engine braking question

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vw100
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engine braking question

Post by vw100 » Fri Dec 11, 2015 7:13 am

say you are doing 30km/h in N and shift to 2nd. Now no matter how slowly you let the clutch out (feather/pause your foot) you will experience engine braking (at least in my tdi, maybe high compression).Not a good feeling, you can even risk getting rear ended. Because the engine is at idle and the clutch will make it spin up to 1500rpm (for my car)
Now the question is if you apply just the slightest amount of gas (and hold it) and do the same thing with the clutch (be really careful letting it out) do you get the same braking effect, or will it be really smooth ?
ps: i know rev matching (a blip at 2000rpm and slowly dragging them down with the clutch ) will be butter smooth and the best choice but i just want to know what happens when you feed just a little bit of gas to the engine end haven't got the chance to try it.

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theholycow
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Re: engine braking question

Post by theholycow » Fri Dec 11, 2015 8:05 am

vw100 wrote: you can even risk getting rear ended
In practice I've heard no examples of this nor experienced any indication that it's a real risk, but I do agree that for safety's sake your brake lights should light up when braking. I would add my usual note that the car's service brakes are excellent at braking. That's their job, they were designed to do it. They do it extremely well in an automatic, so much so that nobody ever asks about engine braking, and manual cars aren't equipped with sub-standard brakes. Of course that's not to say that engine braking is useless (and it's just as useful in an automatic as in a manual), just that it seems like people suddenly discover it when discussing manuals and think they should do it when they wouldn't have with an automatic.
Now the question is if you apply just the slightest amount of gas (and hold it) and do the same thing with the clutch (be really careful letting it out) do you get the same braking effect, or will it be really smooth ?
ps: i know rev matching (a blip at 2000rpm and slowly dragging them down
If your target RPM (current speed in target gear) is 1500 and you're asking about the difference between blipping up to 2000rpm or 1500rpm, then 1500rpm is rev-matched and 2000rpm is a sloppy mess. A blip to 2000rpm then slowly dragging them down is a rev-unmatch. 25% error is a lot to call "match".

So, now on to the question of whether a near-perfect rev-match (for which you can and should dump the clutch quickly, rather than slowly easing it) to 1500rpm will produce engine braking and how it will compare:

- Compared to just entering that gear and engaging the clutch at idle, it will produce less engine braking immediately.

- Compared to entering that gear at 2000rpm, it will produce more engine braking. Entering that gear at 2000rpm will produce acceleration, at least for a second.

- Assuming you keep your foot off of the accelerator pedal and you're descending a mountain, all 3 options will eventually even out, it's just that first moment of clutch engagement that differs.

Note that electronic throttle controls (as well as some older systems that meddle too much with their electronic idle speed controls) can mess this up, and such behavior will vary by model and even individual car. My 2008 VW Rabbit had rev hang; often if I rev-matched to a certain RPM it would hold that RPM for a long time as if I had kept my foot on the accelerator pedal. When rev hang happened there was no engine braking.
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potownrob
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Re: engine braking question

Post by potownrob » Fri Dec 11, 2015 10:23 am

vw100 wrote:say you are doing 30km/h in N and shift to 2nd. Now no matter how slowly you let the clutch out (feather/pause your foot) you will experience engine braking (at least in my tdi, maybe high compression).Not a good feeling, you can even risk getting rear ended. Because the engine is at idle and the clutch will make it spin up to 1500rpm (for my car)
Now the question is if you apply just the slightest amount of gas (and hold it) and do the same thing with the clutch (be really careful letting it out) do you get the same braking effect, or will it be really smooth ?
ps: i know rev matching (a blip at 2000rpm and slowly dragging them down with the clutch ) will be butter smooth and the best choice but i just want to know what happens when you feed just a little bit of gas to the engine end haven't got the chance to try it.
sounds like a wasted rev-match if you add gas then let the clutch up slowly - you lost the benefit of the rev-match by not letting up the clutch quickly. 30 kph =~19 mph which is usually well within 2nd gear terrortory, but you probably will slow down from engine braking unless you get back on the gas. I sometimes shift into 3rd in this situation, and there's probably no reason you can't with a Diesel engine.
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Re: engine braking question

Post by Reverence » Fri Jan 08, 2016 5:11 am

Now the question is if you apply just the slightest amount of gas (and hold it) and do the same thing with the clutch (be really careful letting it out) do you get the same braking effect, or will it be really smooth ?
I believe the OP was asking if there would be engine braking effect if you'd apply gas and hold at target rpm, i.e. 1500rpms while carefully releasing the clutch.
the answer is: there will be zero braking if 1500rpm is the target, regardless of how you release the clutch pedal, however controlled fast release is the way to go just in case you are slightly off.

Reference to rev matching by blip to 2000 is not that bad, guys. if target is 1500, and the car has no rev hang, rev matching by bliping to 2000 is fine, by the time he releases the clutch pedal, he would be at or near target.

In this forum there are 2 type of blippers: 1) the high 2) the right.
1) the high: bleep to higher rpm and catch the falling rpms at target
2) the right: bleep to target and quickly release clutch.
no one way is better

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Re: engine braking question

Post by theholycow » Fri Jan 08, 2016 10:05 am

Reverence wrote:In this forum there are 2 type of blippers: 1) the high 2) the right.
1) the high: bleep to higher rpm and catch the falling rpms at target
2) the right: bleep to target and quickly release clutch.
no one way is better
3) the low: catch rising rpms at target
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AHTOXA
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Re: engine braking question

Post by AHTOXA » Fri Jan 08, 2016 12:51 pm

theholycow wrote:
Reverence wrote:In this forum there are 2 type of blippers: 1) the high 2) the right.
1) the high: bleep to higher rpm and catch the falling rpms at target
2) the right: bleep to target and quickly release clutch.
no one way is better
3) the low: catch rising rpms at target
That's me, the low-rising blipper.
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Re: engine braking question

Post by Rope-Pusher » Fri Jan 08, 2016 4:24 pm



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Re: engine braking question

Post by Reverence » Sat Jan 09, 2016 10:36 am

been a while since I lurked in here, yeah, there are 2 types of blippers, 1) and 3).
2) is only described for academic purposes :D

I 1) when slowing down and get ready to get going, I sort of 3) when downshifting to flash accelerate, but that's not bleeping anymore, as foot remains on the gas pedal.

LoL Rope, bleep, I forgot! :D

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