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I have a 2004 VW Passat with a manual transmission. I have heard that "blipping" the throttle can be a good thing to do in order to achieve smoother downshifts whilst driving. What does this mean? How do you do it? What are the advantages and disadvantages to doing it on a regular basis?

marked as duplicate by Pᴀᴜʟsᴛᴇʀ2, Bob Cross Jan 8 '16 at 23:37

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  • I have added more details to my question regarding the transmission and I don't think that driving conditions would make much of a difference anyway – Max Goodridge Jan 8 '16 at 20:35
  • Although I thought a manual transmission would be inferred because as far as I am aware you can't "blip" the throttle to achieve the desired effect of a smoother downshift – Max Goodridge Jan 8 '16 at 20:38
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    It does require a manual transmission. – DucatiKiller Jan 8 '16 at 20:47
  • If this question is truly about rev matching the transmission for quicker shifting, then it is off-topic because it's about driving technique. – Pᴀᴜʟsᴛᴇʀ2 Jan 8 '16 at 22:05
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    On that note, I think the increased fuel consumption would be almost negligible compared to the amount that you would reduce the wear and tear on the clutch over time by doing this – Max Goodridge Jan 8 '16 at 22:12
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In a manual tranmission car, the clutch is 'meshed' only when both sides of the clutch - the engine side and the wheel side - are running at the same speed.


Downshifting Example

I'll make up some numbers for an example: Say you're at 3rd gear, going 45 miles an hour and the engine is at 2.5k RPM. If you downshift while still going 45 miles an hour, the equivalent engine speed is now probably near 3-4k RPM for second gear at 45 MPH.

If you were to just naively disengage the clutch, switch to 2nd gear, and re-engage the clutch, all while keeping your foot off the throttle, then you've got a few things going on:

  • The engine speed is going to drop while you're making the switch
  • The new engine speed is going to be a lot higher than what it was before.
  • The clutch is going to have to do a lot of work to bring the engine speed up to where it needs to be.

So the chain of events goes something like this:

  • You're in 3rd, traveling 45 MPH and the engine speed is 2.5k RPM
  • You disengage the clutch
  • You start switching the gear to second gear
  • The engine speed drops to 1.5k RPM in the time it took you to change gears
  • The car is still traveling at 45 MPH
  • 2nd gear at 45 MPH wants the engine to be 4k RPM, but the engine is currently going 1.5k RPM
  • You start to engage the clutch
  • The clutch "grinds" a bunch while it brings the engine up to speed
  • The clutch stops grinding when its transferred enough energy to the engine to bring the engine up to 4k RPM.
  • You continue on your merry way.

The biggest problem is that when downshifting, your engine speed has to go up (a lot) in order to match the change in gear ratio, and the clutch has to be the guy to do that.


Blipping

Ok so with that out of the way, what is "blipping"? Blipping is known by a few other names - "burping" the throttle, rev-matching, etc, but it's all the same idea.

The idea is to manually raise the engine speed when downshifting so that the clutch has to do less work - if I disengage the clutch and give the engine some gas to manually bring it up to 4k RPM, then when I re-engage the clutch after switching to 2nd gear, then the clutch has to do very little work.

In this example, if the engine speed is already at 4k when I re-engage the clutch, then the clutch won't experience as much wear and tear, the shift can complete much sooner (important if you need to shift fast for some reason), and the ride is more comfortable because there's less of a jerk when the clutch re-engages.

All-in-all, rev-matching is a good thing for your car, but it does take some time to master. You have to develop an innate sense for what engine speed you need when you're in X gear and the car is traveling at Y speed; and then actually being able to do it in one quick procedure takes practice.


Competitive Contexts

Rally

In competitive contexts, rev-matching is done, but sometimes for other reasons. Keep in mind that when downshifting, the car might jerk a little as the engine is brought up to speed - if you're a rally driver and you're trying to be very careful about when you make demands of your traction, you might try to rev-match in order to not accidentally cause your traction to break loose, for example.

Power-shifting

Another related technique is power-shifting - applying full throttle while upshifting, ie, never releasing the throttle while trying to accelerate hard and switch gears. This is very hard on the clutch, gearbox, vehicle dynamics, but it can be helpful for your track times - during that split second while you're switching gears the engine is still making power - that power is being stored in the rising rotational energy of the engine. Then when the clutch is re-engaged, that stored energy is transferred to the vehicle, giving you a small boost. The end result is that you can accelerate just a little faster than if you let off the throttle.

Powershifting can have second-order effects too, for instance if the engine uses turbo chargers. Turbo chargers work by using the force of the exhaust gas to spin an impeller that then is used to force air into the engine. Turbo chargers are notorious for 'turbo lag' - they don't help a lot when the engine speed - and thus the volume of exhaust gas coming out - isn't that high. Turbos help make more power at higher engine speeds.

Well, if you are doing things like power shifting, then that helps keep the turbo 'spooled up' - itself turning at a high speed - so that it can be ready to help make more power for the engine.

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"Blipping" the throttle just means giving it a very short (half a second) stab raising the RPM for a short time.

It creates smoother down shifts because it raises the RPM to where it's going to be after the shift so the synchros have less work to do.

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    So you do use the throttle briefly as you bring the clutch up from the downshift? – Max Goodridge Jan 8 '16 at 20:05
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    press clutch, as your moving the shifter, blip the throttle to raise the rpm, release clutch. – rpmerf Jan 8 '16 at 20:17
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Blipping is just what @cdunn stated in raising the RPM for a short period of time ... very short. The main purpose of blipping the throttle is usually to reduce the idle speed of an engine. Sometimes an idle will be slightly higher than it would be (ie: idle at 900rpm instead of a customary 650rpm). By blipping the throttle you can sometimes get the engine to drop the idle down to where it's supposed to be. Really, you should only be doing it when you need to do it. The main disadvantage is you are wasting gas if you don't need to bring the idle down.

To my knowledge, there was never a correlation between blipping the throttle and shifting. There was however a procedure called double clutching, which involved shifting out of a gear into neutral, releasing the clutch pedal to bring down the front half of the transmission down to (or up to, depending on and upshift or downshift) engine speed, quickly pressing the clutch pedal again, shifting into the higher gear, then releasing the clutch pedal once more to gain further speed. This was how you shifted vehicles before modern synchronizes came about.

Blipping the throttle during shifting will have no effect on the transmission due to the fact when the clutch pedal is down the transmission is completely disengaged (power wise) from the transmission. There's no way it could affect it.

  • no way it could affect it Yes that is the case when the clutch is all the way down, but I think @rpmerf is talking about when the clutch is being released possibly – Max Goodridge Jan 8 '16 at 20:30
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    @MaxGoodridge - That verges into driving technique, but not only have I never had to do that before, there isn't a need to do that. You're just wearing your clutch out. That's what synchros are made to do is get your transmission parts to the same speed while shifting. Really it's a waste of time and fuel energy to do so. – Pᴀᴜʟsᴛᴇʀ2 Jan 8 '16 at 20:35
  • @Paulster - I think you've read the question wrong. This question is about RPM matching the two sides of the clutch while downshifting, not about attempting to lower the engine idle speed. – antiduh Jan 8 '16 at 21:48
  • @MaxGoodridge I am referring to blipping the throttle while the clutch is down/being released. heel/toe shifting. this matches the speed of the engine to the speed of the transmission before releasing the clutch for smoother shifts and less clutch wear. This does not effect the transmission/syncros. – rpmerf Jan 8 '16 at 21:57
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    @MaxGoodridge - heel-toe is very similar, but that's about managing all three pedals at the same time using two feet - gas to raise the engine speed, brake to slow the vehicle down, clutch to change the gear. What you're talking about in your original question I've always known as simply "rev-matching" and thus heel-toe would be rev-matching while also trying to brake. If you're just trying to downshift without braking, I'd just call that rev-matching. – antiduh Jan 8 '16 at 22:31

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