Rev-matching

I have a Leon 1.9 TDI PD150 with a manual 6 speed(same platform with Golf IV). If I do not rev-match the engine on downshifting it just jumps to its necessary rpm gracefully without problems.

But recently I've driven an Opel Astra 2.0 dti(2003) where rev matching when downshifting was necessary, otherwise gear shifting would become uncomfortable.

So is rev matching really necessary? I've read a lot of different opinions on this and it probably depends on car. If you don't do it what could be the consequences?

Or is it something that our drive-by-wire system automatically does.

IIRC transmissions with synchronizers don't require this. Those without do. Those with worn out synchros also require it. Those with syncros and no rev matching, shifted properly will have no ill effects.

Since transaxle parts are expensive, and the labor to replace them is also precious, and the above even more so compared to the price of and skills required to replace brake parts, I forego the downshifting stuff and take the car out of gear while slowing down with brakes.

Seems to work rather well.

Cheers,

PH

When I was a semi driver, downshifting had some importance. Not in any modern car I have owned since the early 70's.

If you learn to double de clutch not only will you get a smoother gear change, (and your passengers will notice the difference), your clutch will not be worked as hard and will last longer.


This applies to all manual transmissions whether they have synchromesh, are straight cut, or otherwise.




FF

FlyingFin said:

If you learn to double de clutch not only will you get a smoother gear change, (and your passengers will notice the difference), your clutch will not be worked as hard and will last longer.
This applies to all manual transmissions whether they have synchromesh, are straight cut, or otherwise.
FF

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I agree completely. I think that downshifting is part of driving the car. I used to drive old fire trucks that had non-synchronized crash boxes and shifters that looked like pretzels. I got to the point where I only used the clutch to get going. Any manual transmission can be shifted without the clutch and without grinding of gears. With practice you don't even over rev the engine, you know exactly where to shift from "feel"...Mark

There is really no need for it though. If you choose to do it, that is all well and fine. Our transmissions shift just fine without the need to do that. Knowing how to do things is good, even if they aren't really needed, currently. I have a DSG this time and it is very smooth, although it has its occasional momentary Alzheimer moments.

Lightflyer1 said:

There is really no need for it though. If you choose to do it, that is all well and fine. Our transmissions shift just fine without the need to do that. Knowing how to do things is good, even if they aren't really needed, currently. I have a DSG this time and it is very smooth, although it has its occasional momentary Alzheimer moments.

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Mechanical sympathy is essential for a cars welfare and longevity.

In this case, by double de clutching, it eliminates the need for the gearbox to force the engine revs up, to drag the revs upwards against its will, to a speed where the gears will mesh. This puts unnecessary strain on the Clutch, gearbox and engine.

By double de clutching, whilst the gears are in neutral, you blip the throttle to bring the engine revs up to a speed where they meet the road speed of the gear you are trying to select and, as the engine revs are falling naturally when the gear selection takes place, there is minimal stress and strain put on the components.


Yes, it is not strictly necessary to double de clutch to make a gear change in a synchromesh box, but it does make the change smoother and reduces the stress on the transmission and engine components and will give them a longer life overall.

Its a good skill to learn, if left on a hole, it will enable you to still drive your car home or to the garage, should the clutch cable or hydraulic operating system fail.


FF

I know the skill very well. I drove semis for UPS for many years doing it. I just don't agree that there is a need for it for the most part today, in normal cars. I also don't think there is that much stress on the parts when it is shifted properly without double de clutching. At least not when I drive. Many here have gone hundreds of thousands of miles without doing that, with long clutch/tranny/engine life. Your last sentence confuses me. How do you double de clutch when the cable or hydraulic system has failed? That would entail shifting without the clutch, which is also possible to do.

Lightflyer1 said:

How do you double de clutch when the cable or hydraulic system has failed? That would entail shifting without the clutch, which is also possible to do.

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Your comment is a little confusing.... you say how do you do it??... then say it is possible... so just to clarify...


You push the gear lever from being in gear into neutral without touching the clutch pedal, rev the engine and sustain the revs whilst pushing the gear lever into the required gear, decrease the revs gently and when the revs match the road speed for the gear you are looking to select, it will slip into gear crunch free... Simples! Most upward changes will just slip into gear with out problems if the selection is gentle.

The awkward bit is starting the engine with the car being in gear, it takes practice and from then on, your driving must be adapted to anticipate every single move/hazard and then plan ahead so you don't have to come to a stop.

If you are in stop start traffic, call the breakdown company!

And while you're pushing the gear lever into the required gear, waiting for the revs to match the road speed, you're wearing out the syncros.

KLXD said:

And while you're pushing the gear lever into the required gear, waiting for the revs to match the road speed, you're wearing out the syncros.

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I stress that it is a get you out of trouble method of moving your car, but again, if the gear selected for the road speed and the revs are matched correctly, there will be no difference and no extra wear. Anyhow, that is the purpose of the synchromesh gear system, isn't it?


Again, mechanical sympathy needs to be employed.


If you are ham fisted and cannot 'feel' the car then don't bother trying it.




The purpose of the comments was to show that learning to double de clutch, had more than one advantage. I was not suggesting you should drive your car that way all the time.



I just guess some of us have the skill to do it correctly and some don't.

KLXD said:

And while you're pushing the gear lever into the required gear, waiting for the revs to match the road speed, you're wearing out the syncros.

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If input and output shafts are perfectly timed, no wear. She'll slide right in. The difference is holding and catching it, or just going into gear and no hesitation.


But I agree downshifting is pointless in smaller cars/trucks.


I'll ride 6th, see a corner, not too much throttle, but some throttle, to take pressure off the teeth, gently move shifter , falls into neutral, no clutch, coast to corner, stop, continue.

I do notice hitting the input to flywheel between shifts the engagement is much much easier, from relying less on the synchros. Especially 1st and 2nd. Either lots of rotating mass for the brass to scuff against, or straight up worn out.

I got good luck, I'll vote #2.

FlyingFin said:

Your comment is a little confusing.... you say how do you do it??... then say it is possible... so just to clarify...


You push the gear lever from being in gear into neutral without touching the clutch pedal, rev the engine and sustain the revs whilst pushing the gear lever into the required gear, decrease the revs gently and when the revs match the road speed for the gear you are looking to select, it will slip into gear crunch free... Simples! Most upward changes will just slip into gear with out problems if the selection is gentle.

The awkward bit is starting the engine with the car being in gear, it takes practice and from then on, your driving must be adapted to anticipate every single move/hazard and then plan ahead so you don't have to come to a stop.

If you are in stop start traffic, call the breakdown company!

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Miscommunication. On the term double de clutch. We always called what you call double de clutch, not using the clutch. Sure you can do this with lots of practice. One mistake though and you are grinding gears. Better buy a real SH** box to practice up on till you get real good. Not something you learn to do naturally without lots and lots of practice. When I have taught others to drive/drive a manual, I have demonstrated that this is possible, but not really a preferred thing to do under normal circumstances.

FlyingFin said:

Its a good skill to learn, if left on a hole, it will enable you to still drive your car home or to the garage, should the clutch cable or hydraulic operating system fail.

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My buddy in college lost a clutch cable on his GEO crappbox back in the day. He was amazed when I was able to drive it without a clutch to a place where we could repair it.

Downshifting is a necessary skill when driving a manual - I am constantly going up and down gears (and not just at stop lights). It's a matter of being in the right gear for the circumstances.

KLXD said:

And while you're pushing the gear lever into the required gear, waiting for the revs to match the road speed, you're wearing out the syncros.

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After years of practice, when I pushed the shifter, it slid into gear, no over-reving. It was as if I was using the clutch. Once in a while I'll do it in my dodge just to see if I still have it...Mark

Dannyboy said:

No point rev-matching a synchro, that's the whole point of the synchro, it does it for you. You won't wear out synchro by shifting normally, its when you don't change the gearbox oil or keep smashing gears in will see them wear out quickly.then you'll notice rev matching works better

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Not rev matching downshifts isn't smooth.

Rev matching downshifts are smooth.

Down shifts are smooth if you do them right. There is a reason why transmissions are the way they are today. You can sharpen a pencil with a sharpener or use a sharp rock. Both can accomplish the same task and neither way is right or wrong, just different. Most will choose the sharpener though. Same with manual transmissions. Other than special situations there is really no benefit to not using the clutch.

Lightflyer1 said:

Down shifts are smooth if you do them right. There is a reason why transmissions are the way they are today. You can sharpen a pencil with a sharpener or use a sharp rock. Both can accomplish the same task and neither way is right or wrong, just different. Most will choose the sharpener though. Same with manual transmissions. Other than special situations there is really no benefit to not using the clutch.

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A smooth downshift without rev matching?

Wankel7 said:

A smooth downshift without rev matching?

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Yes. Let the car slow down in gear and when the revs are right downshift. I don't do Formula 1 racing and never had an issue in normal driving.

Lightflyer1 said:

Yes. Let the car slow down in gear and when the revs are right downshift. I don't do Formula 1 racing and never had an issue in normal driving.

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Ah yes that would do it.

I find my self doing heel and toe braking and rev matching. No real reason others than it is fun.

Those are really racing/high performance techniques. They are good to know and fun to do, but aren't required in normal driving.

Lightflyer1 said:

Those are really racing/high performance techniques. They are good to know and fun to do, but aren't required in normal driving.

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Neither are most of things I do to my Jetta wagon but I still do them

I do my best to reduce stress on my synchros by rev matching, better for the clutch friction life, too.
Float it into neutral, bring the revs up and clutch into whatever gear.

But I'm also the type that doesn't even know if my radio works. Whenever I remember to, I'm getting another one of the storage shelves to put in its place.

I feel like there is some confusion between double clutching and rev matching. Sounded like the OP is asking about rev matching which does not do anything for the synchros but eases friction on the clutch and smooths out the timespan of the engine matching travel speed.

Double clutching (clutch in-shift to neutral-clutch out-rev match clutch in-shift to gear-clutch out), while not necessary, will save wear on the synchos and clutch if done correctly. However, some will argue that youre adding wear in other places like the throwout bearing since youre pumping the clutch that many more times over its lifetime.

I will almost always revmatch if I'm doing a moderate downshift where the revs are going to climb over 1000rpm when I release the clutch. Its plain habit and makes it so much smoother. I'm not quite experienced enough to double clutch effortlessly but do it once in a while for entertainment or when the transmission is super cold and tough to get in gear.

I rev match automatically, I cant help it now, I've done it so many years.
Do you have to? No.
What benefits does it have? Saves a small amount of clutch friction plate wear and it could be argued engine mount life. If you were interested in outright speed, the turbo would also be spooled earlier, allowing for quicker acceleration. But for ultimate acceleration you cant beat a flat shift. Damn thats abusive lol.

The biggest benefit though is just for smoothness.