Question about rev matching in real life

[oohhh yeah]

When you downshift and you blip the throttle do you have to release the throttle?

So gas, *ok time to downshift*, clutch in, release gas, step on throttle, release throttle, step on throttle and engage clutch at the same time.

Or can you just do it all in one motion, like you keep on the gas the whole time while you let out the clutch? Do you guys get what I'm asking or is it too confusing?

 

[Famine]

Gas, clutch, gas, gear, clutch. It's about matching the coasting revs to the revs after you've selected the next gear.

As a dirty example, if you're changing from 4th down to 3rd and you're at 3krpm in 4th and will be at 5krpm in 3rd, you need to put the clutch pedal in, pull it out of gear, use the throttle to put the revs up to 5krpm, put it in gear and let go of the clutch. It's best to let off the throttle for the fraction of a second it takes to put it into gear and release the clutch pedal - unless your flywheel weighs nothing at all - to better control your acceleration after you've got drive back.

 

[Ibonibo]

Also you don't need to give "mid" gas with most cars because the gears have sycros.

Some older cars required to gas when clutching otherwise you would blow the gearbox (old fiat 500 i think didn't have syncro rings)

Is that what your asking or:

Full throttle, clutching, but not lifting your foot of the gas. That would not be good because you probably going to hit the rev-limiter a lot then.

 

[Famine]

Also you don't need to give "mid" gas with most cars because the gears have sycros.

Some older cars required to gas when clutching otherwise you would blow the gearbox (old fiat 500 i think didn't have syncro rings)

That's double-clutching (or double-declutching in the UK). It's a slightly different process:

Clutch pedal down, take out of gear, clutch pedal up, throttle, clutch pedal down, put in gear, clutch pedal up.

There's an extra declutch and an extra clutch in there compared to rev matching.


Rev matching simply makes for smoother gearchanges by preventing sudden weight transfer during gearchanges. It makes for a nicer ride for your passengers and a more stable car.

 

[oohhh yeah]

Gas, clutch, gas, gear, clutch. It's about matching the coasting revs to the revs after you've selected the next gear.

As a dirty example, if you're changing from 4th down to 3rd and you're at 3krpm in 4th and will be at 5krpm in 3rd, you need to put the clutch pedal in, pull it out of gear, use the throttle to put the revs up to 5krpm, put it in gear and let go of the clutch. It's best to let off the throttle for the fraction of a second it takes to put it into gear and release the clutch pedal - unless your flywheel weighs nothing at all - to better control your acceleration after you've got drive back.

Ah I see. But when you blip the throttle can you not release and just start to let out the clutch while the whole time your foot is slightly on the throttle? Or is that bad for the clutch?

Kind of like asking if I can shift without fully releasing the gas pedal. That would be bad for the clutch right? But wouldn't that make the car go faster because when the clutch engages the rpm is higher?

 

[Famine]

You can do that, but it'll achieve the exact opposite effect - it'll make your gearchange jerkier.

Essentially, when you're driving, the throttle tells the engine how much air to take in and it works out how much fuel to take in. If you say "more air!" by pressing the accelerator, the engine takes in more air and more fuel, more bangs happen which drives the crankshaft to spin faster. The crankshaft is directly connected, through the flywheel and clutch, to the input shaft of the gearbox, which drives the output shaft of the gearbox and that drives the wheels. At any given time, your roadspeed determines how fast the output shaft spins, which determines how fast the input shaft and crankshaft spin (at the same rate as each other, since they're connected line-astern). With the clutch pedal pressed, the crankshaft and gearbox input shaft are separated and can spin at different rates - now the throttle controls the crankshaft rotation (since it's free of the load of the drivetrain, it'll change speed faster) and the roadspeed controls the gearbox output and input shaft rotations.

The idea of revmatching is to get the crankshaft - controlled by you - spinning at the same speed as the input shaft - controlled by the road speed so that when the clutch is engage and they're brought back together, there's no sudden force and everything's smooth and jerk-free. If you keep your foot on the throttle, you're telling the engine "more air!" and that will result in the wrong amount of acceleration after the clutch is re-engaged and you'll get a little jolt.

Revmatching isn't really a sports/fast driving technique - it can be helpful stabilising a car under hard-braking while cornering (which shouldn't really be done together) and heel-toe is used for this purpose - rather a smooth driving technique. It's more about comfort and mechanical sympathy than pace. Double clutching (or double de-clutching) is similar and used to be sports/fast driving technique but it's redundant in modern cars with synchromesh gearboxes.

 

[homeforsummer]

There'd also be limited point to keeping your foot on the gas when rev matching as you slow down, as once drive is engaged the car would immediately want to speed up again. Dunno about anyone else, but when I'm slowing down towards a corner, I prefer not to get a sharp jolt of acceleration.

To be honest, even if you're rev matching for a quick downchange to overtake there's still limited point. You're better off just doing it smoothly and being smoothly back on the power just as you come off the clutch. Better mechanical sympathy, better for passengers and ultimately, probably quicker.

 

[Danoff]

Ah I see. But when you blip the throttle can you not release and just start to let out the clutch while the whole time your foot is slightly on the throttle? Or is that bad for the clutch?

You can do that, but it'll achieve the exact opposite effect - it'll make your gearchange jerkier.

...you'll get a little jolt....

Actually I think the jolt would go in the opposite direction. This is clearly a downshift-for-immediate-acceleration case, rather than a braking scenario (in which it would be silly to let the clutch out while on the gas and brake simultaneously).

Let's say you're traveling at 50 mph. You want to pass the guy in front of you so you go down a gear. Let's also say to maintain speed at 50 mph you need 10% throttle in the gear you're in, and 20% throttle in the gear you're going to. Otherwise engine braking will slow you down.

With the clutch in, you raise the RPM to the target for the lower gear, but you probably only needed 7% throttle to do that (or something less than cruising was requiring). Now as you release the clutch, you're going to start braking if you aren't on the gas. In fact, you have to increase throttle from 7% in order to avoid engine braking at speed.

So staying on the gas is actually smoother right? Or have I mixed something up?

 

[Famine]

This is clearly a downshift-for-immediate-acceleration case, rather than a braking scenario

I wouldn't recommend it - not least because that's poor roadcraft. Since the point of rev-matching is ride comfort, you don't really need to worry about it if you're about to stamp on the noisy lever

It's a largely redundant technique in any case. You can achieve similar results by treating the clutch pedal as progressively as you treat the other two, rather than an on/off switch.


Edit: No, I've thought of a couple of uses. Modern DSGs will give a throttle blip during a downchange (while the clutch[es] is[/are] disengaged) to revmatch - though they don't keep their foot in (so to speak). It'd also be useful on a high-revving car with a very light flywheel (or a DMF) prone to losing revs while coasting, but you're not looking at many cars there.

 

[stanepoke]

http://www.drivingfast.net/car-control/heel-toe-shifting.htm#axzz1bjMeAf1H

Good explanation of the Heel-Toe Technique on the page linked above with pictures and a wee video too:tup:

 

[Danoff]

http://www.drivingfast.net/car-control/heel-toe-shifting.htm#axzz1bjMeAf1H

Good explanation of the Heel-Toe Technique on the page linked above with pictures and a wee video too:tup:

Pretty sure this guy is not talking about braking at all. So Heel-Toe doesn't apply. If he's talking about braking, it's a very strange question.

 

[wfooshee]

Downshifting while slowing, Famine explains it well. Clutch, bump the throttle for some revs while selecting the next gear down, and releasing the clutch as the engine coasts down to the revs needed for the gear. Done well, there is no lurching forward or dipping the nose as you shift. If you're stopping, don't bother with all of that. It's unnecessary and annoying to your passengers. Just coast, depress the clutch as the car slows, and get neutral for while you sit at the light. If you're slowing for a turn, again don't worry about every gear on the way down from 5th to 2nd. Slow, use the clutch, shift straight to second, give it enough gas to come out of the turn and release the clutch, drive on your merry way.

Downshifting to drop a gear and nail it, it's still pretty much the same, but no "coasting." Once you've learned the car's rev ranges and gearing, you can downshift while giving throttle, and nailing throttle as the clutch is engaged, and accelerate away, on your passing maneuver or whatever.

While learning, always complete your shifts before giving a lot of throttle. Your (or whomever's) car and clutch will last longer, and people won't begin to fear riding with you. As you get experience, you will develop muscle memory and a feel for where the revs are for each gear, how much space there is if you drop one gear or two gears, or even three gears. Don't try to watch the tach and think, "I was at 2500 RPM, and I need to be at 5,000 when I release the clutch." You'll drive into something that way. Use your ears, and listen to the engine.

Most importantly, you don't want to slip the clutch very much, which is the whole point of rev-matching. If the engine is turning quite a different speed from what it would while in the gear you're selecting, then the clutch has to absorb that difference through friction, and thus wear. If the engine's way too slow, letting out the clutch drags it up to speed, and jolts the car a bit as if the brakes had been tapped. If the engine's way too fast, then the clutch is slipping to drag the engine down to speed, and if severe enough, snapping everyone's neck back as the car lurches forward.

All this assumes a traditional H-pattern stick. Sequentials are different. And there are a few sequentials out there with clutch pedals, not just paddle shifters.

 

[oohhh yeah]

Great answers guys. But I don't think I asked my question clearly enough

Yes this is H pattern gearbox, and I am wondering about downshifting for passing power. I understand toe heel, but I still don't understand the throttle thing.

Let's say you wanted to do a 1/4 mile run. And you're not worried about the longevity of your car. Would it be faster to shift with your right foot glued to the floor for the entire run? Or at least not let off gas all the way. Since when the clutch hooks the engine is already revving high so it'll make the car go faster. This is bad for the clutch but it will make the car go faster right?

So basically this is my question of "if you keep your foot down after rev matching" just explained differently.

 

[nk4e]

So during downshifting its, clutch, tad bit of gas, into gear, gas?

 

[Danoff]

Let's say you wanted to do a 1/4 mile run. And you're not worried about the longevity of your car. Would it be faster to shift with your right foot glued to the floor for the entire run? Or at least not let off gas all the way. Since when the clutch hooks the engine is already revving high so it'll make the car go faster. This is bad for the clutch but it will make the car go faster right?

So basically this is my question of "if you keep your foot down after rev matching" just explained differently.

That's actually much less clear. You shouldn't be downshifting during a quarter mile run. There will be no rev-matching with the throttle. Regardless, I believe the answer to the question you asked is no, you should not keep your foot on the throttle throughout the quarter mile. I'm sure somehow it slows you down to bang the rev-limiter while the clutch is in because when you dump the clutch there's bound to be a slowing hiccup from something or other. (Also terrible for the car)

 

[Famine]

So during downshifting its, clutch, tad bit of gas, into gear, gas?

Yep.

That's actually much less clear. You shouldn't be downshifting during a quarter mile run. There will be no rev-matching with the throttle. Regardless, I believe the answer to the question you asked is no, you should not keep your foot on the throttle throughout the quarter mile. I'm sure somehow it slows you down to bang the rev-limiter while the clutch is in because when you dump the clutch there's bound to be a slowing hiccup from something or other. (Also terrible for the car)

Yes to all of that too.

 

[wfooshee]

Since when the clutch hooks the engine is already revving high so it'll make the car go faster. This is bad for the clutch but it will make the car go faster right?

3 or 4 times, yes. Then the clutch will refuse to hold when any power is applied, as it continues slipping and grinding and burning its way into oblivion.

Clutch-slippage is not the way to make a car accelerate faster. It's very very very very bad for everything involved! There will obviously be some slippage at launch, but even then you want the clutch fully engaged (pedal fully released) as soon as it's possible to be that way. Do NOT slip the clutch during shifts, breathe the throttle as you upshift.

Purpose-built drag racers are different. They have multi-stage clutches that are designed to slip and increase their grip during the run. If you listen to a Top Fuel car you notice a fairly steady engine note. The engine is running at the max torque RPM as the clutch stages run through their programming to grap more and more as the car accelerates. Of course, those guys rebuild entire cars between every run, too.

 

[oohhh yeah]

So during downshifting its, clutch, tad bit of gas, into gear, gas?

Damn I made the question even more confusing.

Basically, the 1/4 mile example was just to ask the question in a different way.

Basically the question is if you can engage the clutch while you're on the throttle the whole time. Not standing on the gas, but on the gas to hold it at whatever downshift rpm it needs to be at. So let's say I need to be at 4k revs in the next gear down.

So after I blip the throttle to 4k, do I have to release the throttle all the way, and then engage the clutch and throttle like normal? Or can I just keep the throttle at 4k revs and let out the clutch? (This is describing a downshift for power)

Sorry for all the confusion:tup:.

 

[Famine]

So after I blip the throttle to 4k, do I have to release the throttle all the way, and then engage the clutch and throttle like normal? Or can I just keep the throttle at 4k revs and let out the clutch? (This is describing a downshift for power)

No. While the clutch plates are separated, the throttle governs the engine (crank) speed. When they are not, the roadspeed governs the engine (crank) speed - by driving the gearbox output shaft which drives, through gearing, the input shaft which is joined to longitudinally (through the clutch) and rotates at the same speed as the crankshaft. If you keep your foot pressed down during the gearchange you're trying to drive the crank faster than the output shaft, resulting in clutch slipping - exactly the opposite purpose of rev-matching which is an attempt to bring the crank to the expected speed after the gearchange before you bring it together with the input shaft so that the clutch doesn't slip.

More importantly, the throttle isn't a device with a set progressive input resulting in a set progressive output. It's an air inlet switch. 0% travel is no air, 100% travel is as much air as it can take in. How much fuel added to the engine this results in - and as a result how much power you make and how much acceleration you get - depends on hundreds of factors, not least of which are local air density, current fuel:air ratio settings (is it running rich or lean? Is it designed to do so during certain throttle conditions), loading (look at the power curve, torque curve, current engine speed in gear, current speed) and you get the idea.

In any case, to peg the engine at 4krpm in neutral requires almost no throttle travel. To do it in 3rd might require none (you're going downhill at... errr... 85mph?) or loads (you're doing 25mph uphill). Simply keeping your foot in during the change might overestimate or underestimate how much throttle you need after the gearchange, but it will result in a slipping clutch. Which is bad for everyone.


In fact, if you sit back and analyse this like that, you'll be utterly amazed just how much driving you do with muscle memory and how many parameters you subconsciously monitor. Scary, isn't it?

 

[nk4e]

So muscle memory isn't a term from the Fighting Genre...YAY!

 

[Kievit]

OP, you recently got your driver's licence? I did the same. Enjoyed learning new driving technics eventhough they aren't very usefull (in normal driving anyway), it did add to the driving pleasure. After rev matching and heel-toe, I learned to shift without using the clutch (up AND down).

Just don't try the latter on a car you like/don't want to wreck, because the gearbox dousn't like the learning proces Have fun

 

[oohhh yeah]

In any case, to peg the engine at 4krpm in neutral requires almost no throttle travel. To do it in 3rd might require none (you're going downhill at... errr... 85mph?) or loads (you're doing 25mph uphill). Simply keeping your foot in during the change might overestimate or underestimate how much throttle you need after the gearchange, but it will result in a slipping clutch. Which is bad for everyone.


In fact, if you sit back and analyse this like that, you'll be utterly amazed just how much driving you do with muscle memory and how many parameters you subconsciously monitor. Scary, isn't it?

So theoretically let's say 4k is the perfect rpm, and I blip to 4k exactly. And let's pretend I'm god and I won't overestimate or underestimate how much throttle after the gear change.

So the clutch will slip if I keep my foot in? So the only way to not slip the clutch is if I fully release throttle and *clutch out throttle in. using the typical timing method*?

But I thought the clutch ALWAYS slipped a bit. Because in city driving when you're not hard on the gas you're going to HAVE to slip the clutch. The more gas you give the faster you can release the clutch (less slippage) and vice versa.

 

[Famine]

It always will, a bit. Unless you can control the throttle opening by tenths of a percent (you can't) and account for all of the hundreds of atmospheric and mechanical factors (you can't), you won't perfectly match the rotational speed of the crank to the rotational speed of the input shaft.

The point of rev-matching is to bring the crank up to approximately the post-change speed as near as you can manage, to minimise the slip and make life smoother for your passengers and drivetrain. Trying to spin the crank faster than the input shaft while the two are separated acheives the exact opposite - so you may as well not bother trying to rev-match in the first place.


If you film your feet as you drive along, you'll notice that you always feed out the throttle as you feed in the clutch and then feed in the throttle as you feed out the clutch - and that after a gearchange you're always providing exactly the right amount of throttle to maintain the engine at the new crankshaft speed for the same roadspeed. All that rev-matching does is add a brief throttle press (your big ol' flywheel will take the first couple of tenths of a second of it, and keep it spinning a couple of tenths after you've let go) just to smooth out the violence of bringing together two spinning rods at different speeds. It's not about performance driving.

 

[oohhh yeah]

It always will, a bit. Unless you can control the throttle opening by tenths of a percent (you can't) and account for all of the hundreds of atmospheric and mechanical factors (you can't), you won't perfectly match the rotational speed of the crank to the rotational speed of the input shaft.

The point of rev-matching is to bring the crank up to approximately the post-change speed as near as you can manage, to minimise the slip and make life smoother for your passengers and drivetrain. Trying to spin the crank faster than the input shaft while the two are separated acheives the exact opposite - so you may as well not bother trying to rev-match in the first place.


If you film your feet as you drive along, you'll notice that you always feed out the throttle as you feed in the clutch and then feed in the throttle as you feed out the clutch - and that after a gearchange you're always providing exactly the right amount of throttle to maintain the engine at the new crankshaft speed for the same roadspeed. All that rev-matching does is add a brief throttle press (your big ol' flywheel will take the first couple of tenths of a second of it, and keep it spinning a couple of tenths after you've let go) just to smooth out the violence of bringing together two spinning rods at different speeds. It's not about performance driving.

What do you mean when you mentioned "big ol' flywheel will thake the first couple tenths of it"?

Also, I now understand what you meant in the bolded. But what if you DON'T spin it faster while it is separated? And the second part in bolt is exactly what my original question was. Do I have to feed out the throttle as I feed in the clutch and then feed in the throttle as I feed out the clutch, or can I just keep my foot down after blipping(but not revving it excessively high). Will that make the clutch slip more?

Thanks man

 

[Famine]

What do you mean when you mentioned "big ol' flywheel will thake the first couple tenths of it"?

Between the clutch and the engine block, attached to the crankshaft, is a flywheel. Depending on the car, this thing can weigh several pounds - my MX-3 one is a lightweight affair at just 8lb and that's standard.

The flywheel always provides resistance to acceleration. Give it gas in neutral and you'll see the revs rise a couple of tenths of a second afterwards, because you have to provide enough energy spin up the flywheel too. Let off and the revs will drop off a couple of tenths of a second afterwards as the flywheel loses energy.

The lighter the flywheel, the more responsive the engine is to acceleration but the more prone it is to losing engine speed. Heavier flywheels make engines harder to accelerate but they don't drop off so easily either.

Do I have to feed out the throttle as I feed in the clutch and then feed in the throttle as I feed out the clutch

Yes. That's normal driving.

or can I just keep my foot down after blipping(but not revving it excessively high). Will that make the clutch slip more?

That'll make it slip more and undo the good work you did in the middle of the change.

If you're downshifting to overtake right now, absolutely keep your foot in ("flat shifting"), but that's relatively poor roadcraft as all overtakes should be planned. That's for a different thread.

 

[Danoff]

or can I just keep my foot down after blipping(but not revving it excessively high).

I think actually that the answer to this is yes (Famine knows more about this than I do, so he can correct me). Try to imagine your gear change in terms of 3 throttle settings. I'm making these numbers up to help the discussion.

Cruise - 30%
Rev-matching - 5% (hypothetical perfect throttle with clutch-in to rev match)
Downshifted Cruise - 40%

You're asking if you can keep your foot at 5% while you let the clutch out in the downshifted gear. The answer is yes, and it's fine, but it's not enough. You actually need to feed in to 40% throttle in order to not slow down once the clutch comes out. So to do the shift right, you never need to let off the gas entirely. You need to reduce the throttle very quickly from 30% to 5% while punching the clutch to the floor. Then feed in from 5% to 40% while letting the clutch out.

What this will look like if you film your feet moving around is you letting off the gas when the clutch goes in, and getting back on the gas when the clutch comes out. What's actually going on is that you're rev-matching in between and never off the gas.

*waits for Famine to tell me I'm wrong*

If you do it right, your RPMs go immediately to the target value the moment the clutch goes in, and stays pegged there all the way through clutch out. If you do it wrong, you'll either over-rev and feel the car lurch as the clutch slips out, or you'll slow down after the shift.

 

[Grayfox]

I have issues with downshifting being new to manual.

 

[nk4e]

I have issues with downshifting being new to manual.

What I did is practicing while the clutch is engaged. ( Parked and in a empty lot)

 

[Grayfox]

Well i don't take my foot of the brake so i don't relly rev before i downshift, i just bring the clutch out slowly so i build RPM.

 

[nk4e]

When I down shift its: ( I am usually running around 2k-3k)
Clutch in, shift down, rev a bit, let go of the clutch and move on to the gas.

I am currently trying to rev match...