to doing a little math in laying out your transmission.

BTW, that gear in the eBay listing appears to have some wear / abuse to its dog ring. With 915 gears, keep in mind that they do wear out with use and age. Make sure anything you purchase can be returned. And look carefully at the condition of the gear teeth, especially at the tooth root which is the area of greatest stress.

Cheers,

[GH85Carrera]

I still have my OEM 85 911 5th gear for sale. I will never go back to it. It looks great and was 100% fine when removed to put in the TL 5th.

Evidently there is not much market for it, so it will just stay on the shelf wrapped in an oily rag.

[porschedude996]

Still working on my parts list for my 915 overhaul. I find most everything here on our host's site. I did find a bundle of parts from someone else and it contained "Dog Teeth Gears". I have searched the P-Car parts manual and it does not show the part numbers. So are they part of another piece or an assembly?

[3literpwr]

I opted for a 26:26 5th and the short 2nd through 4th to match. Different direction, but sure is nice.

[Matt Monson]

Originally Posted by porschedude996

Still working on my parts list for my 915 overhaul. I find most everything here on our host's site. I did find a bundle of parts from someone else and it contained "Dog Teeth Gears". I have searched the P-Car parts manual and it does not show the part numbers. So are they part of another piece or an assembly?

Those are the replacement synchro gear teeth. Originally Porsche didn’t offer them, and you won’t find them in PET, though you can find them in Pelican’s directory of parts for sale.

[GH85Carrera]

Originally Posted by 3literpwr

I opted for a 26:26 5th and the short 2nd through 4th to match. Different direction, but sure is nice.

I am sure it runs through the gears quickly!

I drive on the highway for 900 mile per day trips every summer. The real beauty of a 915 is all the choices of gears sets!

We have sections of highway here that the legal limit is 80! Easy cruising speed. And as a guy that lived through the bad old days of 55 MPH limit, I really relish and enjoy driving past a cop running radar as I go by at 80. A speed that would have meant handcuffs and jail back in the bad old days.

[porschedude996]

Originally Posted by Matt Monson

Those are the replacement synchro gear teeth. Originally Porsche didn’t offer them, and you won’t find them in PET, though you can find them in Pelican’s directory of parts for sale.

Thanks Matt for your continued help.


Keith

[porschedude996]

I’ve finally got my parts ordered and three things i would like to pass on what I found.

1. I found that most every part I needed was found here at our hosts site. Plus I applied my PCA discount.

2. Found needle bearings on Amazon 1/5th the price. Most sell for $60, but one seller bundled 5 bearings for the cost of one. I’ll definitely inspect the quality.

3. I found a well known vendor who bundled 5 Synchro Rings, 5 Dog Teeth, 2 Sliders, and a gasket/seal kit.

[Old H2S]

Just got my TL 5th gear yesterday from Matt...Out today to New England Gear Polishing for a REM job...Cheap. Thanks Matt.

[porschedude996]

Originally Posted by Old H2S

...Out today to New England Gear Polishing for a REM job...Cheap.

So if you would, tell me about your expectations or experiences on this gear repolishing. Post before and after pictures if you can.

[Matt Monson]

Originally Posted by Old H2S

Just got my TL 5th gear yesterday from Matt...Out today to New England Gear Polishing for a REM job...Cheap. Thanks Matt.

Don’t do it. You’ll ruin it. It will wear out in less than half the life.

[Old H2S]

CNC God caught me in time..

[Old H2S]

REM finishing is a polishing process for metal, that is very accurate compared to impact blasting, micro blasting, mass finishing, wheel polishing. It is reducing the Ra peaks rather than etching the whole surface which includes the valleys also..Rb. The media used is walnut shells as the rubbing away of the smut that is left from the chemicals added to the bath producing a smoother surface Tham polishing alone. The smoother surface has less friction which lowers the heat produced which make the lubricant work better to reduce wear so parts last longer... a circle of improvement system rather than run to failure. It is used to get maximum improvement in DLC coatings that work best the better the surface finish is. Comp cams is using it now but I have only seen Chevy LS cams done so far so us slugs must pick and choose how and what to get finished and coated..

[PeteKz]

Doesn't "break in" accomplish the same thing?

[Old H2S]

Look at any gear set with magnification and the surface will show waves up close. It is easiest to see on the ring and pinion because they are much bigger wear surface and you can see where the noise is coming from, same thing on roller bearings but in a much smaller area.
NEGP has polished bearings in stock which would be fun to try out?