danglerb

Unless that is a fresh rebuild or a G28/55 out of a GT $3500 is a bit of a premium, as in more than 928Intl charges for a rebuilt incl core charge (last I checked).

brutus

Got started before syncronizers were invented. It is how you MUST spin up the gear sets in order to be able to shift. Ever notice highway signs on a grade telling truckers to hold it in a lower gear ? Semi trucks if you miss shifting and get rolling too fast you may not be able to get it into another gear.


Old USA cars and trucks three on the tree manuals. You could only get first if you stopped (none syncro) unless you learned to shift into neutral spin the gears in neutral depress the clutch a second time while RPM matching the engine to the speed you are traveling, make the gear selection with just a minor crunch and let out the clutch without chirping the tires. Another fun trick with manuals is float shifting where you ignore the clutch and just pull the shifter into neutral (hold trottle so there is no load no acceleration no decelleration) then rev the engine and transmision gear to the correct RPM for the new gear and firmly pop it into gear.
If your clutch hose lets go on your 928 and it will. You can drive it home by starting it in first gear and then float shifting it until you get it up to fifth. You can freeway drive it that way pretty easily if you know how to RPM match.
The silly affectation is blipping the throttle on an UPSHIFT just because it sounds cool.
So double clutching is a real and useful technique and it will greatly extend the life of the syncronizers in a good gear box and will allow you to get into second even if the sycros are shot. Given that syncros are wear items just like clutch discs and brake pads it pays to be nice to them.
Heal and toe allows you to brake and hit the gas pedal at the same time with your right foot so you can do that RPM matching while braking even if not double clutching. Like if you need to downshift in a corner and you are near the traction limit by reving the engine you are able to let out the clutch and have no engine braking which might upset the car enough to cause the rearend to step out.

outbackgeorgia

Brutus,
Correct! You have explained float shifting perfectly. I once watched Bob Tullius post lap times within a few seconds of normal without a clutch in a Trans Am car.
My kids were amazed when I drove my 911 home after the clutch cable snapped. Once out on the Interstate, it was really no issue. Have not yet had to in the 928, but have practiced, seems like it will be fine.
Dave

GregBBRD

Yes it is, especially with the "Porsche" design syncro found in the early transmissions.

The syncros in these transmissions are "compressed" by the slider, when the gearshift lever is moved. There is an "angle" on the leading face of the syncro and a similar "angle" on the inside of the slider. When these two "angles" meet, there is friction which results in wear from this friction on both the syncro and the slider.

(This is the reason why these transmissions, which get rebuilt with "syncros" only, with total disregard for the sliders, seldom shift properly for very long, if they ever shift properly. The "list" of people that have put only syncros into these transmissions and had very poor results is very, very long.)

Both surfaces wear.

If you look/think about the design and what is wearing and how it wears, it becomes obvious, instantly, that the longer period of time these two surfaces are "in contact", without the engaging teeth coming together, the more wear there is. Reduce this time and the syncros/sliders actually last longer!

Let me put this in English:

Shift these transmissions as quickly as possible, without the "engaging teeth" grinding, and you will have the least amount of wear possible. Double clutching or slow shifting kills this style syncro!

BTW...shifting one of these style transmissions without the clutch is just about the worst thing you could ever attempt to do! If you can perfectly match the required rpm for the gear exchange, there isn't a problem. However, the "learning curve" to be able to do this is high enough that by the time you "learn" how to shift without a clutch, you will have probably destroyed the syncros and the sliders. Again, look/think about the design and what is wearing and how it wears. If these is no way for the syncro to slow or speed up the gear, when shifting, and the gear continues to spin at a different rate that the slider, the wear on these two pieces will be extremely high. This is the main reason why these vehicles always have a 1st or 2nd gear syncro problem...because the clutch didn't always completely release. Never "try" to get one of these cars home, with a broken/disfuctional clutch. Call for a tow.

stvracing

Let me clarify what I said.I was only after the trans. The WHOLE car is costing me $3500.