915 Gearbox conversion from 4 sp to 5 sp
 

Anyone out there ever convert a 4 speed 915 transmission to a 5 speed. Is it more trouble than it's worth..???

I converted my car from a 4-speed 901 to a 5-speed 901, then to a 5-speed 915, but that's probably not what you want to know.

I think it's a change in the main shaft and added parts (5th gear) in the nose section of the gearbox, but that's from faint memory. Others will have better info.

Sherwood

Assuming you could "convert" your existing 4-speed at all, your present gear ratios would probably be all wrong for the first 4 speeds of a 5-speed. You may be better off selling your 4-speed and picking up a used 5-speed.

I'm also interested in doing this converson. Any suggestions???

subscribed

id be interested to hear how difficult the swap would be too

After doing some web surfing, I found this info: www.911t.org/wklyreprint/type_915_transmission

A 915 4-speed is the same as a 5-speed, except that a spacer on the shaft is substituted for fifth gear, and the associated shift fork, linkage, etc, is omitted.

I believe the conversion is possible. I did read somewhere that, first gear for both 4 and 5 speed trannys are the same, also the final gear (4th or 5th) are the same.

Sorry, try this...

http://www.911t.org/wklyreprint/type_915_transmission.htm

A quick look at my '74-77 Factory parts book indicates that the main shaft and ring and pinion shaft have the same part #s, which suggests there should be no really large obstacles here. Yel911's researches confirm this.

For small obstacles, I'd suggest you look at a PET (I think you can look at those from kind folks who have put them on line) and do a closer look. Leaving gearing aside, it may amount to little more than purchasing the 5th gear parts.

But that's leaving out a lot. First, of course, is the same as between the 4 and the 5 speeds (has to be, since the mainshafts are the same). Second is close if you have a 72-3 4 speed, but gets pretty tall for a 5 speed with the later ones. And 3d and 4th get really tall, so at a minimum you'd be looking at swapping out those two gears if you wanted a balanced transmission. If you just wanted a taller top gear, you could plop just a 5th in there at relatively little cost compared with the alternatives. But on the '75 and later 4 speeds, their 4th was as tall as the 5th on the 5 speed transmissions. You don't say what year your 4 speed is, so no use guessing about details (different gears for different years, and 7/31 vs 8/31 R&PS).

So you'd want to compare the costs: sell your 4 speed and buy a used 5 speed versus the parts cost of the conversion. Of course, by converting and changing gears you'd have the box apart, and could replace (at some cost, to be sure) the wear items you find inside, so you'd end up with a rebuilt box rather than a used box of unknown internal condition.

Walt Fricke

My transmission is a 74 915 4-speed. It has the 7:31 R/P.
Thanks for you input!!!