2nd to 1st grinding on a 915 at speed

[LBEEMING]

Hi Team,

Recently did my first motorkhana / autocross event.

I learnt my lesson on the first run that my 85 with 915 doesn't like going into first from second above 20mph.

I tried to rev match and it didn't seem to help.

Is this my box needing a rebuild, or should I be changing into first at a much slower speed?

Thanks

[john walker's workshop]

Bordering on abuse, really. They don't like that.

[nickelplated5s]

Somewhere on here Matt said 901 boxes in 1st aren't built for that. I'll paraphrase from memory "if you want to launch it use 2nd." At a guess a 915 isn't much different. Better question is why 1st above 20 MPH? Listen to John, he's more curt than I which is saying something. Best in person quote I was there for from John was " drill a hole and put a bolt in it."

[dannobee]

First things first. What kind of car did you drive before this one? Because a 915 sure isn't like any other transmission. Finicky at best. It could be that it needs an overhaul. Have you changed the oil in it? Any fuzziness on the drain plug? Does the magnet look like a sea urchin? Or have any small bits attached to it?

Or it could be abuse, as John said.

[kenzinger]

Mine doesn't like to go into first most of the time, let alone at 20mph. I find the best bet for my '83 915 is to double clutch and then slide it into first around 5 mph as I'm coming to a stop

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[4sd911]

901/915’s shift from 1st to 2nd or 2nd to 1st can be delicate, and more so on an aged trans. It can be a sign of the synchro ring weakening.

[Fixer]

I can shift my 930 4 speed into 1st at 35mph.

The best 915 ive had came out of my 76, it shifts excellent but i sure as hell would never try going into 1st above 5 mph.

[stownsen914]

The early gearboxes just aren't that great going into first while the car is moving. Sure a rebuild would make it better to some extent. Double clutching (not just blipping / rev matching) should make it doable though. Personally I avoid shifting into first with the car moving whenever possible, and double clutch if the car is moving.

[4sd911]

Both my 901 & 915 can be shifted from 2nd to 1st at around 10 mph but need to let go in easy no gear jamming.

[wswilburn]

I only shift into first from a dead stop, regardless of the transmission.

[A930Rocket]

1st gear at 0 MPH for me. I probably need to rebuild the transmission, specially new synchros.

4th to 1st at 100 MPH at turn 1 at Road Atlanta years ago.

[Bill Douglas]

Something wrong with it. One particular corner I do, it's a downhill hairpin, I go from third to first and it works fine every time.

[917_Langheck]

Originally Posted by A930Rocket

4th to 1st at 100 MPH at turn 1 at Road Atlanta years ago.

The things nightmares are made from. That is why I put Karsten's spring on my shifter. While I tried a 4 into 1 one or twice, the trans was kind enough to tell me it was a bad idea at the time. The K spring keeps that 3-4 plane nice and straight.

[Hi_Fi_Guy]

When my father thought me to drive, one of the first lessons was never shift into first until the car is at a complete stop. Right or wrong it has served me well.

[proporsche]

Originally Posted by wswilburn

I only shift into first from a dead stop, regardless of the transmission.

^^^^^^^^^yup--this^^^^^

Ivan

[Bill Douglas]

If you pop the clutch out when in neutral (yup, doubling the clutch) at about 5mph it will go into first immediately.

[Schulisco]

It varies from box to box, some do, some not. Also depends on wear, previous driver talent, oil, etc. Mine (from 1977) works while rolling onto a corner at low speed, never tried it at higher speeds, won't recommend it. It has a factory short shift kit and I added the Seine Systems Gate Shift kit.

The 915 is an old fashioned design, the Porsche patended synchronization mechanism is original from 1953. But for the era the 915 is a very modern construction. It's synchonization mechanism is blocking the gearchange as long as both gears are not on the same rotation speed. So the 915 urges for drag torque from the transmission oil to speed up the gears and to make the synchros work, but modern synthetic oils won't provide that much as the 915 requires. The G50 uses a Borg-Warner synchronization like most todays cars. Different story.

The original filling was mineral SAE90 GL5. And no - we won't hijack this thread as another oil thread. But in respect to the topic it needs to mentioned imo. A workshop filled in mine a modern synthetic transmission oil, result: shifting cold was good, but when warmed up I couldn't find the gears any more and the gearbox denied quick shifting. Switched back to the original SAE90 GL5 mineral oil showed cold shifting needs to be more careful. But when the oil warms up, it shifts pretty nice. There were many discussions on that here before. Use the search function.

Double clutching may work as well as engaging into fourth or second before going into 1st gear, but both are methods of convincing the transmission to do something the 915 was not designed for...

Thomas

[stownsen914]

Schulisco makes a good point - use an oil that's appropriate for a 915. It can make a difference. There are plenty of threads on this forum with specific recommendations if you are looking for that.

[Matt Monson]

Originally Posted by nickelplated5s

Somewhere on here Matt said 901 boxes in 1st aren't built for that. I'll paraphrase from memory "if you want to launch it use 2nd." At a guess a 915 isn't much different. Better question is why 1st above 20 MPH? "

Two different things/situations here. The reason you don’t launch in 1st on a 901 is because 1st is out in the tailbone cantilevered out on the end of things. The splined section is skinny, and on a higher powered cars you can snap the shaft. See attached photo from the Pelican tech archives.





As for the answer to the question you’ve posed, I think it’s because he wants to Autocross the car. I’ve attached a gearchart (thanks Bill V).



Clearly, if you get down to 20mph on course, you’ve got to get it back down into 1st. As Scott T has said, you can’t just rev match with the clutch to the floor. You’ve got to de-clutch, blip, clutch, and slide it into gear. The goal with the blip is 3500rpm, or so, as you can see by crossing the lines on the chart. If you manually spin the main up to that speed in neutral, the synchros won’t have to do any work, and the gear will slip right in.

I suggest practicing it in slow motion outside of a race environment. It isn’t that hard, but it takes a bit of practice. I have done it for so many years, it’s just second nature.

[safe]

At 20, or less, you should be able to downshift to first. Its necessary for tight corners, I do it, no rev matching just rams it in... But it is abusing the gearbox a little.
Mine is rebuilt and if I do it enough there is probably another one in the future. But there has been a few years and no grinding.