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Old 07-10-2005, 08:54 PM   #1
GravelRash
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Lightbulb Rev matching, double clutching, synchros

Rev matching...double clutching...syncros...wear...efficiency...this combo seems to have generated a weekly-thread-of-confusion for the last month or more. Most of those threads end up including some very knowledgable, and useful, entries - amidst the confusion.

So here's a synopsis of basic functional truths:
  • To start with, it's illegal in most states to coast with the car in neutral. As noted in other threads on this topic, if you have a sudden need for power to maneuver it won't be there. (These laws were probably written long ago, when even normal cars, let alone big rigs, couldn't always make it down steep grades w/o smoking their brakes.)

    The opposite end of the spectrum - downshifting through every gear as you come to a stop - is great practice if you're learning to rev match or double clutch, but outside of that isn't very efficient. The only true upside is that you'll always be in an appropriate gear should you need power to avoid an emergency: 2k revs in top gear isn't going to be optimum for that

    Coasting to a stop over a short distance? Probably not a big deal. Longer distances? Pretty worthless in most cases; see below...
  • Efficiency: in most fuel injected cars you'll waste gas - not save it - by going to neutral too soon, or by coasting in neutral. Why? In gear, with the throttle closed, fuel is usually cut entirely. Coasting in neutral? Engine is idling - which uses gas, and not very efficiently.
  • Basic driveline stuff: for this discussion we can break the drivetrain into 3 groups, 1) engine/flywheel/clutch pressure plate, 2) clutch disk (just "clutch" in the discussion below)/transmission input shaft (which the clutch disk is attached to)/any gears splined to the input shaft, and 3) transmission output shaft/any gears splined to it/diffs, driveshafts, wheels. (Note: yes, reverse gear requires another shaft and idler gear, there is a (or >1) layshaft involved, but all that's immaterial to this discussion.)

    Also note that in a modern ("constant mesh") transmission the gears themselves are always meshed; you're not engaging the actual gear teeth when you shift gears. Instead, one of each pair of gears is fixed to its shaft and can't rotate on the shaft, while the mating gear on the adjacent shaft spins freely on its shaft - until it's time to shift into the "gear" where that meshed pair of gears is used.

    When you shift into that "gear" the shifting forks move locking rings into engagement with the freely spinning gear of the desired gear pair, and at the same time make sure that all the other free gears in the other gear pairs are not locked to their shafts. The result is that there is only one meshed pair of gears where both gears are locked to their shafts at any one time.

    The locking rings (which may have other names) are splined to their shafts; when the locking dogs on the ring are slid into the mating engagement recesses of the freely spinning gear then that gear also becomes locked to the shaft.

    And this is where the syncros come in: they're mounted between the lock rings and the gears themselves. When the ring is being shifted into engagement with its gear the syncros, use spring loaded friction to speed up or slow down the shaft - and therefore the locking ring splined to it - so that the ring and gear are at or near the same speed and the locking dogs can engage the gear, instead just spinning against the gear and chewing at it.

    Driving: all 3 groups are tied together, the first 2 by the flywheel/clutch/pressure plate sandwich, that latter 2 by the single set of gears linked together for the "gear" you're driving in, one on the input shaft, one on the output shaft.

    Clutch disengaged: (clutch pedal pushed down) first group is disconnected from 2nd and 3rd groups, which are still connected.

    Neutral: 2nd group is disconnected from 3rd group; 1st and 2nd groups connected. If the clutch is also disengaged then all 3 groups are disconnected from each other.

    For diagrams and more detail see http://auto.howstuffworks.com/transmission.htm, which has links for more detailed stuff...and I'm sure there're lots of other good sites out there. Or you can just look in the Subie factory shop manual.
  • Rev matching: this alone has no effect, good or bad, on syncros; but it does save wear on the clutch and the rest of the drivetrain. By rev matching during a downshift you're providing exactly the same function - in reverse - as occurs naturally during a normal upshift.

    On the upshift, when you lift off the gas momentarily engine revs drop...and if you time it correctly the drop at least approximates the difference in engine revs between the previous gear and the one you just shifted up to. So when you let out the clutch the engine/flywheel, on one side, and clutch disc/transmission on the other side are spinning at (or close to) the same speed; no lurching, bucking, etc. as you release the clutch.

    On the downshift the "natural" factors that help the upshift are now working against you. So you have to supply the speed matching - by revving the engine - to get the same match across the clutch interface.

    If you don't rev match on the downshift, it's just like holding the revs at the same level during an upshift: the clutch has to slip, just like when starting out from a stop, until the engine speed and trans speed match.

    All of this has no effect on what happens when you move the shift lever between gears: the syncros take the hit, doing the internal speed matching between gear set in the trans - exactly analogous to what the clutch does between engine and trans when you don't rev match the shift. Since the clutch is disengaged while you're performing the gear shift you can't affect the internals of the trans, except via the shift lever.

    So what you gain by rev matching is smoother driving, and reduced clutch wear, plus reduced wear on the rest of the drivetrain if you weren't slipping the clutch much . Which brings us to...
  • Double clutching: all the factors discussed above for rev matching are exactly the same. The difference is that now you disengage the clutch - and shift to neutral instead of the next gear - and release the clutch. Now you're connected to the trans internals in the middle of the shift, and can affect the speeds of the internals. Now you can save wear on the clutch and the syncros and rings.

    Now you rev the engine - just like rev matching alone - only this time you're increasing the speed of the trans input shaft along with the engine/flywheel to match the appropriate rev level for your next gear. So when you disengage the clutch and move the shift lever...the syncros have little or no work to perform since the bits they're trying to "rev match" for engagement are already rev matched due to your skilfull revving while in neutral.

    So now the trans is seamlessly into the next gear - but you still need to perform the second, "external", rev match - the one you'd be doing with the clutch disengaged for a rev matching only downshift - before releasing the clutch to complete the shift.

    This is all complicated somewhat in practice by the fact that the time necessary to complete each of the 2 steps - even very quickly - means that the relative revs needed at each stage will vary depending on the decelleration if any - taking place during the shift. It's much easier to learn while traveling at a constant speed, in the mid range of both gears, e.g. on a slight downhill (so the car won't slow when rolling with clutch disengaged) shifting from 3rd to 2nd at ~35mph.

    It's significantly more difficult to get it all right downshifting under hard braking - which is where "heel and toe" comes in, outside the scope of this discussion - where the appropriate relative speeds you're trying to match are a rapidly moving target. But it's just a higher level of skill.

    Just to note, double clutching can also be applied to upshifting (but the rev relationship is reversed), but now we're definitely talking an area of diminishing returns with modern transmissions. However, if you drive a vehicle with a bad syncro(s) you can work around it on the upshift just like on the downshift.

    This time, with double clutching, the benefits are smoother driving, reduced clutch wear - and reduced syncro wear.

    Side note: for those who haven't already noticed, Steve McQueen double clutches all his up and downshifts in Bullitt. Very cool...
  • Wear and tear: Not downshifting as a method of saving wear on the drivetrain is theoretically valid, but in practice it will be unnoticeable. Really want to save wear and tear? Do one less hard launch and you'll save more than 5yrs of not downshifting!

    Do you disengage the clutch every time you slow down while driving, and engage it only to maintain speed or accelerate? I didn't think so And backing off the gas to slow for a turn, traffic, whatever, is no different than slowing - in gear - towards a stop. And the former action occurs orders of magnitude more often than the latter...unless you're trying to drive in Manhattan

    As described above, downshifting w/o rev matching results in increased clutch wear; the greater the difference in gear speeds, e.g. shifting from 5th to 2nd at 50mph, the greater the extra wear on the clutch. And if you "minimize clutch wear" by re-engaging it quickly after the downshift then you've only shifted the wear and tear to the rest of the drivetrain.

    Downshifting w/o double clutching results in increased syncro wear. Now, modern transmissions are - as noted many times - designed to shift w/o double clutching, with the syncros doing all the work of rev matching the trans internals. But syncros and rings are a wear item, just like a clutch, though - presumably - designed for much longer life.

    Don't want to learn to double clutch? Fine; it's not a sin...at least not a mortal one

    So why bother? less wear and tear as noted, it's one more way of being in tune with the functions of your car, and it can be an enjoyable driving skill that results in smoother driving. And last but not least it's a skill that can allow you to drive around trans problems that could otherwise make driving difficult if not impossible.
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Last edited by GravelRash; 07-23-2005 at 03:51 AM.
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Old 07-13-2005, 12:53 PM   #2
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Im not sure if it was discussed in your above post but the question I have that I have been searching for an answer for is this... While going through normal driving (say on the freeway with minor acceleration) One can drop the shifter out of gear and into another gear higher (2nd to 3rd) providing, of course, that the revs are matched. As I understand the definition of rev matching it has more to do with downshifting using the clutch and the gas pedal (usually during heel toe shifting) to give you the sensation that you really didnt even make a shift (if you are good enough). Now, how bad is it for the syncros, transmission to upshift w/o using the clutch?

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Old 07-13-2005, 12:58 PM   #3
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I've read many times, if done correctly, that its better for the synchos because they do less work when you rev-match and double clutch.
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Old 07-13-2005, 03:53 PM   #4
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Quote:
Originally Posted by blitzpb
Im not sure if it was discussed in your above post but the question I have that I have been searching for an answer for is this... While going through normal driving (say on the freeway with minor acceleration) One can drop the shifter out of gear and into another gear higher (2nd to 3rd) providing, of course, that the revs are matched. As I understand the definition of rev matching it has more to do with downshifting using the clutch and the gas pedal (usually during heel toe shifting) to give you the sensation that you really didnt even make a shift (if you are good enough). Now, how bad is it for the syncros, transmission to upshift w/o using the clutch?

Nick
rev matching is usually for downshifting, the engine/transmission speeds between upshifts aren't so great, so there's no longer a need to rev match. On the downshift however, the input/output speeds are a lot different, so you need to rev in order to execute a fast and smooth downshift without any shock in the drivetrain.

It is VERY bad to do clutchless shifting, it ruins your transmission pretty good. Pro racers can get away with it because they get a new transmission every other race.

Quote:
I've read many times, if done correctly, that its better for the synchos because they do less work when you rev-match and double clutch.
Rev matching theoretically does preserve the life of your synchros, at the expense of some wear on your clutch. Double clutching prevents excessive wear on the clutch.
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Old 07-13-2005, 05:00 PM   #5
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Great write up. Worthy of a sticky in Noob/Faqs forum.
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Old 07-14-2005, 01:51 AM   #6
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Quote:
Originally Posted by eltrouble
Rev matching theoretically does preserve the life of your synchros, at the expense of some wear on your clutch. Double clutching prevents excessive wear on the clutch.
You got that backwards. You rev match and you don't wear your clutch because the engine speed is where it's suppose to be, you let out the clutch and it stays there. What you wear is your syncros because you're not double clutching on that downshift.

When you double clutch w/ rev-matching, you don't put wear on anything; clutch and syncro.

You let the clutch get the engine speed up to the gear and you wear the clutch down. You let the syncro match the gears speed and you wear them down. You help save clutch wear by revving engine speed up to gear speed so when you let go of the clutch, it doesn't have to wear to get the engine speed and the gear speed up to the same speed.

you double clutch and you save the syncros by matching the gear speed via engine speed/revv.
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Old 07-14-2005, 04:34 PM   #7
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Thx, REDrum

However, we have some good examples of the confusion I'm trying to address, right in "this heah' thread"... In order:

blitzpb: (the 2nd/3rd paragraphs under "Rev Matching" address this) Since the issue with just rev matching is trying to match engine/flywheel speed to clutch/trans speed the upshift works for you - as long as you back off on the gas as you shift: off gas / in clutch...engine/flywheel speed is dropping as you shift the trans into the next higher gear, which lowers the speed of the clutch/trans(input shaft)...let out the clutch and by happy accident, if you timed it correctly , the engine/flywheel revs have dropped to approximately the new speed of the clutch/trans.

It helps to remember (and I'll update the original for this) that the clutch and trans input shaft to which it's attached are linked to the wheels whenever the trans is in gear.

When downshifting you don't have the "natural" factor of engine speed falling working for you, since the relative speed change is in the other direction: you have to add engine/flywheel speed while the clutch is disengaged to match the now higher clutch/trans speed produced when you shifted to the next lower gear. Hence blipping the throttle to increase the engine/flywheel revs on the downshift. (I realize this is just kind of expanding on what you stated...)

In general eltrouble is correct that shifting w/o the clutch is not conducive to long and happy transmission life In fact, get it really wrong and said life could be measured in seconds...

On the other hand, if you can do it exactly right every time then you will indeed save wear and tear on the clutch and syncros. But that just ain't gonna happen; here are some factors:
  • In order to shift w/o the clutch you have to accomplish 3 things: 1) disengage the current gear set, moving the trans to neutral, 2) match engine speed to wheel speed for the next intended gear, and 3) shift into that next gear.
  • With many, if not most, modern transmissions it's fairly difficult to shift into neutral with any load whatever on the engaged gearset. Which means you have to precisely match engine speed to wheel speed for the current gear so that there's no load at all on the gearset; not that easy to do, since you can't feel the tiny difference between slight gear loading - either accel or decel - and no load at all, since the car's momentum masks any sensation in that zone.

    You can put a bit of pressure on the gear lever toward neutral and then try to feather the gas pedal until you hit that sweet spot, but it's not easy to do, especially not in a hurry, every time, under varying speed and load conditions! <insert smiley for understatement here>

    If you don't get it right you'll be putting abnormal loads on the shift forks, the slip rings on the gears, and the engagement dogs on the gears...which pretty much moves you from advantage to disadvantage right there.
  • But, assuming you've gotten to neutral w/o any problem at all, you've now got a much bigger problem: to complete the shift correctly you now have to precisely match engine speed to the trans speed for the next gear you want to engage - just as precisely as you did to get to neutral, but now w/o any "feel" clues to tell you when you're at that point.

    If you don't get it right, then you're using the syncros to force the engine speed to match the trans speed, with the full momentum of the car involved.

    Take 2 friends; hand one of then a drill motor w/ a phillips bit in the chuck, the other w/ a phillips screw in the chuck. Have them hold the drill motors pointed at each other, 1/4" apart or so and spin them up. Now grab one of the chucks w/ each hand, and using the tension of your grip try to adjust the relative speed of the 2 chucks so that you can bring the bit into engagement w/ the screw head w/o any slippage or damage to either. Oh, and post the video, too (This isn't a totally accurate analogy, since your hands don't get to spin freely like the syncro rings do, but I hope it's illustrative.)

    Back to reality, every time you even try to move the shift lever into the next desired gear when the speed isn't precisely matched you're asking the syncros and rings to overcome the mismatch between the momentum of the car and the rotating inertia of the engine/flywheel - while it's generating power. Miss it be even a little bit and you've put more wear on the syncros than in weeks, or maybe months, of normal shifting.

    That said, I used to do this with a 3spd, column shift, car, that didn't have syncro in 1st iirc. But it was a bulletproof trans, I was careful...and my mom totalled the car before the trans showed any signs of a problem

zombiedog: as I tried to explain above - with emphasis , rev matching alone has no effect, good or bad, on the syncros. They're simply not part of the mechanical picture when you're just rev matching. (Is there a way I can edit to make this clearer?)

But double clutching - when done correctly - definitely saves syncro wear: in the excercise w/ 2 friends above it would be like having an electronic speed control for each drill motor so you could dial up exactly the same speed on both before engaging the bit w/ the screw. Except you have to do it by ear, experience, and feel...(unless you want to design a computer interface that will give you a graphical display of the relative speeds of the engine/flywheel/trans-input-shaft and the calculated desired engine speed for the next intended gear )

eltrouble: I covered the upshift vs. downshift stuff above re blitzpb's post, but there's something to add here... Let's say I'm driving down a gentle slope - just enough that if I were coasting in neutral the car speed would be constant at say 35mph. If I'm in 2nd gear, and shift up to 3rd - no change in car speed - there's an rpm diff of (not exact, but fine for example) 1000, and I let the engine revs drop by that 1k while I'm shifting into 3rd, then let the clutch out...no problem.

Now I want to go back down to 2nd gear - still @ 35mph, and the engine speed difference between the gears is the same as for the previous shift - 1000 revs, but in the opposite direction. This time I have to raise the engine revs by that 1k to do the matching (same for double clutching).

The point being that there's nothing inherent in upshifts that makes the rev difference between gears any less than for downshifts. I all depends on the required engine speed in any particular gear for the current speed of the car, and that doesn't change based on shifting up or down. But, as covered in the original post, and above in this one, it seems easier on the upshift because the engine revs naturally fall as you back off on the gas while shifting.

And Fuji K caught the other problem with your post quite nicely
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Old 07-14-2005, 05:43 PM   #8
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Coasting is illegal? What a weird law. How would they ever prove that? It's not, btw, illegal in Ohio as far as I can tell.

A question about your "Efficiency" section. Wouldn't what you've written only be true if you coasting to a stop. For example, if you coasted for a while and then needed to accelerate again, wouldn't you actually save gas because you would be going proportionately faster without the time spent transmission braking?

I'm not sure this would make much of a difference, was just curious about your input.
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Old 07-14-2005, 07:35 PM   #9
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Quote:
Originally Posted by makanai
Coasting is illegal? What a weird law. How would they ever prove that? It's not, btw, illegal in Ohio as far as I can tell.

A question about your "Efficiency" section. Wouldn't what you've written only be true if you coasting to a stop. For example, if you coasted for a while and then needed to accelerate again, wouldn't you actually save gas because you would be going proportionately faster without the time spent transmission braking?

I'm not sure this would make much of a difference, was just curious about your input.
They can't prove it...unless of course you feel guilty enough to admit it. At least I don't think.

Can someone explain to me how coasting in neutral for a long time actually wastes more gas than coasting in gear? I'm not contradicting it, I'm just interested in the thoughts behind that theory. And what's with this strange 'save more gas' theory about going WOT in a high gear in order to save gas? Somebody posted a link to that website and swore by it, but I can't possibly see how going WOT in high gear would save gas.
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Old 07-14-2005, 08:24 PM   #10
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I think the theory behind saving gas by coasting gear is based on the idea that our cars are smart enough not to feed fuel to the motor under those conditions.

Not sure if that's really the case or not, but that's what I read somewhere aorund here.
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Old 07-14-2005, 08:32 PM   #11
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Quote:
Originally Posted by eltrouble
They can't prove it...unless of course you feel guilty enough to admit it. At least I don't think.

Can someone explain to me how coasting in neutral for a long time actually wastes more gas than coasting in gear? I'm not contradicting it, I'm just interested in the thoughts behind that theory. And what's with this strange 'save more gas' theory about going WOT in a high gear in order to save gas? Somebody posted a link to that website and swore by it, but I can't possibly see how going WOT in high gear would save gas.

didn't GravelRash answer this in the "efficiency" section of the first post?
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Old 07-14-2005, 08:33 PM   #12
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Quote:
Originally Posted by stanh04wrx
I think the theory behind saving gas by coasting gear is based on the idea that our cars are smart enough not to feed fuel to the motor under those conditions.

Not sure if that's really the case or not, but that's what I read somewhere aorund here.
The way I think about it, it takes less air/fuel to keep the car at idle in neutral, as opposed to slowing down in gear at 2k or 3k...which would require slightly more air/fuel?
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Old 07-15-2005, 02:39 AM   #13
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The key to the coasting debate is, to quote my original post , "Why? In gear, with the throttle closed, fuel is usually cut entirely. Coasting in neutral? Engine is idling - which uses gas, and not very efficiently." (Emphasis added.)

The original discussion concerned coasting to a stop, e.g. stop sign/light, not "coasting for a long time", but we can expand on the topic .

We'll start with the case where you're coasting down a looooong hill, in top gear, throttle closed...and the car is maintaining your desired speed. This is the optimum: at desired speed, using no fuel (most (all?) modern fuel injected cars; carbureted is a totally different story), and not having to apply the brakes.

Next best case is the same, except the slope is steeper, so you have to use a bit of brake to not exceed your desired speed. Still not using any fuel...

The other side of the coin shows up when the slope isn't as steep, and coasting with the throttle closed means enough engine braking that you aren't maintaining desired speed. This is the problematic case; should you open the throttle, lessening the engine braking to maintain desired speed? Or should you shift to neutral, and use the brakes to control speed (if necessary)? Like much of life, "that depends..." In this case, largely on your desired speed.

If you want to cruise at 45mph in top gear, and need to just barely open the throttle to do so, my guess is it's probably just as efficient as neutral and idling - and no need to use the brakes to control speed, which may tip the balance in favor of in gear travel.

Need to open the throttle a bit to maintain, say 70mph? Now it's probably more efficient to go to neutral, since the open throttle RPM would be so much higher, and even at very little throttle would use more fuel than idling.

eltrouble's remaining question, re wide open throttle efficiency, is waaayyy outside the transmission topic, but it is related to efficiency, so a quick digression...

The whole idea here is to minimize pumping losses, which occur when the engine has to do excess work just to move air past an intake restriction, i.e., a partially open throttle. So let's go up the hill we were just coasting down

Let's say we want to maintain 50mph up a moderate hill. We could use any gear from 2nd to top gear to do so. Let's say the hill is just steep enough that in top gear we need wide open throttle to maintain 50mph. We're also at low rpm, and combined with the free air flow unrestricted by the throttle we're making the necessary power fairly efficiently.

Let's choose 2nd gear instead. This time we're at much higher RPM, but we need only a fraction of the power we could generate at that RPM, so the throttle is barely open. Now the engine is trying to breathe, at high RPM, through a very restricted intake, and has to do more work in the process just to move air through that restriction.

Think of breathing slowly, with your mouth wide open, vs. breathing very quickly through a straw; that's the general idea.

However, all the discussions I've read on the topic have only used NA engines as the example. What happens with a turbo engine? I don't really know. The turbo may help offset the pumping losses...but maybe not, since with a mostly closed throttle the turbo won't be spooling up very much. But what if, in the current example, we use 3rd, or 4th gear? Lower RPM, a bit more open throttle...and maybe there's a point where the turbo assist cancels out the pumping losses. Maybe; as I said I haven't seen that one addressed.

Oh, and when you shifted down to 2nd for the experiment up the hill, you did double clutch, didn't you?
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Old 07-15-2005, 02:15 PM   #14
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Quote:
Originally Posted by eltrouble
rev matching is usually for downshifting, the engine/transmission speeds between upshifts aren't so great, so there's no longer a need to rev match. On the downshift however, the input/output speeds are a lot different, so you need to rev in order to execute a fast and smooth downshift without any shock in the drivetrain.

It is VERY bad to do clutchless shifting, it ruins your transmission pretty good. Pro racers can get away with it because they get a new transmission every other race.



Rev matching theoretically does preserve the life of your synchros, at the expense of some wear on your clutch. Double clutching prevents excessive wear on the clutch.
No, unless the car is in neutral, rev matching alone DOES NOT spin up the synchros. Rev matching WITHOUT double clutching will do nothing, repeat....NOTHING to help synchro life...
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Old 04-05-2006, 04:16 PM   #15
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Quote:
Originally Posted by eltrouble
They can't prove it...unless of course you feel guilty enough to admit it. At least I don't think.

Can someone explain to me how coasting in neutral for a long time actually wastes more gas than coasting in gear? I'm not contradicting it, I'm just interested in the thoughts behind that theory. And what's with this strange 'save more gas' theory about going WOT in a high gear in order to save gas? Somebody posted a link to that website and swore by it, but I can't possibly see how going WOT in high gear would save gas.
someone correct me if im wrong but when your off throttle and in gear, the drivetrain is moving the pistons, so there is no need for gas and air, but when in neutral like said it needs to idle so it needs air and fuel
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Old 04-05-2006, 04:43 PM   #16
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about fuel on engine braking / coasting, we're discussing that exact problem in the exact thread over on page 7 of an identical thread over at wrxforum.com

http://www.wrxforum.com/cgi-bin/ulti...;f=13;t=000294
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Old 04-05-2006, 04:44 PM   #17
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Originally Posted by FuJi K
You got that backwards. You rev match and you don't wear your clutch because the engine speed is where it's suppose to be, you let out the clutch and it stays there. What you wear is your syncros because you're not double clutching on that downshift.

When you double clutch w/ rev-matching, you don't put wear on anything; clutch and syncro.

You let the clutch get the engine speed up to the gear and you wear the clutch down. You let the syncro match the gears speed and you wear them down. You help save clutch wear by revving engine speed up to gear speed so when you let go of the clutch, it doesn't have to wear to get the engine speed and the gear speed up to the same speed.

you double clutch and you save the syncros by matching the gear speed via engine speed/revv.
+1...

I agree with you.
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Old 04-05-2006, 05:16 PM   #18
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Hello,

Holy crap! GravelRush makes my typical 5-10 screen-full posts seem like cliff notes!

Yeah, I "know" that you don't fuel on decel, but is that 0% duty cycle and is there no fuel entering the cylinders?

Joel, subscribing
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Old 04-05-2006, 11:20 PM   #19
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Quote:
Originally Posted by keithwrx8
someone correct me if im wrong but when your off throttle and in gear, the drivetrain is moving the pistons, so there is no need for gas and air, but when in neutral like said it needs to idle so it needs air and fuel
Most modern cars cut fuel until rpms are down to the target idle when the throttle is fully closed. It's called "coasting fuel cut."

- Jtoby

ps. I know that coasting downhill out of gear is illegal in many states, but could someone please point me to a statute that makes coasting out of gear always illegal
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Old 04-07-2006, 04:33 PM   #20
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Default Racing and clutchless shifting

Just a note, racers often don't have constant mesh trannys and have straight cut gears. The drill for shifting at speed then is to lift the throttle slightly and shift for an up****, lift throttle then rev motor as you pass neutral into the lower gear, for downshift.

Takes a while to get it right, which is why Bondurant has you do loops around pylons practicing before you get onto the track, when you do the Formula Ford part of the racing school. I think this is standard drill in the schools to acclimatize street drivers.

Done right, the shifts take half or less as long. Done worong and...it is a mess.

George
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Old 04-07-2006, 05:02 PM   #21
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Hello,

Just thought I'd test out the fuel on decel thing while driving yesterday. I was cruising on Hwy 5 (long, straight, and sometimes no cars anywhere near by) and I would let off the throttle, then turn the engine off, then turn the key to on, and do that a couple times. I felt absolutely no change in decel with the engine on or off and turning the engine on (key on, not "start" - no need to zing the starter to a million rpms ) simply turned the gauges back on, I felt no difference and the fuel computer went straight to 99 mpgs.

So, the answer is, the injectors do turn off on decel with the stock ecu (and, incidentally, with my Cobb Stage 2 custom map (by Christian@Cobb)).

And yes, that would mean that decel in gear is more fuel efficient than "running" the motor at idle.

I didn't see if it was answered already, but the reason for a law saying 'no coasting in neutral' is pretty simple - if the motor dies, you don't have power steering, which can surprise you in an emergency situation. In gear, the accessories are turning and you keep power steering, even if the engine isn't firing, so long as it's being rotated by the wheels.

Joel
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Old 05-07-2006, 10:01 PM   #22
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hello,
Probably a dumb question, but how do the cylinders keep moving when there is no fuel being injected into the cylinders on deceleration while in gear? Is it just the tranny kinda reverse running the engine?
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Old 05-07-2006, 10:06 PM   #23
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from the crankshaft which is connect to your drive train

BTW i was thinking about this the other day and although you use more fuel being in neutral you really dont because what about when you rev up to rev match, that has to use more fuel than idling
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Old 05-08-2006, 12:09 AM   #24
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Quote:
Originally Posted by The Deliverator
I think the theory behind saving gas by coasting gear is based on the idea that our cars are smart enough not to feed fuel to the motor under those conditions.

Not sure if that's really the case or not, but that's what I read somewhere aorund here.
When your coasting down in gear but comepletely off the throttle most cars cut fuel delivery completely, simply turning the engine into a big air pump thats uses the cars inertial to power it. Thus no fuel is used, if you coast down in neutral your engine must use gas to maintain idle, thus using some gas... my thought has always been i am saving gas AND brake wear, the engine is lubricated and is in a very LOW stress situation simply pumping air with little compression on rings and rotating parts so coasting down IN gear makes great senseot me... if you can rev match correctly...
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Old 09-21-2006, 07:48 PM   #25
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Default Teh NASIOC

it's sites like NASIOC and all the great members herein that are making semi-capable noobs like me capable of saving thousands of dollars on maintenance and repairs, not to mention drastically enhancing the subby experience through perfoming one's own repairs. Thanks to all the knowledgeable memb's and their posts. I wish you guys reproductive success; we need more intelligent co-operators in this world...
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