[JamesNoBrakes]

Originally Posted by Norm Peterson

Very doubtful it'd amount to anything you'd be able to measure - all it is as far as engine wear is concerned is a few extra crankshaft revolutions at only the low power needed to accelerate the moving parts up to the rev match target rpm. Peanuts compared to accelerating the whole car even mildly.


Norm

That wasn't really my point, my point was that if you are "slowing the car" by downshifting WITHOUT rev-matching (manual or automatic), you are basically slowing the car with the clutch material, once it's matched and fully-engaged the clutch the engine compression is what's slowing you down, mainly the pressure would be on the rings I'd think, but not like they can't take it, since like you say mild acceleration would be just as much stress.

Otherwise, downshifting WITH rev-matching bypasses the whole "clutch friction to slow down" issue and relying mainly on the engine compression.

I always noticed that my low-compression turbo WRX had crap for engine compression (as in very low, because the turbo makes up for it), so downshifting wouldn't slow you down much, despite the short gearing.

[Norm Peterson]

Originally Posted by JamesNoBrakes

That wasn't really my point, my point was that if you are "slowing the car" by downshifting WITHOUT rev-matching (manual or automatic), you are basically slowing the car with the clutch material, once it's matched and fully-engaged the clutch the engine compression is what's slowing you down, mainly the pressure would be on the rings I'd think, but not like they can't take it, since like you say mild acceleration would be just as much stress.

Keep in mind that you re-engage the clutch on a rev-matched downshift just like you do on a normal (unhurried) upshift. It's not like when you're starting up from a full stop at all.

Otherwise, downshifting WITH rev-matching bypasses the whole "clutch friction to slow down" issue and relying mainly on the engine compression.

Emergency "help-I've-lost-my-brakes" situations aside, using clutch friction to specifically slow the car down is not why you should be downshifting. Remember that the process of downshifting is not so you can get a braking effect of any amount, let alone a maximum possible braking effect. It's only to match the engine revs to what the next gear wants them to be at, for the speed you're traveling at. Done right, you should hardly feel the clutch re-engage - if you feel it at all.

I always noticed that my low-compression turbo WRX had crap for engine compression (as in very low, because the turbo makes up for it), so downshifting wouldn't slow you down much, despite the short gearing.

The Subie in my sig probably has less engine compression braking available than your WRX. It's never felt like its compression effect was inadequate here in flatland S. Jersey, and 4th gear seems to have been low enough to maintain speed down most of the longer and steeper downgrades I've been on with it so far (though I would not be opposed in the least to using 3rd if that's what it was going to take).


Honestly, this whole business of shifting and rev-matching became second nature a long time ago, with little or no conscious thought ever involved until I try to describe it to somebody else.


Norm

[Bluecyclone]

It's a "feel" for the car one achieves when, "You are one with the car". This develops over time. Like my paddle shifting "timing" of giving the next shift throttle, understand?

[torqueaddict]

Originally Posted by Norm Peterson

.


Emergency "help-I've-lost-my-brakes" situations aside, using clutch friction to specifically slow the car down is not why you should be downshifting. Remember that the process of downshifting is not so you can get a braking effect of any amount, let alone a maximum possible braking effect. It's only to match the engine revs to what the next gear wants them to be at, for the speed you're traveling at. Done right, you should hardly feel the clutch re-engage - if you feel it at all.


Norm

Here, I disagree a bit. Prepping for the next gear is definitely one use, and a big one, but engine braking to slow the car down is a common use for downshifting and is fine as long as the rev-match is timed right. For example, approaching a stop that's a little ways off gives me time to downshift and use engine braking. It doesn't have to be a skip gear downshift, just one gear to let the engine handle some of the braking.

Approaching a corner is another reason to downshift, engine brake before the corner, then accelerate out. Again, it doesn't have to be aggressive, and if done right, should be smooth with hardly any impact on the clutch.

[Norm Peterson]

Originally Posted by torqueaddict

Here, I disagree a bit. Prepping for the next gear is definitely one use, and a big one, but engine braking to slow the car down is a common use for downshifting and is fine as long as the rev-match is timed right.

It may be common, but it's poor practice to downshift specifically for the purpose of slowing down - that's what the brakes are for.

For example, approaching a stop that's a little ways off gives me time to downshift and use engine braking. It doesn't have to be a skip gear downshift, just one gear to let the engine handle some of the braking.

Approaching a corner is another reason to downshift, engine brake before the corner, then accelerate out.

I'm very familiar with both of these situations and encounter both of them virtually every time I drive anywhere at all. In neither case do I rely on engine braking for anything; my right foot is either on the brake pedal for slowing down or on the throttle for speed maintenance once I get slowed down to the necessary speed (or acceleration once that becomes possible). Very seldom is my right foot not on one pedal or the other, unless maybe idle rpm in whatever gear just happens to provide exactly the necessary vehicle speed. Yes, I do realize that I'm getting small amounts of engine braking 'help' - it's strictly incidental and you're going to get a little of that in any braking short of fairly hard regardless of whether any downshifting took place.

Again, it doesn't have to be aggressive, and if done right, should be smooth with hardly any impact on the clutch.

"Aggressive" is a word that should never need to be used to describe a non-emergency downshift. Done right, like you just said, should be the goal every single time.


Norm

[torqueaddict]

Originally Posted by Norm Peterson

It may be common, but it's poor practice to downshift specifically for the purpose of slowing down - that's what the brakes are for.

Depends on who you ask. I've done it for 11 years on my last car and have had no ill effects as a result. 160k miles, same clutch, engine runs strong. If it's poor practice, I'll take the gamble.

I'm very familiar with both of these situations and encounter both of them virtually every time I drive anywhere at all. In neither case do I rely on engine braking for anything; my right foot is either on the brake pedal for slowing down or on the throttle for speed maintenance once I get slowed down to the necessary speed (or acceleration once that becomes possible). Very seldom is my right foot not on one pedal or the other, unless maybe idle rpm in whatever gear just happens to provide exactly the necessary vehicle speed. Yes, I do realize that I'm getting small amounts of engine braking 'help' - it's strictly incidental and you're going to get a little of that in any braking short of fairly hard regardless of whether any downshifting took place.

Maybe I wasn't clear. When downshifting to slow down, I use engine braking as assistance to regular braking. I don't take my foot completely off the brake or throttle depending on the situation. Though, there have been instances in a lower gear, where the engine did most of braking. Still, my foot was on the brake pedal.

It is impossible to drive a manual and not engine brake often. Let off the throttle even slightly in the lower gears (1-3) and the car engine brakes noticeably.

"Aggressive" is a word that should never need to be used to describe a non-emergency downshift. Done right, like you just said, should be the goal every single time.

For maximum acceleration, skipping a gear on a downshift isn't out of the norm. One can call that aggressive. They can also call it 'dumb' depending on the situation. Agreed on your last sentence though.

[DC5]

I typically downshift once when slowing down from higher speeds; for example, when exiting the highway and encountering a stop sign or red traffic control light, I'll shift from 6th gear to 5th gear, which allows me to stay in gear longer before coming to a complete stop. Otherwise, I keep the vehicle in gear as long as possible without lagging the engine and then proceed to put it in neutral.