Porschephanatic

I, well, really my wrench and I, just installed a rebuilt 915 w/ 7:31 r/p, new coupler and bushings, and factory short-shift kit into my '81 SC targa . I love the new-found acceleration! <img border="0" title="" alt="[Eek!]" src="eek.gif" /> Thank you Jeff Curtis! It really does feel like a 50HP increase. The wrench in my estimation has installed 30+ 915s since I've known him, so he has experience. Could something internally be forgotten or backwards to cause this? I double checked the part# on the short shift kit. This one has us stumped! <img border="0" alt="[crying]" title="" src="graemlins/crying.gif" /> Any ideas?

ironspider

out of curiosity, not being a transmission or gearing expert... what exactly did u do with your transmission? changed the ratios? i will need to rebuild mine at soem point....

Bill Gregory

The first thing I would look at is the shift adjustment, which is done at the shift rod/transmission junction under the panel on the rear floor. The procedure is well documented in both Bentley and the Factory manual. It may just need a tweak to make everything OK.

JackOlsen

I would guess that he got the 'extra 50 hp' from switching from an 8:31 ring and pinion, to a 74-and-earlier 7:31 model 915. It lowers your gearing across the whole range, producing an effective 14% increase in torque. Of course, your top speed is lower, and your cruising RPMs on the freeway might be a little more annoying.

Transmissions kept getting taller gearing for mileage and noise restrictions, as well as the pursuit of higher top speeds. None of those things serves the enthusiast well. (The earlier 915's are less robust than the later trannies, though.)

Porschephanatic

Ironspider, Jack's got it right. I did have Vertex rebuild my tranny and swap my 8:31 ring and pinion (per Jeff Curtis' recommendation) to a 7:31 r/p and it makes an incredible difference in acceleration. <img border="0" title="" alt="[Eek!]" src="eek.gif" /> We're taking Bill's advice and playing with the adjustment. If that doesn't work, it won't take long to reinstall my old shifter(compared to dropping the tranny again). At least everything is clean underneath now. The new short shift is a factory kit, but we just don't know if there might be a slight defect in it. Thanks for the responses! I'll let everyone know what the problem was when we figure it out. <img border="0" alt="[byebye]" title="" src="graemlins/wave.gif" />

Bill Gregory

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"><strong>The new short shift is a factory kit, but we just don't know if there might be a slight defect in it. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I would be very surprised if there is a defect in the factory kit that would cause it to jump out of 5th gear. Assuming, of course, it's installed correctly, it's basically just springs that apply pressure to the shift lever that is itself modifed to provide different leverage (hence the shorter shifts) - the "gating" is controlled by the transmission itself, and the shift adjustment described above.

Porschephanatic

You're right, Bill. It is very unlikely there is any defect in the factory kit. I was just hoping it was something simple that we overlooked. We walked away from it for a while, as the weather here is nasty, anyway. Hopr to get back to it this week!