Today I learned what double-clutching is. Double-clutching explained.
The site says:
The double clutching method is only used when downshifting.
So my question: why would someone be downshifting during a drag race?
I think you are thinking of “power shifting”:
They wouldn’t, of course. The only question is if the screenwriters were clueless or brilliant.
It says as such in the linked article:
While the technical theory behind double clutching as an advantageous move in a manual transmission is not entirely wrong, it’s entirely misplaced within the movie. In the case of Diesel and Walker, the double-clutching technique would not have served him at all since it’s only really possible to use it when downshifting. For the purposes of the movie, it’s just another in a long line of examples of Hollywood getting things wrong; using cool-sounding terminology even when it’s not remotely accurate. Whether or not anyone told the producers that they were incorrect in their assertions is unclear.
I don’t think double clutching is necessary with modern manuals. When I downshifted on all my previous manuals (2006 VW Jetta, 89 Prelude, 81 Accord), I would put the clutch down, hit the gas while downshifting, and then lift the clutch pedal. No need to go down up down up with the clutch.
What was the point of down up down up? Is that like what syncro-mesh does automatically?
On my current car (2016 Mini S), if you depress the clutch and shift to a lower gear, it hits the gas for you and revs the engine.
My '66 MGB does not have synchromesh between 2nd and 1st. (That came out in '67.) I can double-clutch, but why bother? The only time I go from 2nd to 1st is when I’m coming to a stop, and I can just shift into first when I’m stopped.
I’ve been driving manuals my entire life and now you’re making my wonder if I’ve been doing it wrong for the entire 25 years I’ve been driving.
My standard procedure for upshifting is to simply depress the clutch while simultaneously letting off the gas, shift gears, then let out the clutch smoothly but quickly. I do not and never have revved the engine while the clutch is depressed.
For downshifting when I know the the engine will have speed up considerably (I do a lot of mountain driving so going from a steep dowgrade to a steep uphill grade is common) I let off the gas and depress the clutch as normal, put the transmission in neutral, let out the clutch, rev the engine, push in the clutch and shift into a lower gear. I have always found that this makes for a much smoother shifting experience. If I fail to do so I can feel the synchromeshes doing their thing as I shift into whatever lower gear I’m aiming for which actively slows me down, if just for a split second. Doing it this way is also less jarring when I let out the clutch because the engine usually hasn’t returned to idle yet (I’m guessing this is why your Mini does it automatically). Failing to do this process isn’t a bad thing necessarily – the synchros are doing what they’re designed to do – but it feels better to me. My first car was a VW bus with a 4-speed and shifting was kind of a game with each gear playing hide-and-seek so maybe that’s why I do it: it was more of a necessity then.
It sounds like you’re double-clutching when downshifting, which is what they were doing in that movie (I’m led to believe), and you’re doing it out of habit because of whatever awful clutch the 60s VW bus had.
I’ve never had to do that to downshift smoothly, even back when I had the '81 Accord. That is, I’ve never had to let out the clutch between gears when downshifting, and I don’t see how it would help me sync the engine speed any better than regular downshifting.
I think it’s more about synchronizing the speeds of the various gears in the transmission rather than the engine speed. I have only a rudimentary understanding of how transmissions function so take this with a grain of salt but my understanding is that by letting the clutch out and revving the engine between gear changes when downshifting (and this is only necessary when downshifting) you’re using the engine to speed up all the spinny bits of the transmission so that when you make your gear change into a lower gear the transmission is already spinning at the speed it "wants’ to be in to make the gear changes smooth. If you don’t do that extra step then the synchomesh gear needs to match the speeds of the two gears as they start engaging. If the two gears are already spinning at similar speeds then the synchromesh ring has much less work to do, essentially only needing to act a guide to mate the two gears together.
You’re likely right that at this point I do it out of habit based on the needs of a vehicle I haven’t driven in 20 years but when I don’t do it I notice a bit of a difference: a less smooth shifting motion.
Double clutching is for downshifting, and it’s from when there weren’t synchromesh transmissions.
I do practice downshifting without the clutch;. Pop it into neutral, raise the rpms until the transmission matches and pop it into the lower gear. Upshifting is easier: pop it into neutral, let the trans slow down, and pop it into the next higher gear.
And despite how good a car chase it was, Bullitt did not need to double clutch upshift.
Remember, Dominic Torretto is a career criminal who smack talks to rile up his opponent. He’s not someone you can trust.
His opinion of nitromethane should be viewed in the same light.
The problem using RPM matching for shifting on a transmission with synchros is it apparently can increase wear on them (i.e. they’re still tied in to the transmission even if you’re effectively bypassing them), and synchro replacement will cost you more than a new clutch, so you want the clutch to wear out first.
Granted, I’m not a mechanic, just someone whose been using manual transmissions through most of the time he’s had a license to drive. And this is just what I’ve heard from other sources so take it with the appropriate varying grain of mileage.