911SC Transmission Build

[nparnell]

The synchronizers have been going out in my 1981 911SC for sometime now. I have officially passed the point of shifting around the 5 speed manual transmission problem. I do some street driving and an occasional track day, however, my primary use for the car is autocross. I am looking for set up advise. Would a taller first gear improve autocross performance? While the transmission is out I would like to replace the clutch and I am exploring the idea of a short shift kit. All advise and setup tips for performance improvement is greatly appreciated.

[Dmitry at Pelican Parts]

Welcome to the forum! If you're planning on replacing your clutch, check out the tech article below from our website. I've also attached a general link to our tech section that I hope you find useful in troubleshooting and addressing any issues.

Clutch Replacement - Porsche 911 Clutch Replacement | 911 (1965-89) - 930 Turbo (1975-89) | Pelican Parts DIY Maintenance Article

Porsche 911 Tech Articles - Porsche 911 (1965-1989) Technical Articles - Pelican Parts


-Dmitry

[Walt Fricke]

NP
What 915 transmission problem? With the simple addition of a Seine shift addition to spring load the lever in the 3/4 position in neutral, you can avoid missing shifts or having trouble finding gears. This should be backed up with a tight round coupler in the rear by replacing the stock oval bushings with round ones.

You will find a tall 1st gear to be awfully expensive. Typical autocross courses are 1st and 2d gear. But you start in 1st, and a tall gear is going to cost you coming out of the start box. If you get to the point where gearing is what holds you back, I suggest maybe considering a shorter 2d and 3d, or swapping in a 7/31 ring and pinion with the stock gears.

But you will get more (and perhaps better) advice if you post this on the racing and autocross forum here. This is where the guys who rebuild (or at least try to work on) their own air cooled engines reside. You don't need to know how to rebuild an engine to be a super autocrosser.

[Walt Fricke]

Oh - the short shifter I like is the factory short shift. It shortens the fore and aft throw, but keeps the side to side distance the same. This is important in creating a larger distance for your muscles to memorize. The cheap ones (maybe some of the expensive ones also?) neglect this aspect.

[Trackrash]

Walt nailed it.

I am using a factory short shifter with a Seine gate addition. I also have solid bushings in my coupler. This is the way to go IMO. No play or vagueness in my shifter.

A 7:31 R&P may not be the best for an SC. Do you ever hit your rev limiter in 2nd gear?

I have a 7:31 R&P, but my motor will hit 7200 rpm, so my gearing works for my motor.

Some guys run 15 inch wheels for AX, it gives lower gearing and lower CG.

Depending on the tracks you run, close ratio 2nd and 3rd gears may make sense.

[arbita1]

Gordon beat me to it. Drop to 15" wheels with 205/50 & 225/45 tires. Leave gearing as is. Do a stock rebuild. Get into 2nd gear asap and leave it there while on the course.

[Walt Fricke]

How about low aspect ratio 245s in the rear on 9s, and 225s - or 245s - in the front on 8s. Hoosiers. 15" rims.

You can shoehorn in 275s on the rear, but have to grind one of the alignment nuts and its bolt, and shim out some, and do extra rolling, and will still get some light rubbing. This may be too much tire for autocross - there is a suspicion that it is too much for the track, because if you have too much tire it won't heat up to its optimum range, or will take a long time to do so. Autocross doesn't really give you that time, does it?

[Trackrash]

Originally Posted by Walt Fricke

How about low aspect ratio 245s in the rear on 9s, and 225s - or 245s - in the front on 8s. Hoosiers. 15" rims.

You can shoehorn in 275s on the rear, but have to grind one of the alignment nuts and its bolt, and shim out some, and do extra rolling, and will still get some light rubbing. This may be too much tire for autocross - there is a suspicion that it is too much for the track, because if you have too much tire it won't heat up to its optimum range, or will take a long time to do so. Autocross doesn't really give you that time, does it?

Depends. I was having trouble getting RA-1s up to temp on an AX course. Went to Khumo 710s, problem solved.
I just got a set of BFG R1-Ss. One AX with them and so far they are great. (710s are NLA)
AX tire question

[G450X]

915's are expensive to rebuild, make sure you budget appropriately. I just picked mine up from a shop and it was near $5k with tax. I supplied quite a few parts and heavily modified it which increased the cost. Mods: one piece bearing retainer, reinforced side cover installed, Wevo internal gate shift, short 2nd through 5th gears (15:30, 21:31, 24:27, 28:24), and a Wavetrac LSD/TBD. I dropped the 915 off, no removal or installation costs were involved.

I had sourced a really nice 7:31 R&P, but I decided to re-gear to keep the stronger 8:31 based on my research. I picked up parts over the course of a few years to "lessen the pain," but it still hurt to pay the bill! I opted to rebuild and modify a spare 915 that I had sourced (late '77 lightweight mag box with electric speedo), I will keep the original '82 box stock.

At a minimum in a build I would add the one piece bearing retainer and reinforced side cover. If you just shorten 2nd and 3rd gears I don't think you will be happy on the street. I almost went with a short 2nd, 3rd, and 4th and kept 5th stock, but I decided I did not want too steep of an rpm drop in the 4th to 5th shift. Be prepared for some gear whine over stock if you re-gear with non-stock gears (I used GT and Albins gear sets). IMO forget the 1st gear swap - it's low enough and very expensive to swap compared to the other gear sets.

Good luck with the decision making, it's only money - it's very easy to spend another person's coin!

[Porsche84]

nparnell

I have a bunch of 915 parts, many used in excellent condition. I have quite a few good gear sets too.
If you change first gear you'll need to replace the input shaft and matching gear, and it is only available aftermarket, probably above $1500. Let me know if you want to "freshen up" with some good used parts. I also prefer the Porsche "short" shifter.