Engine Braking

DaMonk · 31-07-2008 2:58pm #1

Hi, i recently remapped my VW and am very conscious of trying to keep the clutch in one piece! just wondering if cycling down through the gears when stopping is bad for the clutch and would I be better off coasting to save the clutch?

HungryJoey · 31-07-2008 3:00pm

What engine did you have remapped that has you so worried? Why not just have an uprated clutch put in if you are that worried about the extra power harming the OE clutch.

2 Espressi · 31-07-2008 3:09pm

You're not going to do much damage to the clutch as long as you let the revs drop fairly low before downshifting, and rev matching:

wrote:

REV MATCHING is a driving technique for shifting a manual transmission smoothly. One simply memorizes approximately how many RPMs different the various gears are from one another at the same speed, and then matches the engine's revs with the gear they are shifting in to. When upshifting, you just let the revs drop and slip into gear. When downshifting, you tap the throttle (or stomp on it depending on how long it takes to rev up.) This is often referred to as 'blipping' it. This works easily primarily because of the syncromesh gears in your transmission; if your transmission does not have them, this is very, very difficult to do without grinding. Even so, it takes the right touch.

For example, in one car the gears might all be approximately 1,000 RPM apart at the same speed. When downshifting from third to second, for example, you would apply accelerator pressure to raise the engine RPMs by 1,000 (say, from 3,000 to 4,000) and make your shift. You can then use engine braking to reduce your speed.

This practice results in reduced brake wear, less of a lurch when shifting from a higher gear to a lower one, less clutch slippage when shifting from a lower gear to a higher one, and a smoother shift without having to slip the clutch so much.

In auto racing, one often uses the more advanced form of this activity, heel-toeing (Alternately heel-toe braking or heel-toe downshifting) in which one's heel is on the brake pedal, decelerating, while the toe is on the gas pedal, accelerating. This allows the use of both the brakes and the engine to perform braking.

Mr.David · 31-07-2008 3:16pm

Hi,

When I learned to drive I was taught to cycle down through the gears and combine engine braking and hydraulic breaking! However, I now work for Jaguar and have since done a few advanced driving courses. The instructors teach you not to cycle instead to brake down to the point at which the car feels as if it will soon cut out and then clutch and change to the next gear you need. i.e. Brake from 60mph (5th gear) down to 20mph (still 5th) clutch and change to 2nd for the corner.

This saves the clutch (less use) and allows you to have both hands on the wheel for control and 100% concentration on the road ahead. Engine braking is just a throwback to the days of huge trucks with brakes that were insufficient. Modern cars do not require engine braking.

Hope that helps!!

Mr.David · 31-07-2008 3:23pm

Regarding Rev matching, the easiest way to try it is not to let the accel out when downchanging. i.e. travelling at 35mph in 4th, with your foot on the accelerator (not gaining momentum just staying constant) clutch in and change gear swiftly (keeping foot on throttle the whole time). This has the effect of blipping the revs to match the required revs for the lower gear. Its much smoother once you get the hang of it.

Once you have mastered that you can try powershifting i.e. changing gear without clutching at all, it only requires you to match revs carefully. Its faster (was employed by rally drivers for instance) and does not damage the car as long as done well.

Alanstrainor · 31-07-2008 3:26pm

Always wondered about power shifting, too afraid to try it though.

green-blood · 31-07-2008 3:44pm

good lord lads, MECHANICAL SYMPATHY

or if you prefer WALLET SYMPATHY

Engine brekaing is all well and good, and if you can match your revs correctly go ahead, but breaks are for breaking... consider the replacemtn cost of new pads vs the cost of a new clutch... can you heel and toe??

ditto "powershifting"...on the road... when woudl that ever be neccessary (other than after a spot of engine brekaing that eh, breaks your clutch)

Zube · 31-07-2008 3:47pm

Mr.David wrote: »

The instructors teach you not to cycle instead to brake down to the point at which the car feels as if it will soon cut out and then clutch and change to the next gear you need.

That's what my driving instructor told me (20+ years ago), as brake pads are cheaper than clutches. However, he also told me to use all the gears on the test, as testers believed in it. Don't know what current tester thinking is.

Mr.David · 31-07-2008 3:52pm

As far as I know in Ireland you still should cycle down for the test but in UK they have changed it. Typical. One of the many ridiculous things they teach you!!

peasant · 31-07-2008 4:11pm

DaMonk wrote: »

Hi, i recently remapped my VW and am very conscious of trying to keep the clutch in one piece! just wondering if cycling down through the gears when stopping is bad for the clutch and would I be better off coasting to save the clutch?

The fact that your engine was remapped has no influence on engine braking ...after all, you're not on the gas when engine braking, so the increased power makes no difference.

What's going to kill your clutch quicker is all the heavy acceleration, now that ou have more ooomph to play with.

DaMonk · 31-07-2008 4:23pm

peasant wrote: »

The fact that your engine was remapped has no influence on engine braking ...after all, you're not on the gas when engine braking, so the increased power makes no difference.

What's going to kill your clutch quicker is all the heavy acceleration, now that ou have more ooomph to play with.

I know that ya. Just wondering if I'm wearing my clutch needlessly by engine breaking when I could just be breaking normally. Engine breaking is a habit by now and I hate coasting but since my clutch is going to get worn more due to remapping I'm trying to keep all other wear at a minimum.

On a side note, is it better or worse for the clutch to come off it faster when accelerating gears? From 1st for instance?

peasant · 31-07-2008 4:28pm

there's a golden middle between shiftiing down all the gears and coasting ...only changing gears when necessary,as described further above.

What really wears a clutch is slippage ...so be careful only to stomp back on the gas when the clutch has fully engaged when changing up.

I wouldn't worry too much about matching engine speed and gearbox speed ...that's what syncromesh is for.

Dirty_Diesel · 31-07-2008 4:29pm

DaMonk wrote: »

Hi, i recently remapped my VW and am very conscious of trying to keep the clutch in one piece!

I dont know what engine you have but when I remapped my PD130 passat I was told to never floor it below 1900rpm in 4th/5th/6th or the torque will wreck the clutch. My brother bought the car off me and didn't heed my advice. After a few months he had to fork out €1000 for a new clutch :mad:

DaMonk · 31-07-2008 4:31pm

peasant wrote: »

there's a golden middle between shiftiing down all the gears and coasting ...only changing gears when necessary,as described further above.

What really wears a clutch is slippage ...so be careful only to stomp back on the gas when the clutch has fully engaged when changing up.

I wouldn't worry too much about matching engine speed and gearbox speed ...that's what syncromesh is for.

Ya I was a bit confused as to what slippng is exactly. Am I right in saying that its accelerating while having the clutch not fully let out?

peasant · 31-07-2008 4:34pm

DaMonk wrote: »

Ya I was a bit confused as to what slippng is exactly. Am I right in saying that its accelerating while having the clutch not fully let out?

yepp ..that would be it.

Also, if the engine is too strong for the clutch, applying full power from the beginning would cause the clutch to slip all by itself, you've got to ease it into it, especially with a diesel that has all that low down torque.

Biro · 31-07-2008 5:01pm

The savings on the clutch by coasting when slowing down will be so minimal that the fuel you'd save by engine breaking might be a bigger saving. In any case coasting might only save 1000 miles on a clutch over 100,000, meaning that chances are you'll have to replace it one way or another at some point.

mickdw · 31-07-2008 6:52pm

The idea of slowing down on the brakes to a point where you cannot slow down anymore without stalling before changing down is a bad idea. It means you are using a lowest rpm at any given speed but as far as car control is concerned, Id certainly like to have a little revs/power available to balance the car with.

Esel · 31-07-2008 9:25pm

mickdw wrote: »

The idea of slowing down on the brakes to a point where you cannot slow down anymore without stalling before changing down is a bad idea. It means you are using a lowest rpm at any given speed but as far as car control is concerned, Id certainly like to have a little revs/power available to balance the car with.

It's not for when you are slowing down, it's for when you know you are coming to a stop.

ianobrien · 31-07-2008 10:38pm

I remember reading in a tuning guide (admittedly it was for classic Ford engines) and they were on about clutches.

They said that the strength of a clutch is due to the clamping forces of the springs on the friction forces in the clutch being stronger than the torque produced by the engine. When the engine is producing more power than originally designed, and when the torque is more that the clamping forces in the clutch springs on the friction surfaces, clutch slip results.

Oh, the clutch spring provide the force that keeps the clutch plates together. It's the friction between these plates that makes the clutch work

When the clutch is worn, the level of friction between the clutch plate & flywheel is reduced. Eventually, the level of friction is so low that the torque produced by the engine is more that the friction can handle, meaning a slipping clutch and it's time to change.

So, to make the clutch last, you minimise the wear on the friction surfaces. To do that, basically make sure that the clutch pedal is fully out before you stamp on the loud pedal, no grand prix starts and pressing the clutch doesn't wear out the clutch friction surfaces, it wears the clutch release bearing.

cyclopath2001 · 01-08-2008 8:09am

Interesting thread. I never understood so much about what goes on under the hood.

My driving instructor educated me in engine braking, his rationale was that you were more in control of the car than when using brakes alone. He also insisted that I tap (press) lightly on the brake pedal (no...not morse code, just engage the brake light to warn others that I'm slowing down).

Nothing annoys me more than someone dragging a car through a turn while in too high a gear.

I think the wear & tear thing depends on how gentle you are on the pedal and listening to the revs. But, I'm no advanced driver.

Mr.David · 01-08-2008 8:49am

Its somewhat down to personal preference. I often brake down from 5th to 3rd for instance. Coasting should definitely be avoided and it actually uses more fuel than leaving the gear engaged and taking the boot off the throttle. Being in control of the car means having 2 hands on the wheel as much as possible in my opinion but whatever you are comfortable with. Regarding clutch wear as a rule of thumb, if its smooth its not wearing if its jerky it is!

DaMonk · 01-08-2008 11:56am

Mr.David wrote: »

Its somewhat down to personal preference. I often brake down from 5th to 3rd for instance. Coasting should definitely be avoided and it actually uses more fuel than leaving the gear engaged and taking the boot off the throttle. Being in control of the car means having 2 hands on the wheel as much as possible in my opinion but whatever you are comfortable with. Regarding clutch wear as a rule of thumb, if its smooth its not wearing if its jerky it is!

Is this true because I thought by pressing the go accelerator before the clutch is released is slipping and thats smoother than waiting to take your foot off the clutch fully?

mickdw · 01-08-2008 12:21pm

esel wrote: »

It's not for when you are slowing down, it's for when you know you are coming to a stop.

Thats still no way to be driving. You are always more in control of the car with some power available. Fair enough you are intending to stop but if you do this down through all the gears from speed, it is possible that on occasion you may have to take evasive action of some kind before you stop the car and then you need the car to be responsive. Also if you do this when coming to a stop it will become habit and you would probably start to drive continuously in that manner thus ruining your driving style. I would challenge any instructer (advanced or not) who recommended that.

ianobrien · 01-08-2008 1:30pm

mickdw wrote: »

Thats still no way to be driving. You are always more in control of the car with some power available. Fair enough you are intending to stop but if you do this down through all the gears from speed, it is possible that on occasion you may have to take evasive action of some kind before you stop the car and then you need the car to be responsive. Also if you do this when coming to a stop it will become habit and you would probably start to drive continuously in that manner thus ruining your driving style. I would challenge any instructer (advanced or not) who recommended that.

I'd agree with that. Having some power available is handy for when the back steps out on a greasy road:D

(Yes, before I get shot here about speeding, poor driving, etc, I do like driving, and regularly have the back of a 05 Almera out sideways and not from the handbrake. I blame the wet roads and crap tyres, rather than the speed! And yes, it's only on quiet roads and private lanes I do it, not the main road)

peasant · 01-08-2008 1:49pm

ianobrien wrote: »

I'd agree with that. Having some power available is handy for when the back steps out on a greasy road:D

(Yes, before I get shot here about speeding, poor driving, etc, I do like driving, and regularly have the back of a 05 Almera out sideways and not from the handbrake. I blame the wet roads and crap tyres, rather than the speed! And yes, it's only on quiet roads and private lanes I do it, not the main road)

two things wrong with this post

a) last time I checked the almera was FWD

b) it's miles off topic

Zube · 01-08-2008 1:55pm

mickdw wrote: »

You are always more in control of the car with some power available.

That's the conventional wisdom the Irish testers are looking for.

Does the same logic apply when cruising along? Do you hang on to 3rd at 60 in case you need a burst of power? No, you drive in the lowest appropriate gear. Whipping through all five or six gears on the way down to a stop is just a pointless ingrained habit.

Honey-ec · 01-08-2008 2:01pm

Intersting article on engine vs hydraulic breaking here: http://www.straightdope.com/classics/a980227.html

Eddie Cunningham also addressed it in his column in the Indo a few years back - his advice was "Gears to go, brakes to slow" - snappy, eh?

Esel · 01-08-2008 8:19pm

Mr.David wrote: »

Being in control of the car means having 2 hands on the wheel as much as possible....

Ergo, automatic gearbox!

echo hotel · 02-08-2008 12:28am

Mr.David wrote: »

Regarding Rev matching, the easiest way to try it is not to let the accel out when downchanging. i.e. travelling at 35mph in 4th, with your foot on the accelerator (not gaining momentum just staying constant) clutch in and change gear swiftly (keeping foot on throttle the whole time). This has the effect of blipping the revs to match the required revs for the lower gear. Its much smoother once you get the hang of it.

Once you have mastered that you can try powershifting i.e. changing gear without clutching at all, it only requires you to match revs carefully. Its faster (was employed by rally drivers for instance) and does not damage the car as long as done well.

ruins the syncronising rings in the box if you screw up though then you'll never get gear

ianobrien · 02-08-2008 12:30am

peasant wrote: »

two things wrong with this post

a) last time I checked the almera was FWD

b) it's miles off topic

1. lift off oversteer and left foot braking will put an almera sideways in the wet 2. The advantage of having some power left in the was mentioned for control. I also said power was handy for controlling slides 3. If you want to see an almera sideways, pm me and we'll meet up and I'll show you how it's done

echo hotel · 02-08-2008 12:37am

DaMonk wrote: »

I know that ya. Just wondering if I'm wearing my clutch needlessly by engine breaking when I could just be breaking normally. Engine breaking is a habit by now and I hate coasting but since my clutch is going to get worn more due to remapping I'm trying to keep all other wear at a minimum.

On a side note, is it better or worse for the clutch to come off it faster when accelerating gears? From 1st for instance?

the less your foot is on the pedal the less you're goin to wear the clutch also when not using the clutch make sure your foot isn't resting on the pedal as there is a few millimeters of play to protect the release bearing any preasure on the pedal will cause this to wear quicker

guil · 02-08-2008 10:25am

Mr.David wrote: »

Regarding Rev matching, the easiest way to try it is not to let the accel out when downchanging. i.e. travelling at 35mph in 4th, with your foot on the accelerator (not gaining momentum just staying constant) clutch in and change gear swiftly (keeping foot on throttle the whole time). This has the effect of blipping the revs to match the required revs for the lower gear. Its much smoother once you get the hang of it.

thats the most ridiculous thing i have ever heard , the clutch would last no time doin that , and you say you're a mechanic

Mr.David · 06-08-2008 9:00am

guil07 wrote: »

thats the most ridiculous thing i have ever heard , the clutch would last no time doin that , and you say you're a mechanic

And you are qualified to say that this is "ridiculous" how?! I am not a mechanic I am a design engineer for Jaguar Land Rover. The above technique is an advanced driving skill taught to police drivers etc. If done properly it is the optimum way to change gear.

guil · 06-08-2008 9:39am

so ur tryin to say that when changin gear keep the accelerator in the same position while pressin the clutch, the engine would just rev high and then would gradually wear the clutch after time, i may have picked u up wrong but thats the way i see it

Zube · 06-08-2008 10:01am

guil07 wrote: »

so ur tryin to say that when changin gear keep the accelerator in the same position while pressin the clutch, the engine would just rev high and then would gradually wear the clutch after time,

When changing down, particularly changing down several gears in one jump, you need to increase engine revs to match the lower gear or the clutch will slip and wear until the speeds match (or you may just stall the car).

guil · 06-08-2008 10:16am

yeah i know that i read it wrong the first time i thought he meant all the time as in changin up aswell,

Mr.David · 06-08-2008 10:26am

No not for changing up definitely not!!

If driving at 40mph in 4th at 2,000rpm and want to change to 3rd then the revs will be higher for the same road speed. Suppose they would rise to 3,500rpm. Then keeping foot on acc and popping in the clutch will let the revs rise to 3,500rpm then engage the gear by letting out the clutch. Less wear and more comfort!

peasant · 06-08-2008 10:32am

Zube wrote: »

When changing down, particularly changing down several gears in one jump, you need to increase engine revs to match the lower gear or the clutch will slip and wear until the speeds match (or you may just stall the car).

What do you do then when you change up?
Slow down the car so that he gearbox speed will match the lower engine revs?

You match your speeds before you change gear ...i.e you make sure that you are going fast enough / slow enough for the gear that you want to select next so that the engine won't stall or over-rev.

Other than that you let the clutch in and out quickly without lingering on the pedal and that's that ...it's what it's designed for. No need to confuse yourself and fiddle with the accelerator.

Rev matching was necessary on non-syncromesh gearboxes, otherwise you couldn't engage gears ...but that's different from the workings of the clutch

Mr.David wrote: »

No not for changing up definitely not!!

If driving at 40mph in 4th at 2,000rpm and want to change to 3rd then the revs will be higher for the same road speed. Suppose they would rise to 3,500rpm. Then keeping foot on acc and popping in the clutch will let the revs rise to 3,500rpm then engage the gear by letting out the clutch. Less wear and more comfort!

Why would you need to do that?

If you're comfortable at the speed you're going, stick in fourth ...if you want to slow down and use engine braking, why speed up the engine first and loose the braking effect?

The only time that rev matching makes sense is when you're changing down to speed up. And even then it's not to save the clutch, but to keep the momentum going.

Mr.David · 06-08-2008 10:38am

peasant wrote: »

What do you do then when you change up?
Slow down the car so that he gearbox speed will match the lower engine revs?

You match your speeds before you change gear ...i.e you make sure that you are going fast enough / slow enough for the gear that you want to select next so that the engine won't stall or over-rev.

Other than that you let the clutch in and out quickly without lingering on the pedal and that's that ...it's what it's designed for. No need to confuse yourself and fiddle with the accelerator.

Rev matching was necessary on non-syncromesh gearboxes, otherwise you couldn't engage gears ...but that's different from the workings of the clutch

Why would you need to do that?

If you're comfortable at the speed you're going, stick in fourth ...if you want to slow down and use engine braking, why speed up the engine first and loose the braking effect?

By changing to 3rd you would increase the engine braking not decrease it. You dont have to do any of it of course you can simply change gear in the normal manner, but it is something interesting to try none the less.

peasant · 06-08-2008 10:51am

Mr.David wrote: »

By changing to 3rd you would increase the engine braking not decrease it.

Yes, by changing down you increase the engine braking ...but if you rev the engine up first before engaging the clutch there won't be much braking left.

Mr.David · 06-08-2008 11:53am

http://www.telegraph.co.uk/motoring/main.jhtml?xml=/motoring/2001/07/02/emrask30.xml

http://www.ehow.com/how_2273367_stop-using-engine-braking.html

Zube · 06-08-2008 12:01pm

peasant wrote: »

Yes, by changing down you increase the engine braking ...but if you rev the engine up first before engaging the clutch there won't be much braking left.

Simply selecting a lower gear and banging the clutch out is not engine braking! This would be awfully hard on your clutch.

For engine braking, you'd match engine speed to the lower gear, release the clutch, and then the engine, in a lower gear and foot off the accellerator, will slow the car.

The one place engine braking is still recommended is on very long downhill stretches, but it certainly doesn't mean riding the clutch all the way down!

daveharnett · 06-08-2008 12:08pm

DaMonk wrote: »

I know that ya. Just wondering if I'm wearing my clutch needlessly by engine breaking when I could just be breaking normally.

Not in a big way, but yes. Brakes are better at braking.

DaMonk wrote: »

Engine breaking is a habit by now and I hate coasting

There's nothing wrong with coasting, provided you are in gear and you're not on the clutch. As all the economy people are telling us, it saves money (and polar bears).

Bottom line, drive in such a way that all gear changes are smooth, and don't stomp on the accelerator at low revs, especially in higher gears.

peasant · 06-08-2008 12:18pm

Zube wrote: »

Simply selecting a lower gear and banging the clutch out is not engine braking! This would be awfully hard on your clutch.

For engine braking, you'd match engine speed to the lower gear, release the clutch, and then the engine, in a lower gear and foot off the accellerator, will slow the car.

The one place engine braking is still recommended is on very long downhill stretches, but it certainly doesn't mean riding the clutch all the way down!

What are you on about?

Once the clutch has made full contact, there is no more slippage. Not using engine braking to "save" the clutch would be akin to not accelerating either because that would also "save " the clutch. Just keep the car parked ...that'll save the clutch.

The clutch was desigend to transfer more than the maximum torque of the engine at any time...that's its job. Yes there will be wear on it, that's the nature of the beast, but with normal driving a clutch will easily last for 100,000-200,000 km depending on the circumstances. The only thing that will wear it prematurely is to let it slip for longer than necessary.

If you match the engine speed when shifting down, you get next to no engine braking anymore, only that of the engine slowing down when you lift off the accelerator.In case you're driving a diesel and are going downhill, you will get no engine braking after matching the speed ...the engine will just run away with you, no matter how far off the accelerator you lift your foot.

And whatever gave you the idea of riding the clutch when using engine braking going downhill??

Just to clarify: I'm not promoting to use engine braking via downshifting gear by gear when you're planning on stopping anyway ...that's pointless.

I'm talking about engine braking during normal driving ...when going downhill, approaching a bend, etc

Zube · 06-08-2008 12:30pm

peasant wrote: »

If you match the engine speed when shifting down, you get next to no engine braking anymore, only that of the engine slowing down when you lift off the accelerator.

That is only kind of engine braking anyone would recommend!

What you are talking about is dropping the clutch when the engine speed is slower than required for a lower gear, and dumping the braking energy into the clutch. That is a mad idea.

In case you're driving a diesel and are going downhill, you will get no engine braking after matching the speed ...the engine will just run away with you, no matter how far off the accelerator you lift your foot.

Generations of truck drivers will be fascinated to learn that engine braking doesn't work. All those signs on the French motorways I passed this summer, telling trucks to use engine braking on 5% gradients, all pure fantasy!

And whatever gave you the idea of riding the clutch when using engine braking going downhill??

You did: when you say that engine braking doesn't work if you match engine speed to the appropriate gear, you're talking about slowing the car using the clutch while the engine is spinning slower than required. The only way you can do that continuously for a 10 mile downhill stretch is by riding the clutch.

I'm talking about engine braking during normal driving ...when going downhill, approaching a bend, etc

No, you're talking about slowing the car with the clutch, a completely seperate thing from engine braking, and a really bad idea.

peasant · 06-08-2008 12:44pm

Zube wrote: »

That is only kind of engine braking anyone would recommend!

What you are talking about is dropping the clutch when the engine speed is slower than required for a lower gear, and dumping the braking energy into the clutch. That is a mad idea.

Just about as mad as dumping the accelerating energy into the clutch when you step on the loud pedal

Generations of truck drivers will be fascinated to learn that engine braking doesn't work. All those signs on the French motorways I passed this summer, telling trucks to use engine braking on 5% gradients, all pure fantasy!

Try to read what other people write. Engine braking does of course work going downhill ...but the brake force is reduced when speed up the engine first.
In the case of a diesel engine there actually is a point where it will run away from you ...that's why trucks have motor brakes and/or retarders to assist with braking (btw ..I have driven trucks)

You did: when you say that engine braking doesn't work if you match engine speed to the appropriate gear, you're talking about slowing the car using the clutch while the engine is spinning slower than required. The only way you can do that continuously for a 10 mile downhill stretch is by riding the clutch.

That just doesn't make any sense

No, you're talking about slowing the car with the clutch, a completely seperate thing from engine braking, and a really bad idea.

I never said to slow the car with the clutch ..I don't know where you are getting that from

daveharnett · 06-08-2008 12:53pm

peasant wrote: »

The clutch was desigend to transfer more than the maximum torque of the engine at any time...that's its job.

I wouldn't be too sure about that, given that the maximum torque has just been increased beyond the manufacturer's specs.

Zube · 06-08-2008 1:01pm

peasant wrote: »

Just about as mad as dumping the accelerating energy into the clutch when you step on the loud pedal

Not so.

Here's my idea of engine braking:

I'm tipping along at 100 km/h in 5th on the flat. I reach a long downhill stretch. Instead of braking all the way down, I drop into 4th (matching engine speed as I change), and if that doesn't hold my speed, I drop into 3rd. Now I can cruise down the hill at a steady speed (with a bit more noise from the engine) without touching the brakes.

OK?

Here's yours, as I read it: when I reach the hill, I change down to 4th without matching engine speeds, and the car slows as the engine revs are dragged up when the clutch is engaged.

If that's not what your saying, I can't imagine what you mean when you say engine braking is reduced if I match engine speeds when changing gears. If that is what you're saying, then yes, you are slowing the car using the clutch, and it's a mad idea.

peasant · 06-08-2008 1:19pm

daveharnett wrote: »

I wouldn't be too sure about that, given that the maximum torque has just been increased beyond the manufacturer's specs.

Yepp ...once the engine is chipped, the new found torque may be too much for the clutch. That's why I would also change to a higher rated clutch if chipping the engine

Zube wrote: »

Here's yours, as I read it: when I reach the hill, I change down to 4th without matching engine speeds, and the car slows as the engine revs are dragged up when the clutch is engaged.

correct ...only that I let out the clutch quickly, so it isn't slipping. Once fully engaged it isn't subjected to any force bigger than it would be under full acceleration ...so no problem there

I can't imagine what you mean when you say engine braking is reduced if I match engine speeds when changing gears. If that is what you're saying, then yes, you are slowing the car using the clutch, and it's a mad idea.

If you rev up the engine while changing down you're loosing part of its braking power. The spooling up of the engine (while your foot is off the accelerator) is what brakes the most. By revving up beforehand you're loosing out on that bit.
Particularly in a diesel and going down hill, that may just be that bit that's needed to stop the engine from running away with you, as the hill may be too steep and you still end up having to use the brakes for too long.

Just to clarify ...of course I'm not advocating to do this at brutally mismatched speeds. Your road speed would have to be in the correct band for the next lower gear. But if it's braking force that I'm after, I'd rather use the foot brake to slow the vehicle down further before shifting down than use the accelerator to rev up the engine.

Hope you get now what I mean.

Pete67 · 06-08-2008 2:07pm

Lots of different views here. I'll add mine, from an engineers perspective. Engine braking is a useful tool to control vehicle speed, particularly on downhill gradients. However it should not be used to reduce vehicle speed suddenly by transferring the kinetic energy of the vehicle into increased engine rpm. This will cause additional wear of the clutch friction plate as it must slip until the two speeds are matched. The footbrake should be used for this purpose - it's far cheaper and easier to replace brake pads than to change out a clutch. If the driver changes down matching engine speed to road speed for the new gear before engaging the clutch then there is no slip and hence no wear on the clutch. In fact, the only time a skillful driver will slip the clutch is when starting the vehicle from rest at the lowest engine speed possible to provide enough engine torque to get moving without stalling.

Zube · 06-08-2008 4:30pm

peasant wrote: »

correct ...only that I let out the clutch quickly, so it isn't slipping. Once fully engaged it isn't subjected to any force bigger than it would be under full acceleration ...so no problem there

OK, I understood you correctly, so my earlier comments stand. This is not engine braking as anyone else understands it, this is just mechanical abuse of your clutch.

But, hey, it's your clutch, feel free.

Engine Braking

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