915 Gearbox Operation...Your Experiences!

[CVDH]

Originally Posted by universeman

+1 for baby the transmission and spank the engine

I am running stock 915 in my 82, 305k miles (rebuilt motor/trans at 295k by PO so pretty fresh)

When I bought the car, shifting was sloppy and vague. Rod bushing was totally shot, so I replaced it and the shift cup bushing. Have not replaced coupler bushes yet, due to stripped set screw that I have not wanted to deal with.

Replacing bushings above made massive improvement in shift handle tightness.

And here I thought my situation was unique. What is up with people spending cubic dollars to rebuild the transmission, and then forgetting to change out ~$40 worth of bushings?!

[pmax]

Originally Posted by 911SauCy

Only strange thing I experience, is sometimes I go to engage reverse (car idling still) push in the clutch and i'll get a little toothy clunk noise. Not like a grind, but like one of the gears was spinning slowly when i engaged it.

Anyone else on that?

Yes. That's the 915 reverse gear crunch. The 915 reverse gear is not synchronized.

The manual says to pause for a few seconds before engaging reverse.

[TheSt|G]

Originally Posted by pmax

Yes. That's the 915 reverse gear crunch. The 915 reverse gear is not synchronized.

The manual says to pause for a few seconds before engaging reverse.

Or just go 2->R.

[RSTarga]

If you are using more than 3 fingers to shift, something is wrong. I've got 145k on mine, no rebuilds. Bushings replaced and factory sport shift using Swepco forever.
Always double clutch and match revs on downsifts, it's fun.

[911SauCy]

Originally Posted by word

Mine is the same. I have the WEVO kit installed with the reverse lockout. Sometimes if I try to go straight from N to R it will block me all together. I have to be patient with reverse, similar to a first gear downshift when coming to a stop. Still gives a hearty clunk once it accepts the shift.

Yes. It's always from N to R, as any forward motion into R is a HUGE no-no

[911SauCy]

Originally Posted by pmax

Yes. That's the 915 reverse gear crunch. The 915 reverse gear is not synchronized.

The manual says to pause for a few seconds before engaging reverse.

Confession:

I posted it here as it seemed like an opportune time, but as silly as it may sound, I think it sounds the age of the trans and I wouldn't fix it even if it were broke

Like old race cars whose transmissions whine and crunch gears...I love it!

[Geary]

The OP's 915 is probably typical of most 915s. Dino oil needs to warm up a bit for shifts to become smooth.

The guys at Porsche were pretty good engineers & usually had appropriate designs.

I think someone at Porsche was being VERY stubborn, as one of those engineers HAD to have noted, "Hey, VWs (with Borg Warner synchros) shift great!"

I read somewhere(probably Pelican) that 1st gear was designed only for leaving the pits.

Probably in reference to the 901, with dogleg 1st. At times, they'd even install identical ratios in 4th as in 5th, to ensure finishing a 24-hour race with dinky 901 gears.

[Speed Buggy]

When I got mine, I had to talk nice to it to get into first. I changed the bushings and it got better but still not great. I "freshened" the tran, 1 and 2 dog teeth and sliders and blocks etc. now it goes into first gliding into a stop with no issues. Course it's not running now, but that's a different story.

[Bob Kontak]

Originally Posted by ClickClickBoom

95k, Stomski shift coupler, short shift kit w/ lengthened lever, Seine Shift Gate and a dose of Swepco. It is the nicest, smoothest, most precise shifting transmission I have ever driven. The Rebel Racing Sport trans/engine mounts tighten things up as well.

Just for fun, I am going to ask if short/performance shifters place additional stress on the synchronizers that the long throw original shifter in the 1978 915 does not.

I have heard it does but I have no basis aside from what I have heard/read.

[universeman]

Bob, can't imagine why that would be. Length of travel should have no bearing (no pun) on the wear factor for the synchros. I think people want there to be a "tradeoff" for the benefit they get from short shifting.

PS - the video linked below is pretty good, for being from the 1930s. Simplest explanation of synchros I've ever seen. They have another one regarding the rear differential on Hemmings that is also quite good.

Tech 101 – How manual transmission gears work | Hemmings Daily

[universeman]

Also this: Busting the "shortshifters cause excess synchro wear" myth - The Tuners Group

[PooShee]

Originally Posted by pmax

Yes. That's the 915 reverse gear crunch. The 915 reverse gear is not synchronized.

The manual says to pause for a few seconds before engaging reverse.

My story is no different, a few things that have helped "learning" how to shift a 915...FWIW

I learned from my dad to go into second before engaging first or reserve

From first to second, take the shifter to the middle then left again into second, not just strait back from first to second

PITA but no grinding

[Neel]

Originally Posted by ClickClickBoom

95k, Stomski shift coupler, short shift kit w/ lengthened lever, Seine Shift Gate and a dose of Swepco. It is the nicest, smoothest, most precise shifting transmission I have ever driven. The Rebel Racing Sport trans/engine mounts tighten things up as well. It makes the Japanese transmissions seem like a straw in a milkshake.

Almost the exact same on mine.. Except I will be changing back to Kendall soon.. Gear oil choice is very subjective, just like engine oil.. Lots of threads on both when you do a search.. Some people swear by one and swear at another...

[daleflesburg]

My 915 is from an 86 Carrera, and is installed along with a 3.2L engine in my 74.

Mine shifts fine when warm into and from 1 2, 3 and 4. Sometimes finding 5th when rolling along at 55 or 60 is troublesome, as I cannot seem to find my way over to the 5th/R gate. I have to be careful because quite often I am now in 3rd. We are going to convert back to a stock shifter and away from the aftermarket short shift unit (which my mechanic thinks may be my problem). He has had trouble with it too. If this does not help, we will have to remove it and take it apart.

[Bob Kontak]

Originally Posted by universeman

Also this: Busting the "shortshifters cause excess synchro wear" myth - The Tuners Group

Many thanks.

I have the middle one on my 81. Back when it was just over $100 in the 1800's.

Myth is probably "justifiable" if you slam the short throw shifter as fast as you can vs. the long throw.

I shift like a little girl after the $2,500 nut to rebuild.

[KTL]

I think the 915 operation and bad rap has been inflated by many poor repair approaches and abuse.

By poor repair approaches I mean "rebuilds" that ignore blunted dog teeth, replace only the synchro friction band and reuse of the slider sleeves/rings. I think the sleeve/ring part is arguably the most important to replace because it engages the dog teeth AND the synchro friction band. But yet this part seems to be the one most people look at and assume it looks OK visually and they put it back in.

Another bad repair approach is so many people hope to fix the shifting with external fixes and fluid change. Sure, shifter bushings and coupler bushings are important. So is a fresh gear oil. But often by the time those items need fixing, the internals of the trans have also suffered. Yet most are only willing to look at the external stuff and the fluid and hope that fixes things. Transmissions are really neglected by a lot of people. People will change engine oil all day long and debate what oil to death. But transmission oil is almost an afterthought to a lot of these same people.

Regarding abuse, I think some people believe the 915 is a tainted trans and there's just no chance of it ever shifting good. Reality is a lot of shifting is done really poorly and the trans can only compensate for that so much. A lot of people are notorious for yanking the shifter before the clutch is fully disengaged and the 915 doesn't like that. Over time that bad shifting habit can distort the dog ring by trying to pull its press fit off the gear.

Another abusive behavior that a lot of people are completely unaware of is matching RPM on downshifts. Not only does it wear out your clutch when you don't match RPM, but you also put more wear on the synchros. Sure the more modern cone style synchro G50 transmissions are better at managing unmatched RPM, but even they will eventually start crunching. I had an 83K mi. G50 in my '87 that was clunky going into 2nd gear when cold and would crunch 3rd gear during fast shifts when hot. So it had obviously seen a lot of heavy shifting to be crunchy at *only* 83K mi.

The reverse crunch is definitely related to the non-synchronized reverse gear assembly. It's not a problem with the trans in terms of it being something amiss. It's due to the lack of synchronization to slow down the straight gears as they're trying to mesh.

With the engine running, clutch out and trans in neutral, the shafts inside the trans are all spinning. Push in the clutch and those shafts begin to stop spinning. If you quickly go for reverse while they're still spinning somewhat quickly, you'll get the crunch every time since the reverse slider hub has nothing but bare, albeit rounded-edge, straight teeth on it that are attempting to engage with the same type of teeth on the reversing gear pair. But if you select another gear like 2nd or 4th or whatever other gear before you go for reverse? You're using that other gear's synchros to slow down the spinning shafts and that's what lets you get into reverse without crunching.

[impactbumper]

Now you guys are making me want to take the transmission out and apart.... :d

[universeman]

^^^Great stuff Kevin

[Matt Monson]

Originally Posted by universeman

Also this: Busting the "shortshifters cause excess synchro wear" myth - The Tuners Group

Short shifters don't cause excessive synchro wear. Drivers do. A short shifter encourages a driver, who can already shift faster than the synchros can synchronize, to shift even faster. If the driver doesn't do that, the short shifter will have zero impact on synchro life.

[Matt Monson]

Originally Posted by pmax

Yes. That's the 915 reverse gear crunch. The 915 reverse gear is not synchronized.

The manual says to pause for a few seconds before engaging reverse.

The reason to pause is because you are waiting for the internals of the gearbox to stop spinning. You should be at a complete stop and clutch pedal to the floor. In neutral with clutch out, the mainshaft will still be spinning the gears.