Rennshift install / 901 spring pressure, Are the springs pressing on bits in the tranny? Options

[smdubovsky]

I installed the Rennshift last night. Nice piece of kit to be sure. It spring loads the 4/5 plane just like the factory shifter spring loads the R/1 plane. My question is, what are the springs pushing against in the tranny?

The stock 915 (in a 911) uses a little tab on the 5/R side to keep the springs from pressing the shift linkage against stuff in the tranny. Here is a pic from Seine systems (he makes a 915 kit that also springs load 1/2 and adds the tabs to both sides).



You can see the little tabs and the hook on the shift lever that relieves the spring pressure when your anywhere other than the center plane (3/4 in a 915, 2/3 on our 901s). The reasoning is that you can spend alot of time in 5th gear in a 911 and its putting pressure on the linkage & couplings. The stock 914's stock shifter doesnt have it, but you'd never spend alot of time in 1/R either.

What is the spring loading in the 4/5 plane doing to the tranny? Are the 901 shift fork internals that much different from a 915 (doesn't look like it in the pics I've seen)?

Just looking to keep the tranny healthy,
SMD

[JWest]

What are you thinking is going to fail from this pressure?

I don't really see those other devices as "relieving" the pressure, they are simple taking it up in a different spot, and in the case of the factory 915 shifter it is a poorly supported, imprecisely produced, non-lubricated device that can easily be out of sync with the transmission location due to the linkage and the trans/engine mounts.

To be specific, the 901 rides against the neighboring shift rail. This is very much up to the job, covered with gear oil, and is out of plane with any other motion (i.e. it won't cause any other parts in the trans to be loaded). You could put another part in the trans to do the same thing, but you don't gain anything over resting it against the already well supported rail except more parts to potentially wear and have to align.

I built my first sprung 901 shifter in the mid-80's, and I have never seen a wear problem from their use. If it was to wear after many, many, many miles, I'd rather replace the one shift rail with a good used part rather than replacing a custom unit that may no longer exist 20 or more years later (my projection as to how long it would take to see any difference).

There will be more load on any plastic bushings, but I would say the shifting loading will push them beyond service before the side pressure loading.