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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Tracking, Autocrossing, Dragstrip, Driving Techniques > Rev matching on ZF auto trans?



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      06-04-2012, 10:50 PM   #1
Meeni
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Rev matching on ZF auto trans?

Hi all,

To my despair, I have an auto transmission. It is the 330i ZF auto trans. It does an ok job, and I do not have to complain too much about it as long as it is in sport mode (anyway I set it to semi auto almost all the time).

However, it gives me a hard time revmatching. I have always been doing heel-toe on my previous cars, but it seems to not work well with this one (my first auto). The delay between the action on the lever and the actual downshift is anything between .3 to a full second. It is unpredictable, and makes "blipping" very hard. I resorted to toe-heel (heel on the brake, toe on the gas steady at 4k5 rpm), which does a great job of rev matching but also adds significant torque on the rear while braking and probably increases significantly stopping distance.

What do you fellow AT inmates do as revmatching?
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      06-04-2012, 11:17 PM   #2
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I've never seen a need to rev match an auto trans but if it works for you then fine.
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      06-05-2012, 11:25 AM   #3
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There really is no point on an auto trans, since you aren't engaging syncros like that on a manual trans. you are probably just wasting time.
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      06-18-2012, 12:39 PM   #4
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Auto isn't supposed to rev match

Get a DCT or a Manual
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      06-19-2012, 01:28 PM   #5
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There is a benefit it rev matching an auto. There is a speed difference to be made up right? The clutch packs on the gears take the ware just like a syncro does on a manual. During hard braking, it will shift-lock if you downshift too early. I was under the impression that the all autos in the e90 did it automatically. The last 328 loaner I drove definitely rev-matched on the downshifts, but those have GM transmissions. Not sure if the ZF has that ability.
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      06-20-2012, 03:49 PM   #6
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The DCT trans rev match, not auto trans. An auto trans has a torque converter but a DCT does not so rev matching helps reduce clutch wear. There is no advantage in a auto trans to rev match.
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      06-20-2012, 04:21 PM   #7
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Quote:
Originally Posted by chad29 View Post
The DCT trans rev match, not auto trans. An auto trans has a torque converter but a DCT does not so rev matching helps reduce clutch wear. There is no advantage in a auto trans to rev match.
Not entirely true. If you're braking at the threshold and downshift the rear tires will lock. The torque also converter does have a lock up mechanism. It's a wet clutch pack just like DTC's clutch packs and it slips just like any clutch. There are also clutch packs on the gears as well; that's how they engage. I've driven an E90 328 that did rev match. The Corvette, Camaro, Q7, 370Z, and F10 528 are cars that I have personally driven that have rev matching autos. None of those have DTCs.
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      06-22-2012, 12:21 PM   #8
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Rev matching on an auto trans does NOT bring the internal gears up to the same speed like it does in a manual trans. A true auto trans uses planetary gears.

A DCT is two manual trans in a single case with a computer controlled clutch - thus rev matching is useful in a DCT unlike in a fully auto trans with a torque converter. There is no reason for an auto trans to be locking the rear tires on downshifts under hard braking.

If you're using neutral to "rev match" in a fully auto trans, you're not rev matching at all, you're using the torque converter to cushion the downshift when you place it back in gear. Closed throttle downshifts should not be harsh if the trans is programmed properly.

With modern 4+ speed auto trans the gear spacing is much closer than with older 3-speed auto trans. As a result there is even less reason rev matching, but it makes enthusiasts think they are driving a real "race car", aka marketing at it's best.

If "pretend" rev matching of a fully auto trans makes you happy, then so be it. There is no way that you are actually rev matching however because of the internal design of a true auto trans.

Last edited by chad29; 06-22-2012 at 12:28 PM.
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      06-22-2012, 12:39 PM   #9
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You guys are forgetting that the torque converter has a lockup mechanism which IS a clutch. In a manual or DCT the internal speed difference is also taken up by the syncro only regardless if you rev match or not (assuming your aren't double-clutching). If you are braking at the limit it takes very little additional decel to lock the rear tires. When the lockup mechanism engages, there is decel from trying to spin the engine up. It's physics.
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      06-22-2012, 02:44 PM   #10
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Quote:
Originally Posted by e90pilot View Post
You guys are forgetting that the torque converter has a lockup mechanism which IS a clutch. In a manual or DCT the internal speed difference is also taken up by the syncro only regardless if you rev match or not (assuming your aren't double-clutching). If you are braking at the limit it takes very little additional decel to lock the rear tires. When the lockup mechanism engages, there is decel from trying to spin the engine up. It's physics.
Not forgetting anything. The torque converter clutch disengages when you shift into neutral to "fake rev match", so all you have is a fluid coupling when you go back into a lower gear. This cushions the engagement.

The reason why the rear wheels will tend to lock under extreme braking when you downshift is because you are downshifting a little too soon when the vehicle speed is not appropriate for the lower gear you are selecting. The proper technque is to wait a half-second or so longer until the vehicle speed is correct for the gear and then just downshift instead of shifting into neutral and buzzing the engine for nothing.

Remember:

Brakes are fow WHOA'ing

Engines are for GO'ing

Don't use the engine for braking if you want the best performance. Any additional braking of the rear wheels by the engine can be obtained from the brakes and this is far better for the engine and weight balancing of the chassis.

It's worth noting that rev matching is actually intended to match the trans GEAR speeds so that you can select a lower gear, without grinding. In a dog style trans without syncros this is the ONLY way you can actually shift the trans. In a syncro trans rev matching reduces syncro wear. In no case is rev matching intended to compensate for down shifting when the vehicle speed is too high for the gear you are selecting.

Last edited by chad29; 06-22-2012 at 02:51 PM.
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      06-22-2012, 03:54 PM   #11
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That last part is not correct. Rev matching does not reduce syncro ware nor does it prevent grinding in non-syncro gearboxes. If you want to match gear speeds you need to double-clutch. Think about it. If you rev the engine with the clutch in, you are spinning the engine and flywheel only. That's it. So how is it possible to match internal gear speeds if the engine is not connected to the gearbox? If you want to match gear speeds you need to engage the clutch in neutral while you blip. That's double clutching. Rev matching in racing has only one purpose: preventing shift locking. I do it in street driving because is sounds cool. You could say it helps the clutch last last longer but I'm not sure how significant it is.

My brother used to race in the Skip Barber Pro Series when they still ran it. Those cars had sequential dog boxes by Hewland. In order to engage a gear all you had to do was unload the gearbox. Just pre-load the shifter and smack the rev limiter to go up (there was no automatic ignition cut) or lift slightly. No need to use the clutch. The pro car was unusual in the sense that rev matching was not required while downshifting in the dry. Just push the gear handle forward and that was it. Great for left-foot braking. In the wet, however a blip was needed. Most drivers would use the clutch and heal-toe method. If you were good you could execute this without the use of the clutch. Keep your left food on the brake and use your right foot to blip the throttle. Even with a sequential box the car is in neutral momentarily and with a mechanical setup like the pro car, you could feel it through the shifter.

Weather you use it or not, engine braking occurs. In fact one of the many dials on an F1 car's steering wheel is "Engine brake." They don't use it as a primary means of slowing down, but more as a tool to help point the car when lifting. You can actually dial in more or less lift-off oversteer with the engine's software.

The torque converter on the 3 series really likes to stay locked. I've actually drivin some autos the bark up-shifts. If the torque converter locks to quickly it will lock the tires if you're braking hard. This is why a lot of manufactures are making autos that rev match. You can complete the downshift quicker and smoother that way.
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      06-22-2012, 03:57 PM   #12
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Quote:
Originally Posted by chad29 View Post
Rev matching on an auto trans does NOT bring the internal gears up to the same speed like it does in a manual trans. A true auto trans uses planetary gears.

A DCT is two manual trans in a single case with a computer controlled clutch - thus rev matching is useful in a DCT unlike in a fully auto trans with a torque converter. There is no reason for an auto trans to be locking the rear tires on downshifts under hard braking.

If you're using neutral to "rev match" in a fully auto trans, you're not rev matching at all, you're using the torque converter to cushion the downshift when you place it back in gear. Closed throttle downshifts should not be harsh if the trans is programmed properly.

With modern 4+ speed auto trans the gear spacing is much closer than with older 3-speed auto trans. As a result there is even less reason rev matching, but it makes enthusiasts think they are driving a real "race car", aka marketing at it's best.

If "pretend" rev matching of a fully auto trans makes you happy, then so be it. There is no way that you are actually rev matching however because of the internal design of a true auto trans.


I'm surprised someone on this community knows this.
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      06-28-2012, 04:40 PM   #13
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e90pilot-

Just buzzing the engine with the clutch in or the trans in neutral is not "rev matching"...

I'm sharing so you understand the technical issues involved and why there is no such thing as rev matching a fully auto trans and why you should not do what you are doing with a fully auto trans vehicle. The proper technique is to wait until the vehicle speed matches the lower gear you intend to downshift too.

It's OK if you don't understand or don't believe. It is what it is. The information presented was to help you. What you do with it is entirely up to you.

Hack-

Thanks. It's nice that some folks "get it".

Last edited by chad29; 06-28-2012 at 05:28 PM.
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      06-28-2012, 05:41 PM   #14
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Quote:
Originally Posted by chad29 View Post
e90pilot-

I'm sharing so you understand the technical issues involved and why there is no such thing as rev matching a fully auto trans and why you should not do what you are doing with a fully auto trans vehicle. The proper technique is to wait until the vehicle speed matches the lower gear you intend to downshift too.

It's OK if you don't understand or don't believe. It is what it is. The information presented was to help you. What you do with it is entirely up to you.

Hack-

Thanks. It's nice that some folks "get it".
I get it just fine. What you are saying about manuals is WRONG. Rev matching does not match internal gear speed. You obviously didn't read my post. When downshifting waiting until you reach the proper rpm in the next gear won't work because the N52's engine revs fall faster than you can slow down. If you were refering to an auto you can't do this either because if you select the next lowest gear in an auto it just completes the shift. If you put it in neutral then wait it still won't work because you can't select the gear you want until you put it in manual mode.

What I was saying is that there IS a benefit. Most new automatics (not DCT or F1-style automated manuals) DO REV MATCH. I never said that you should try it if the car doesn't do it for you. I'm not sure why your being a dick about it. I always thought that the autos in the e90 already did it for you. There is nothing you are helping me with.
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