I might be completely wrong, but this is how I understood what revmatch downshifting is:
You press the clutch
Bump the accelerator lightly
change the gear down
let go of the clutch
Let’s say I’m driving 50km/h and I’m in third gear, I’m around 2000rpm. If I keep driving the same speed and go down to second gear with regular shifting, the car will jolt and be at 3000rpm or so.
So how does adding even more gas (step two) make any sense?
Is the point in “faking” the third gear (or technically neutral since you pressed the clutch) to have more RPM’s than it actually has, so that instead of 2000 rpm’s you “bump it” to 3000 rpm’s and have those bumped RPM’s go smoother with the real 3000rpm’s of the lower gear?
Might be a confusing way to put it, but I don’t know how to ask it better.
Also how does revmatch upshifting work, are you also supposed to bump the gas between shifting and why?
One side of the clutch is connected to the engine via the gearbox. The other side is connected to the wheels. When you declutch, the two sides disengage and can rotate at different speeds. If you downshift, the side connected to the engine is now going through a lower gear, so it turns more slowly. But the side connected to the wheels is still turning at the higher speed. You need the engine to turn faster to offset the lower gear, so that the engine side of the clutch is brought up to the same speed as the wheel side of the clutch when you re-engage the clutch.
Remember that you have (1) an RPM at the engine, (2) a different RPM at the engine side of the clutch after the gearbox, and (3) another RPM on the side of the clutch connected to the wheels. When you declutch and downshift, (3) stays the same and (1) initially stays the same, so with the lower gear (2) decreases. You need to increase (1) so that (2) matches (3) again before you reengage the clutch.
Not sure quite what you are asking.
The answer is in the name - revmatch.
The entire point is to avoid the jolt where the engine is suddenly forced to change. That jolt is the engine being driven to the new speed via the gearbox from the motion of the car. That puts a lot of strain on the mechanics.
You want to match the revolutions (rpm) so that when the clutch is engaged both sides of the clutch are revolving at about the same rate. revolutions per second matching - hence revmatch. Avoid the jolt.
Upshifting is the opposite, you allow the engine to drop in speed and engage the clutch when the speed either side of the clutch is the same.
Both become pretty natural. Really, any driver of a stickshift that isn’t driving like this hasn’t been taught how to drive properly. Nobody should be feeling a jolt on engaging the clutch.
In principle you can shift without using the clutch at all if you are good at matching revs. You simply use the dwell period between engaging gears to match the revs.
You want to use the throttle to bring up the rpms because the alternative is that the clutch and synchros will do it instead, and those are wear items.
You shouldn’t need to rev match while upshifting as long as the engine doesn’t drop to idle.
None of this should be confused with double clutching, which is generally for transmissions without synchros.
" Not sure quite what you are asking.
The answer is in the name - revmatch .
The entire point is to avoid the jolt where the engine is suddenly forced to change. That jolt is the engine being driven to the new speed via the gearbox from the motion of the car. That puts a lot of strain on the mechanics.
You want to match the revolutions (rpm) so that when the clutch is engaged both sides of the clutch are revolving at about the same rate. revolutions per second matching - hence revmatch. Avoid the jolt."
Obviously I understand this part, the point of my question is - how does revmatching exactly work to coordinate RPM’s and how does it prevent the jolting from happening.
If I understood Riemann, the point is in increasing engine RPM’s while you are declutched, so that they are closer to RPM’s from the wheels once you let go of the clutch.
If we go back to the basic idea of a gearbox and it operation it should be clear.
The car engine can only deliver power over a limited band of rotational speed (aka rpm.) At low rpm your typical internal combustion engine delivers little torque and little power. At higher rpm it delivers more, until it reaches a peak, and performance drops off, and then it reaches a point where it is no longer able to operate safely.
The demands of driving change depending upon need, you need to accelerate, decelerate, and when at a steady speed, you balance power delivered from the engine to losses in the car and most importantly, aerodynamic losses. These also change depending upon speed, and not in a linear manner. Engines also have a minimum speed, below which they stall.
So there is no universal ratio where a typical engine is happy driving the wheels. At low car speeds you will need the engine to rotate more often for each rotation of the wheels so as to deliver useful power and not stall. If you need to accelerate you will want the engine to be rotating even faster so that it can deliver the torque needed to perform the acceleration and power to balance the increased losses. At constant speed you don’t need the engine to be rotating so fast as you are in a steady state. At the cars maximum speed you typically choose the maximum engine power to match the intrinsic and aerodynamic losses, and choose a drive ratio to do this.
When decelerating you can use the engine to apply a braking force on the car by deliberately not supplying enough fuel, and have losses within the engine slow the car. You can accentuate this effect by deliberately changing the gearing ratio.
So you need to change the gearing ratio to match circumstances.
When you change the gearing ratio, you move from a situation where the engine and drive train are rotating at the same rate (as seen at the back of the clutch) to one where the drive train is moving at a different rate to the engine. Dropping the clutch instantly means there is a significant amount of energy to be transferred one way or the other across the clutch to force the engine to match that of the car and its drivetrain (the car is much bigger, it will always win.) So you should control the engine in such a manner that its speed matches that of the drive train (ie both sides of the clutch are rotating at the same speed) before the clutch is engaged again. The only control you have for this is the supply of fuel. So you change the amount of fuel supplied to the engine so as to match the speeds. When downshifting you need to make the engine rev faster, so you blip the throttle. When upshifting, you need the engine to drop in speed, so you lift off the throttle. Engines typically are set up in such a manner that just lifting off the throttle when upshifting gives you pretty much the right drop in revs without needing to think about it. So long as you take a normal amount of time over the upshift. Too long and the engine revs will drop too far and you get the typical judder. Downshifts require you to judge the shift with a bit more effort, with a slight blip on the throttle that matches your shift. That takes practice. Rev counters on the dash are there to help you judge this, but eventually you do it without additional thought.
Eventually revmatching is just the name of the process you apply to achieve all this. There is nothing special about revmatching, it is just correct driving.
This all applies to manual aka stick shift cars. Various forms of automatic gearboxes involve a range of additional automatic capabilities. Automatic throttle blipping on downshift is possible. At the extreme high end that is exactly what a Formula One gearbox does. F1 cars don’t use a clutch to change gear at all. The computer system matches the revs exactly on shifts and the gears just slide between ratios. A clutch is only needed when the car is starting from stationary.
(N.B., “heel-toe” is a race driving technique for rev-match downshifting.)
…in ordinary street driving, in a car with synchronizers, there’s no real reason to heel-toe downshift between consecutive gears, or to downshift through all the gears.
On track (and conceivably on the street) heel-toe is used when you have to accomplish a large drop in speed, like at the end of a straight leading to a low-speed turn. You’re coming out of, say, fifth gear, and the corner calls for second gear, dropping from 130 mph to 40 in a couple of seconds. A lot of stuff is happening all at once, and if you were to let the clutch out in second gear just a little too early, you could over-rev the engine or cause rear wheelspin.
To be more specific for this OP, if you take your foot completely off the gas while braking from high speed in preparation for a downshift, the throttle will drop to near idle speed. The engine speed you will need for the lower gear will be considerably higher; the natural engine speed for the wheels’ speed may be 3,000 RPM or higher. If you simply release the clutch with the engine at, say, 1,000, the wheels will be trying to speed up the engine, which can cause them to lock up (i.e., wheel spin), possibly causing a skid, or over-rev the engine, which can damage it. Even if neither of those things happens, you’re putting serious and unnecessary strain on the clutch.
If instead you blip the throttle to slightly above the right engine speed for the current wheel speed and let the clutch out just as the revs drop to match wheel speed (rev-match), you can make a smooth transition from braking to acceleration without putting undue strain on the engine or clutch.
If you’re really good, you can slip into the lower gear without using the clutch at all. (Do not attempt! Professional driver on closed course.)
This goes back to pre-synchromesh. I believe the full sequence is
Depress clutch
Shift to neutral
Release clutch (the clutch plates are already spinning at about the same rate)
Rev engine to the speed of the downshifted gear
Depress clutch (in theory, if the engine is at the EXACT speed of the new gear you could slip it into the new gear without depressing the clutch and skip step 7)
Shift into downshifted gear
Release clutch in the new gear. (at this point clutch plates should be spinning at about the same speed)
Why? I had a British friend who was a performance driver and he’d double declutch (including heel and toeing) all the time and said it was easier on the synchros if done right. A cursory search seems to agree.
F1 cars use freakball engines with low rotating inertia and super precise electronic throttle control, which is part of what makes clutchless shifting possible. Here’s an old video of one that was programmed to play “God Save the Queen” (fast-forward to 1:00):
AIUI, the other thing that helps achieve clutchless shifting is having sloppy-fit splines on the gears and synchronizers. This makes for more lash in the drivetrain when taking your foot on/off the accelerator, but the increased range of angular slop provides more time for (dis)engagement of any particular gear when the input/output speeds aren’t absolutely perfectly matched.
