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Old 03-06-2013, 12:57 AM   #1
jadawgis732
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Default Question about Rev Matching

Playing around on HowStuffWorks I saw this page, which provides a very good overview of clutch operation. The first little flash app on that page shows the strain put on the clutch by a flywheel. The faster spinning wheel, the flywheel, is connected to the engine. So when I rev match while downshifting, isn't the flywheel going to speed up and therefore cause more friction on the clutch? Isn't that bad? Could someone explain why I'm wrong.
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Last edited by jadawgis732; 03-06-2013 at 01:13 AM. Reason: I think NASIOC hyperlink colors should contrast better with standard text.
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Old 03-06-2013, 01:07 AM   #2
WillGeorge
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I can't answer this but want to know the answer too lol.
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Old 03-06-2013, 01:18 AM   #3
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When you rev match by tapping the throttle. the flywheel will speed up to match the gear you are in. When you rev match properly, there is very little clutch wear because you're not slipping the clutch. It's just like when you upshift and let the RPMs drop to the right level before letting out the clutch, but in the other direction. Downshifting without rev matching requires you to slip the clutch because it forces the engine to match RPMs.
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Old 03-06-2013, 01:33 AM   #4
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Need clarification. Engine speed != flywheel speed? And does the clutch match the road speed or what? I thought if I disengage the clutch the clutch is not spinning at all.
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Old 03-06-2013, 01:37 AM   #5
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From what understand you need to bring the revs up according to gear ratio.
Ie... If you're at 3400 in second you need to bring it to 4800 to get into first.

I am still trying to wrap my heard around it.
And my feet. My brain shirts out when attempting heel toe.
My muscle memory needs to learn it.
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Old 03-06-2013, 02:44 AM   #6
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Quote:
Originally Posted by jadawgis732 View Post
Need clarification. Engine speed != flywheel speed? And does the clutch match the road speed or what? I thought if I disengage the clutch the clutch is not spinning at all.
Your flywheel is connected directly to the crankshaft so flywheel RPM = Engine RPM

Your clutch disk is attached to your transmission input shaft, so when the clutch is engaged, your clutch RPM = flywheel/engine RPM. When your clutch is disengaged your clutch RPM is 0 (or at least spinning down towards zero due to friction/resistance of your tranny spinning with no input power.

To 'rev match' you need to bring your engine to whatever RPM your next gear will be at for whatever speed you are going.

When you upshift this happens naturally - if you time it right, as your engine will be spinning less RPM in the next gear up, and engine RPMs fall as you shift.

To downshift you are in reverse of this so you have to match your transmission RPM to your newly needed engine RPM for the next gear down.

So - for example say you want a 3-2 shift. You are going 25 MPH in 3rd at 2500RPM (for the sake of simplicity I am going to use round numbers) you need to now downshift by:

Clutch in, out of gear, into neutral
Clutch out, now linking the tranny any engine together, but out of gear
Rev match (in this case rev to about 3500RPM)
Clutch in when desired RPM is attained
Select 2nd
Clutch out

This will allow you to get into second easier as the tranny/syncros are already spinning what they need to be for this selection - if you do this without the double clutch action you can actually HEAR the syncros working to spin up (at least I can in my noisy tranny in the STi lol)

Hope that helps
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Old 03-06-2013, 06:58 AM   #7
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^Great description. He's got to be able to understand that.
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Old 03-06-2013, 07:27 AM   #8
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bizzaro is right.

The flywheel is always at engine speed because it's bolted to the crank.

When the clutch is pushed in, it is still connected to the transmission input shaft, so whatever speed the wheels are turning, (through gearing), that's what the clutch is spinning at.

To get the clutch to go to zero, you'd have to have the clutch in and the tranny in neutral.

When you rev match, you disengage the clutch and downshift, throttle to get the engine speed (flywheel) up to match the clutch speed at its new, lower gear speed (faster), then let the clutch out. The speeds are much closer together this way and you get less lurching when you let out the clutch.

If you double clutch, you have an intermediate step where you are in neutral, leaving the clutch unpowered. After shifting to neutral, you let the clutch out, give it throttle to bring the clutch to engine speed as well as the input of the transmission and it's gear, then clutch in, shift and clutch out. This gives you gear speed matching which reduces synchro wear and makes the shift less jarring.
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Old 03-06-2013, 05:58 PM   #9
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Oh, okay, so that's why people double clutch. Thanks!
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Old 03-07-2013, 03:14 AM   #10
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Is this what people are referring to when they talk about synchros or syncromeshes- the ability to match clutch speed and engine speed simply by shifting into neutral?
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Old 03-07-2013, 11:39 AM   #11
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Could someone possibly post a video of this with the STi transmission? I'm curious as to what engine speeds I might be looking at when it comes time for me to try this. Haha
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Old 03-07-2013, 11:44 AM   #12
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Quote:
Originally Posted by bizarro252 View Post
Your flywheel is connected directly to the crankshaft so flywheel RPM = Engine RPM

Your clutch disk is attached to your transmission input shaft, so when the clutch is engaged, your clutch RPM = flywheel/engine RPM. When your clutch is disengaged your clutch RPM is 0 (or at least spinning down towards zero due to friction/resistance of your tranny spinning with no input power.
This is only true if the tranny is in neutral. As Jack pointed out, if the tranny is in gear, then clutch RPM = tranny RPM ~= wheel RPM.


Rev matching basically comes down to this. When the clutch is disengaged and the tranny is in gear, your flywheel RPM = engine RPM and your clutch RPM = wheel RPM (times gear ratio). Rev matching is using the throttle to speed up the flywheel to match the clutch RPM, before releasing the clutch. If you do it right, there is zero wear on the clutch because the two are already spinning at the same speed when you engage the clutch.
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Old 03-07-2013, 11:46 AM   #13
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Originally Posted by Eternaltiger2858 View Post
Could someone possibly post a video of this with the STi transmission? I'm curious as to what engine speeds I might be looking at when it comes time for me to try this. Haha
Why do you need a video? If you're shifting from 3rd to 2nd, then you push in the clutch, accelerate the engine up to whatever RPM it needs to be for 2nd, shift to 2nd, and then release the clutch (the accelerating of the engine and the shifting to 2nd happen simultaneously, so it's really just: push in the clutch, blip the throttle while shifting to 2nd, then release the clutch).

What RPM does the engine need to be for 2nd? Well shift to 2nd and see. It depends on how fast you're going. It's more about feel and timing than anything else, which you can only get with practice.

Heel & toe just takes this one step further. In addition to all of the above, you are also on the brakes slowing down for the corner. So the process goes, use the left half of your foot to hit the brakes to slow down for the corner, push in the clutch, use the right half of your foot to blip the throttle while shifting to the lower gear, then release the clutch while still on the brakes. When you get to the corner and let off the brakes, you're already in the next gear and ready to accelerate out. Don't try this until you get your rev matching down.
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Old 03-07-2013, 12:14 PM   #14
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Originally Posted by the suicidal eggroll View Post
Why do you need a video? If you're shifting from 3rd to 2nd, then you push in the clutch, accelerate the engine up to whatever RPM it needs to be for 2nd, shift to 2nd, and then release the clutch (the accelerating of the engine and the shifting to 2nd happen simultaneously, so it's really just: push in the clutch, blip the throttle while shifting to 2nd, then release the clutch).

What RPM does the engine need to be for 2nd? Well shift to 2nd and see. It depends on how fast you're going. It's more about feel and timing than anything else, which you can only get with practice.

Heel & toe just takes this one step further. In addition to all of the above, you are also on the brakes slowing down for the corner. So the process goes, use the left half of your foot to hit the brakes to slow down for the corner, push in the clutch, use the right half of your foot to blip the throttle while shifting to the lower gear, then release the clutch while still on the brakes. When you get to the corner and let off the brakes, you're already in the next gear and ready to accelerate out. Don't try this until you get your rev matching down.
Wouldn't that actually wear out the clutch more since you're actually spinning the clutch disc at high rpm? I thought the better way to do it was the double clutch/heel-toe because it will let you increase your engine rpm without spinning the clutch. Sorry I'm a noob at manual transmission so lets me know if I'm wrong there.
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Old 03-07-2013, 12:18 PM   #15
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The clutch doesn't care how fast it turns. The clutch cares about how fast it turns in relation to the flywheel as it clamps (ie, whether or not it is slipping). If you approximately match the engine speed to the clutch speed (which, when shifting, will be the input shaft speed of the transmission), the clutch will have a very easy time as it clamps the two together.
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Old 03-07-2013, 12:34 PM   #16
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Originally Posted by luan87us View Post
Wouldn't that actually wear out the clutch more since you're actually spinning the clutch disc at high rpm?
You are correct in that the clutch is spinning at a "high rpm..." but it is disengaged from the surface of flywheel during the "blip" of the throttle, therefore there is no "wear and tear" on the clutch surface.

Then when the clutch surface is reconnected to the flywheel surface (foot lifting off the clutch), the two surfaces are spinning at nearly the same speed so there will actually be less "wear and tear" on both the fly wheel and the clutch surfaces.
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Old 03-07-2013, 12:37 PM   #17
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Huh, when I heel toe/rev match (or this is at least what I thought I was doing) I push in the clutch, shift to the lower gear, then brake with my toe as I rev up with my heel so that there is no "jump" or braking effect when I let the clutch back out. I never knew about letting the clutch out in neutral first, before shifting to the lower gear. Or is this only when shifting into second and first???
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Old 03-07-2013, 12:48 PM   #18
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So if I do not double clutch while down shifting, that is bad?
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Old 03-07-2013, 01:00 PM   #19
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I never knew about letting the clutch out in neutral first, before shifting to the lower gear. Or is this only when shifting into second and first???
That's double-clutching. You can rev match or heel&toe with or without double-clutching.

I typically only double-clutch when going down 2 or more gears. If I'm just going 4th to 3rd or 3rd to 2nd then I don't bother, but if I'm going 5th to 3rd or 4th to 2nd then I do.
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Old 03-07-2013, 01:00 PM   #20
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Don't get confused with heel/toe and rev matching. Heel/toe is nothing more than using the brake while you are using the accelerator (for rev matching into a corner, for example). Forget about heel/toe for now.

It does not wear out the clutch at all because everything will be spinning the same speed when the clutch is engaged. The point of this is not to necessarily save your clutch, either - you can burn that guy up during launches if you want, but up/down shifting is a non-concern. The point is to drive smoothly and to save your transmission from having to do the speed matching using internal synchro's.

I think eggroll is missing a step in his description. Part of the process is to let the clutch out to engage the engine and transmission (in neutral) so that when you blip the throttle to your new speed you are bringing the transmission up to speed, too, and not just the engine. This is really the essence of speed matching double clutching. You will find that when you match the speed correctly that the gear will slide right in with no resistance at all. You should be in the habit of applying virtually NO pressure to downshift (or upshift for that matter). The resistance is the synchro working.

Basic rev-matching:
Step 1 - Push in clutch (dis-engage engine)
Step 2 - Shift to Neutral
Step 3 - Let clutch back out (engage engine and transmission)
Step 4 - Blip throttle (engine/clutch/transmission up to speed)
Step 5 - Push in clutch
Step 6 - Shift to new (lower) gear (LIGHT pressure!)
Step 7 - Let out clutch and accelerate

When you blip the throttle, go for something higher than the speed you need to give you time to shift into gear after things slow a little, don't be scarred to rev a bit high. Just put gentle pressure into the next gear and wait for it to tell YOU when the speed is matched - and it will drop right in.

Advanced rev-matching: ( Do not try unless you are confident!)
Step 1a - Push in clutch without letting up on throttle (or full throttle)
Step 1b - Light pressure on shifter to neutral - will pop out when ready
Step 2a - Let clutch back out (engage engine and transmission)
Step 2b - Wait a moment for engine to get up to speed
Step 3 - Push in clutch, off throttle (or if you're insane - don't)
Step 4 - Shift to new (lower) gear (LIGHT pressure!)
Step 5 - Let out clutch and accelerate

There is also a stupid-advanced version where you don't use the clutch in step 3-5 at all which does a good job of showing you that if everything is truly matched, then you can go into gear without disengaging anything if you're timing and speed matching is impeccable.

Last edited by CrashTestRacer; 03-07-2013 at 01:06 PM.
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Old 03-07-2013, 01:02 PM   #21
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Originally Posted by the suicidal eggroll View Post
That's double-clutching. You can rev match or heel&toe with or without double-clutching.

I typically only double-clutch when going down 2 or more gears. If I'm just going 4th to 3rd or 3rd to 2nd then I don't bother, but if I'm going 5th to 3rd or 4th to 2nd then I do.
Ahh, you are correct, then. They are separate concepts. In my 04 it is pretty much necessary to double clutch every downshift...
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Old 03-07-2013, 01:03 PM   #22
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Rev matching is used to reduce wear on the clutch and make downshifts smoother. Double clutching takes that one step further to reduce wear on the synchros as well. As you pointed out, when the synchros start to wear down double clutching will become more and more necessary when downshifting.
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Old 03-07-2013, 01:08 PM   #23
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I've found that in my 2010 wrx it's pretty difficult to double clutch. This is due to how far the clutch travels in my car. To rev match easier in my car I:
Engage the clutch
As I start lowering my gear from high to low, I blip the throttle
When the gear lever is in the correct gear I disengage clutch and press on the gas normally

If done correctly your car should not jerk, and it should not change speeds.

I only heel-toe when I'm braking during a turn and need to drop in gear before I exit.
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Old 03-07-2013, 01:30 PM   #24
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Originally Posted by abehanna View Post
You are correct in that the clutch is spinning at a "high rpm..." but it is disengaged from the surface of flywheel during the "blip" of the throttle, therefore there is no "wear and tear" on the clutch surface.

Then when the clutch surface is reconnected to the flywheel surface (foot lifting off the clutch), the two surfaces are spinning at nearly the same speed so there will actually be less "wear and tear" on both the fly wheel and the clutch surfaces.
Oh ok I was thinking of the friction point. Lol. So while we're on this topic am I correct to say as long as my clutch is down/disengaged completely I can just rev the car and it wouldn't do any harm to the clutch/drive train?

And yea it's very hard to do rev matching in my 2013 WRX because of how far the clutch travel. You will have to be very quick to get it right. I tried double clutch a few time going from 5th to 3rd and I missed half the time lol.
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Old 03-07-2013, 01:39 PM   #25
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Oh ok I was thinking of the friction point. Lol. So while we're on this topic am I correct to say as long as my clutch is down/disengaged completely I can just rev the car and it wouldn't do any harm to the clutch/drive train?
The clutch only gets worn when the flywheel and clutch disk are spinning at different speeds and the clutch is partially or fully engaged. If the clutch is fully engaged and not slipping, no wear is happening because everything is moving as one. If the clutch is fully disengaged, then no wear is happening because there's nothing touching the clutch disk to cause any wear, regardless of how fast/slow anything is splinning. However constantly riding the clutch pedal will eventually wear out the throwout bearing.
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