If possible to briefly explain to a layman, since they are both synchro designed, why is the 915 design slower to shift than a modern gearbox?

People usually talk about how friction is the enemy to gears and bearings. What gets far less attention is that the way that synchros function is through the use of friction.

When an operating sleeve, aka a slider, is actuated to select a gear, what it is doing is locking it down to give it positive drive. Until that point the gear is free spinning on a needle bearing. The gear needs to be brought up to the speed of the slider or down to the speed of the slider, depending on whether or not it is downshift or upshift. The synchro is the buffer between them that changes the speed of the gear using friction.

The most basic explanation of why is because the old 901/915/930 Porsche proprietary design is not as effective at changing the speed of the gear as quickly. A Borg-Warner cone design synchro is the worldwide standard these days. It changes the speed of the gear so that the slider can lock onto it the most quickly.

Those that have spent time on country roads at high speeds with a V-12 Ferrari or Lamborghini will question why the great unwashed seek to shift as fast as they can.

Trust me, it's true...

JR

Because Vin Diesel?:D

I'm sure he's to blame. Then there are video games, stupid paddle shifters... The youngsters are truly lost.

JR

But Vin only shifts into fourth repeatedly. At least in the F&F franchise that I have seen.

No rev matching double clutching granny shifting like you should...

Thanks Matt. So the bottom line is that modern gearboxes have Borg-Warner cones, and are able to shift much faster. The old gearbox relies more on friction to engage the gears, and one must be a little more patient for the synch to happen.

Matt, hate to steal the thread, but I've got a quick question. 2008 Boxster S, if you had to choose a type of LSD, what would you choose? or would you?

The brown acid, is not too good...

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All gearboxes that use synchronizers rely on friction to shift. You have to match the rotational speeds of several components and without friction, no work gets done.

Two things that are often forgotten about shifting speed are the mass of the components and the spread between the ratios.

I'm not in the camp that the later gearboxes are automatically all that much better than the early ones. A fresh, correct 901 box shifts as quickly as you can move the lever. All of the *****ing about the Porsche syncromesh comes from guys with tired gearboxes and gearboxes that have been rebuilt on a budget.

JR

I call BS.
Try power shifting your 901.

IMO, it's not about "modern". My old T-10 was the slickest shifting trans in my experience.........but unfortunately fragile. Later model GM 4 speeds were not as slick, but more robust.

Power shifting? No idea what that means.

I recently sold my 914-6. It had a freshly rebuilt gearbox and shifted like lightening, especially the first three gears. 4th and 5th were a little slower, because of linkage issues. I ran it with both the tail shifter and side shifter linkage and with the tail shifter linkage the first three gears went by rather quickly. If 4th and 5th hadn't been such a pain, it would have been my favorite arrangement, as the 1-2 and 2-3 shifts were effortless.

JR

I thought that was a possibility.......but didn't want to possibly insult a fellow 914er.

I removed 1, 4, 5 ......and reverse from my 901. Autocrossers are kinda weird...or just overly weight conscience. Hay, if you don't use it, eliminate it.....but you still couldn't power shift it. (right foot flat to the floor, side step the clutch pedal while going for the next gear).

I used to drag race back in the day before decent auto trans became the cat's ass..

A 915 cab shift sweetly. I rebuilt/refreshed mine more than 7 years ago and it's been a dream.

It's not as easy or as fast as my Golf but that's not the point and the 915 is much cheaper to rebuild than a G50.

FYI

http://forums.pelicanparts.com/uploa...1463579999.jpg

I too sold my 914 a few years back...but still have pics.

JP,
Have you seen what Britain has done with the gearbox since? I'll see if I have pictures. 2 Spd 930 dog box now.

It's seem there's only one choice. :) My LSD.

Both very good points. If the splits are right on a 901 or 915 race box, shifting at redline requires almost no synchronization from the next gear up and can yield quite quick shifts.

And double clutch rev matching on the downshift can get you in really cleanly rowing down the gears, especially on the 901 with its light skinny gears.

Two things are going on with the earlier Porsche system:

As the shift sleeve ages, it develops a ridge on each and every engagement tooth. This essentially narrows the gap that the corresponding dog teeth must plug into. Those sharp little ridges slowly replace smooth tooth flanks as the initial contact point.
http://images.thesamba.com/vw/gallery/pix/1062236.jpg

I also believe that while the Borg Warner style synchro remains perfectly square (when pressured) because of the cone design, the early system c o c k s the shift sleeve slightly as it begins to compress the synchro band (especially when forks are worn).
.