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Porsche Transmission Preservation or “The Two Finger Rule”

By Steve Grosekemper


According to the Chinese Zodiac, 2010 was the Year of the Golden Tiger. However, if I had to put a label on the past year I would have to proclaim it to be the year of the Glass Transmission.

After 26 years of working on Porsches, I have found that repairs come in waves. Sometimes you will have 3 cars brought in on a tow truck with bad DME relays in 3 consecutive days, or a month where every other car seems to have a bad power window switch.

For me, 2010 was the year of the broken transmission. It seemed like there was a steady flow of broken, abused transmissions. It almost made me want to start a foundation or air a telethon. (Jerry Seinfeld hosting the Porsche Protection Services telethon perhaps.)

Now I am completely aware that it is my job to repair broken Porsches, but it still pains me to see a car that has been abused or mistreated, even if unintentionally. It is all I can do to manage to fit the “normal” failures into my schedule, let alone the “abnormal” ones.As a matter of course, when a car comes in for a transmission repair I like to quiz the owner about how the failure occurred. Things like whether this has been an ongoing issue that just became severe enough to warrant repair, or an issue that just showed up out of the blue.

One resounding theme lately has been the “loaner” story. I loaned my car to (insert inexperienced, over-enthused, teen-like driver here) and it started having this problem or failure. This is also known as the “Ferris Bueller syndrome.”

Individuals who normally drive mundane egg-shaped people movers can have a Jekyll & Hyde type transformation when they get in a car with the reputation and capacity for performance that a Porsche has. My suggestion is not to loan your car to someone who is not used to driving a car like this. It is much cheaper to rent them a Ford Fusion at Avis.

Sometimes the problem is not a loaned car at all, but a car being driven by a misinformed owner. I had a friend of a friend bring a 993S in for me to look at because he said it made a bad grinding noise. He said the noise happened when he shifted the car from first to second and from second to third. After a very thorough test drive I could find no shifting problems with the car. As a matter of fact, this 32,000 mile 993S was like a new car in every way. So when he came to pick up the car we went for a test drive together with him at the wheel. After the first run through the gears it became apparent what the problem with the transmission was. The driver was shifting the car so fast that the synchros could not possibly do the job they were being asked to do. “How fast?” you ask. So fast that his hand started two inches below the shifter in second gear and got a running start towards the knob in making the 2-3 shift … (I know … almost brings a tear to your eye.)

After I screamed “STOP!” we switched seating positions and I showed him the “two finger rule”—how the car should be shifted using no more effort than can be applied with two fingers on the shift knob. It shifted perfectly without grinding or any other issues. This was his first Porsche and he had been told by a “friend” who knows all about Porsches (but had never actually owned one) that this was how a Porsche was meant to be shifted. I explained how a synchro works and he then understood the “what and whys” of the two finger rule. Now for a bit of what I told him.

“Synchro” is short for synchronizer. The official definition is “to cause to go on, move, operate, work, etc., at the same rate and exactly together”. That is precisely what it does. It makes gears spinning at different speeds match their rotation so they can smoothly engage. It does this with what looks like “Dog teeth” or engagement teeth. These are rings of pointy teeth that engage the gear to the shaft (like intertwining fingers as you bring your hands together). When gears grind, it is these dog teeth coming together, but not at the same speed. The sliders are, well, sliders that move along the input/output shafts. They engage power from the gear to the appropriate shaft.

One common misconception about manual transmissions is that the gears move position when you shift. The gear teeth are always in contact—it is their connection to the input/output shaft that changes. The power moves from the engine to the wheels through these sliders and dog teeth.

As I said, in the transmission there is an input and output shaft that each spin at different speeds according to which gear is selected. In my ’83 911 for instance, the input shaft turns 3.1818 times to spin the output shaft one single rotation when in first gear. In second gear the input shaft only spins 1.777 times for the same single rotation of the output shaft.So to break this down into simpler terms, let’s say we are in first gear at 4000 RPM. We shift to second gear, and while traveling the same road speed, the main shaft has to decelerate from 4000 RPM to about 2500 RPM. As we move the shifter out of first gear position to neutral, the connection between the first gearset and the shaft is lost. As we move from neutral to second gear, the first thing that happens is the synchronizer starts to make contact between gear and shaft. It decelerates the main shaft to the proper speed (2500 RPM in this case) and soon thereafter the dog teeth engage and the slider locks into place creating a connection from shaft to the second gearset. That’s really the easy part, because if you shift from first to second gear with no synchro action, the main shaft will eventually decelerate from 4000 to 2500 RPM. This will allow for the proper engagement speed and the shifter will drop into second gear if your timing is perfect. If not, you might also get some unwanted noise along with it, but it will eventually go in after a few tries. This is how old non-synchronized transmissions work.

Now the tricky part: the downshift. Let’s say we are in fourth gear in my 911SC which has a 1:1 gear ratio, which equals about 3300 RPM at 60 MPH. But I need to get around this big rig in front of me before I get caught behind him on this long grade. So I press the clutch pedal down and move the shifter out of fourth. Then I pass through neutral towards third where the synchros start to come into play. This time they speed up the main shaft. Speeding up the main shaft takes more energy than slowing it down. As the main shaft accelerates to about 4100 RPM the shifter slides into third gear. I let the clutch pedal out, hit the gas and we speed away, passing by the big rig (but all the while not exceeding the posted speed limit, of course).

Porsches have used different types of synchros over the years but they basically all do the same job. They act as a brake to slow down or speed up a shaft to match the speed of the other shaft. So to illustrate this, think about what would happen when you come to a stop from 60 MPH. Braking method “A” would find you lifting off the gas pedal and the car decelerating naturally until at about 35 MPH, when you would gently apply the brakes to bring the car to a smooth stop. Braking method “B” (A.K.A. Breaking method “B”) would find you lifting off the gas pedal as fast as you can, bringing your knee to the back of the steering wheel and slamming down on the brake pedal as hard as you possibly can. Braking method “A” is the two fingered shift method and braking method “B” is the guy in the 993. I am more a fan of the Type “A” shifting for my cars and hope anyone who has been a bit aggressive in the past will change their ways after reading this article. May all our transmissions live long and require only gear oil services.