DemonRX-8

Syncros do a fine job of matching the selected gear rpm's to the engine rpm's . . . but that's not the point of double-clutching. Double-clutching's main purpose is to allow quick and smooth downshifts, whether it's for racing Or just downshifting smoothly in general stop-and-go traffic (or to pass). Also, double-clutching is pointless without rev matching . . . I mean the whole purpose of it is to allow you a moment to blip the throttle with the clutch engaged to match the input and output shaft rpm's during a downshift.

IMO double clutching for an upshift makes no sense since the syncros do a fine job of slowing the high gear down and the engine rpm's will naturally drop as you release throttle. Rev matching is inherent, so the shift is smooth assuming you are not "riding" the clutch out.

Downshifting is a different story. True the syncros will still do a fine job of speeding up the lower gear, but without rev matching simply releasing the clutch does send a significant shock through the drivetrain (as the driveline attempts to raise the engine rpm's to match), which could very well result in the rear wheels locking or worse if you happen to be in a turn - I seem to recall a thread here where someone binned their car by accidentally shifting into 2nd gear from 5th on an offramp. Or you could ride the clutch out slowly as the engine rpms catch up. Either way, it can't be very good for your clutch or drivetrain in the long run. Double-clutching eliminates this "driveline shock" and allows the downshift to happen smoothly and quickly without jerking the car.

Heel-and-toe (please note that the correct word is "heel," not "heal") downshifting is more for "spirited" driving when you are trying to double-clutch downshift while braking going into a turn. If you drive like a grandma then you don't need to worry about this - you can just downshift after your done with the turn and you're going slow enough that there is not a lot of difference between the input and output shaft rpms to send much of a shock through the drivetrain.

One last note: clutching itself is not necessary (except for taking off from a stop) if you rev match properly - and without damage to engine, clutch or transmission. Simply put, if the both the input and output gears are spinning at the proper rpm's the gears will simply slide into (or out-of) place without grinding or jerking. I used to do this regularly on both of my previous cars just to demonstrate to friends by driving for a period of time (both upshifting and downshifting) without using the clutch. I haven't tried it with the 8 yet because it's not as easy to rev-match as my other cars . . . plus it's just too new and nice to do it for no reason! (note to self: try this with the 8 tomorrow!) And before anyone says anything - I've driven several hundred thousand miles with my two previous cars without a single clutch/transmission problem or repair.

msrecant

I agree you need to rev match for smooth downshifts. I don't see where double-clutching is required to do that.


Again, from what I see the rev matching does this, not the double-clutching.


The heel-and-toe technique is used when you want to brake and downshift at exactly the same time. It applies to both normal and double-clutching.


Agreed. This is called speed-shifting, right?

cjkim

I think we're just debating against ourselves...
from my understanding (which could be wrong, please someone correct me if I am)...
rev matching - keeping clutch pressed IN and blipping throttle to match rpms and speed of the gear you'd like to switch to.
heel toe - rev matching, except you're braking at the same time
double clutch - clutch in, shift to neutral, clutch out, blip, clutch in, shift to desired gear...

i haven't seen anyone using their heel and toes to double clutch while downshifting... i believe you are mistaken (though i could be wrong). Heel toe, from what i understand, just involves rev matching and braking at the same time.

msrecant

Sounds right to me.

DemonRX-8

Yes, we are arguing some of the same points.

True. What I meant is that double-clutching helps match revs because the input shaft spools up at the same speed as the engine with the clutch engaged. But the input shaft will also spool up with the clutch in from friction/viscous forces, so rev matching can be done with the clutch in also.

I agree heel-and-toe downshifting does not require double-clutching - I should have said rev match rather than double-clutch. The idea is to get the shift done before the apex and on the power right after hitting the apex. I wouldn't see much use for double-clutching at the track (where you'd be pretty much doing just heel-and-toe and rev matching with the clutch in), but I do it frequently in normal driving. I just find it easier when I'm not shifting for speed (just smoothness) because you can gage directly from the tach when your spinning the right rpm's for the gear you want.

cjkim

that sound's right...
I was just a bit surprised to hear you say you double clutch (or declutch) while heel and toeing on a track...

yiksing

Yes the Demon got it right, there's no necessity at all to double-clutch unless the synchro is busted or one is trying to preserve it.

John Corbitt

I just got back from Sebring. I preped my 8 for a NASA event, but my friend wanted my to run his "Jim Cope" car. It is similar to the TransAm series cars. It has a Jerico transmision with no syncros. Upshifts are done without the clutch. It is a "crashbox". Downshifts can be done with the clutch as well. However I did need to rev match. I tried it both ways. Heal-toe Double clutch downshifting was a lot smother and made for better lap times. This is probably because I am used to double clutching.



John