Don.

Having an SC with worn synchros I've been steadily improving my double clutching skills. I've almost got it so the 3 to 2 downshift is usable. I have heard that double clutching will also help a shift from 1 to 2 but I have no mental model for why this would work, and have not really gotten it to work so far. Does anyone know why double clutching on an upshift is supposed to help?

I know that when your going, for instance, from 4000 rpm in 3rd to 6000 rpm in 2nd, you need to blip the throttle on the way down so that the engine is rev matched on the downshift. If you also double clutch on the downshift ("shift into neutral") and blip the throttle, you're making sure that the input shaft is spinning at the same speed as the pinion (output) shaft. Since the synchros are synchronizing the speed of the input shaft with the output shaft, this makes it much easier for worn synchros to work.

However rev matching should not be necessary when you go from say 6000rpm in 1st to 4000 rpm in 2nd. After you put in the clutch, both the speed of the input shaft and the speed of the engine should slow down, and when the input shaft spins down the 4000rpm, the shfter should just drop into 2nd.

If you double clutch and blip the throttle, it sounds like you're just spinning the input shaft and the engine back up to 6000 rpm, causing you to have to wait again for them to slow down. Somehow I suspect my mental model may be faulty, because it sometimes does seem to help shifting smoothness when I double clutch from 2nd to 3rd, and there seem to be no reason why this should be so. I have yet to have it work well from 1st to 2nd though, since the #2 synchro is much worse than the #3. Can anybody explain why double clutching upshifts help with worn synchros? Or why it doesn't help?

If I'm over about 3000rpm in 1st and shift into 2nd I usually get some sort of crunch (bzzzt, try again!). Would be nice to be able to do some sort of half decent launch...


-Don

911pcars

"However rev matching should not be necessary when you go from say 6000rpm in 1st to 4000 rpm in 2nd. After you put in the clutch, both the speed of the input shaft and the speed of the engine should slow down, and when the input shaft spins down the 4000rpm, the shfter should just drop into 2nd."

Don,
Your theory is accurate. Double clutching is not necessary when upshifting. Your description of double clutching (downshifting) is also very accurate. A lot of people think a blip of the throttle while the clutch is disengaged (pedal down) meets that requirement (it doesn't).

Sherwood Lee
http://members.rennlist.org.911pcars

George H

A blip of the throttle while the clutch is down does, however, keep engine speeds from dropping too much while you are avoiding slamming the transmission up a gear. I always blip with the clutch down on any upshift in my 911 -- the gears engage more smoothly and I can shift more slowly without extra wear on the transmission and clutch.

Since getting in the habit of doing it in the 911 all the time, I frequently do it on instinct in my other car, where it still makes for smoother shifts.

But you're right, thats not a double clutch, but it doesn't mean its not a good thing to do.

Lefty's Deceiver

George,

Why do you blip on an upshift?

Lefty

George H

For just the reason I said -- these transmissions need to be shifted fairly slowly, you can't snap them into gear like modern transmissions, and the slow shifting tends to drop the motor speed too much, which is generally okay, letting the clutch out will spin it back up but that puts too much wear on the clutch. The alternative is to shift faster, but thats hard on the transmission. Its gentler on both if you give it a little blip as the clutch disengages to give yourself a half second more to get the shift done.

I agree double clutching isn't worth it on an upshift, but blipping makes at least my 901 a LOT easier and smoother to shift.

Don.

Thanks, Sherwood/George for the info. Looks like no drag racing for me until I get the transmission rebuilt. I haven't heard before that blipping would make upshifts smoother on the clutch, because I've been focusing on getting into gear rather than what happens later - but I plan to try it, sounds useful. Can't explain why double clutching from 2nd to 3rd seems to make things smoother sometimes - perhaps its just the blipping?

-Don

JackOlsen

It's funny that reviews of the early 911 often cited its transaxle as its single best feature.

Alan Herod

That would probably be early reviews of early transmissions, and the excellent sychros, not necessarily the linkage.

Zeke

Regarding bliping: I keep a little pressure on the gas pedal so that the rpms don't fall as fast. No matter how good you are, it's hard to catch the trans speed on the rpm uptake if you let it fall to idle. High comp engines with lightened flywheels like to do this. I like to look at my passenger and if thier head isn't moving when I'm shifting, I'm doing it right. Bliping is unessesary and actually makes it harder for a puurrrfect engagement.

Don.

Hey Zeke, I have a normal flywheel and I've noticed that although it'll blip to redline in a fraction of a second, it'll take maybe about twice as long to come back down. This makes blipping more feasible for me. But your suggestion is another interesting thing to try. I just have to take the slop out of my throttle linkage to make it more feasible.

All this talk about the "excellent" Porsche baulk ring synchros brings up a related topic for me. I have driven well sorted out (i.e. rebuilt & broken in) transmissions with the baulk ring synchros ('86 911 & '84 928) and thought that those transmissions were as smooth and as fast to shift as a G-50. Does anyone else have the same experience? Those were friends cars, so I wasn't doing redline downshifts, so I'd be interested in hearing if anyone disagrees.

My understanding is that the problem with the 901/915 is not necessarily the speed/smoothness of the downshift, but the longevity of the syncrho. A G-50 with 100K miles on the clock has plenty more years of smooth shifing to go. A 901/915 with 100K miles needs a rebuild. Anyone know how quickly the 901/915 needs new syncrhos (at least #1 & #2)? Every 50K miles? 25K? Again, please enlighten me if you know anything different!

The reason I'm bringing all this up, is if I can convince myself that the 915 is just as good as a G-50 (when rebuilt), my next step up from the '78 I have may be to '84-86, rather than the '87-89 (with G-50) everyone recommends. I've heard that 84-86 may actually have some advantages in engine longevity, as these engines are in slightly lower tune than the 87-89, and (according to Bruce A.) some 87-89 may have premature valve guide wear.

Regards,
Don

CDiercks

I just have to say that after reading this thread I have found new life in my car! The tranny is just about the only thing that frustrates me with my car. My first and second syncro are pretty worn and would baulk at me pretty bad when trying to get into gear at anything other than a very leisurely pace. First, I just replaced my shift bushings (helped a LOT) and installed the factory short shift kit. The combination of those two made my car shift like a 911 should. While driving to and from work today I started playing around. If I let the clutch out for a split second in neutral before downshifting to second, it shifts flawlessly! I was amazed! I drove an extra 15 miles home from work just to get better at it. I understand how transmissions work pretty well, but hadn’t thought about using engine speed to bring the tranny up to the speed of the car before shifting. Makes sense to me when I think about it though. THANK YOU, and my transmission thanks you too!

Zeke

Don't mean to split hairs here, but I think it's the other way around. Your trans is constant with the road speed. You are revving the engine a little to downshift making the engine speed match the traany speed. Great that you discoverd a new technique.

More on the blipping. If you let the clutch out just half way between upshifts while the engine is decelerating, there is a good chance you will "bring the tranny speed up to the engine speed" (not an excact quote) and make life easier for the synchros. On my previous 911, I could actully shift faster w/o grinding between 1st and 2nd (915) using a double clutch technique with no gas pedal input. A very busy left leg. It's just plain awful if you don't get it right! Ouch!

CDiercks

Yes I understand what you mean. I worded it wrong. I have been using the speed of the car to "spin up" the transmission before I down shift. I also blip the throttle a little to make sure the RPMs are equal with speed of the car. It does make for a busy left leg, but if it makes my tranny last longer it's worth it.

lightning

well I used to blip on the accelerator during upshift, not to help the gearbox in any way but to keep the engine running. The original motronic mapping on a US Carrera 3.2 tells the engine to shutdown when the accelerator is released. In my case it resulted in a tiny delay when I accelerated in to next gear and it was not so funny in the bends to feel the shift from oversteer to understeer and then back to oversteer again.

Anyway I replaced my chip with a new one from Steve W and the mapping does not include the shutdown, at least it doesn't't feel that way. The new chip has definitely solved that problem and I don't need to blip the accelerator during upshift any more. A very nice and unexpected side effect.