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      03-04-2010, 12:03 PM   #1
drburton
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Engine Braking

For those of us with a manual transmission, we are all aware of engine braking when downshifting. Generally, I don't do this but do rev matches on downshifts. However, I have noticed with the M3 that the engine braking seems more aggressive than most other cars. If I'm in a higher gear at low RPM and I just lift on the throttle the car slows down pretty rapidly. I'm starting to get concerned this isn't normal.

Thoughts?

I've driven manual cars my whole life including a recent Porsche and have never experienced this (to this degree).
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      03-04-2010, 12:13 PM   #2
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Quote:
Originally Posted by drburton View Post
For those of us with a manual transmission, we are all aware of engine braking when downshifting.
I just want to point out that you also get engine braking with DCT in manual mode, as long as you don't use one of the rev-matching modes. So, S3, for example, will give you engine braking.
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      03-04-2010, 12:15 PM   #3
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Just my thoughts. Having different sports cars, they are all geared differently. I think it is because of the gear ratio having something to do with the way the car slows down quite quickly. Also, the m3 in 6th going 80mph yields almost 4k on the tachometer, which is almost 1k more than I'm used to on some cars.
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      03-04-2010, 12:20 PM   #4
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Quote:
Originally Posted by drburton View Post
For those of us with a manual transmission, we are all aware of engine braking when downshifting. Generally, I don't do this but do rev matches on downshifts. However, I have noticed with the M3 that the engine braking seems more aggressive than most other cars. If I'm in a higher gear at low RPM and I just lift on the throttle the car slows down pretty rapidly. I'm starting to get concerned this isn't normal.

Thoughts?

I've driven manual cars my whole life including a recent Porsche and have never experienced this (to this degree).
The car has very high rear end gearing which causes this and also improves acceleration.
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      03-04-2010, 12:31 PM   #5
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Quote:
Originally Posted by drivendriver View Post
I just want to point out that you also get engine braking with DCT in manual mode, as long as you don't use one of the rev-matching modes. So, S3, for example, will give you engine braking.
Actually it doesn't matter what mode you are in. Engine braking occurs in all modes, and in fact occurs in all cars regardless of what type of transmission is employed. The only time engine braking is not occuring is if you are in a car with a manual transmission and you put the car into neutral when decelerating.
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      03-04-2010, 12:42 PM   #6
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I think its pretty normal on the M3 (and apologies for the following nerd content).

Going back longer than I care to remember when learning about engines, I seem to remember that its also dependent on the compression ratio of the engine (as well as gearing etc) - when you lift off it typically cuts fuel (but the engine still goes through the suck/squeeze/(without the bang)/expel process of a 4 stroke engine) so the usual process of compressing air etc goes on but without the combustion to add power (granted less air than normal as the throttle is closed).

Given a compression ratio of 12:1 of the E9x M3 which is pretty high compared to most normally aspirated engines - and much higher than most forced induction engines - it takes more energy to turn the engine over if its not generating power. The higher the compression ratio - the quicker the car will slow down if your not adding power as compressing the air highly during each cycle is taking more energy from the car.

That plus low gearing means quick deceleration when you lift off. Certainly mine seems to slow down pretty quickly when you lift off compared to anything other than a diesel :-)

Cheers,

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      03-04-2010, 01:32 PM   #7
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Quote:
Originally Posted by tricky View Post
I think its pretty normal on the M3 (and apologies for the following nerd content).

Going back longer than I care to remember when learning about engines, I seem to remember that its also dependent on the compression ratio of the engine (as well as gearing etc) - when you lift off it typically cuts fuel (but the engine still goes through the suck/squeeze/(without the bang)/expel process of a 4 stroke engine) so the usual process of compressing air etc goes on but without the combustion to add power (granted less air than normal as the throttle is closed).

Given a compression ratio of 12:1 of the E9x M3 which is pretty high compared to most normally aspirated engines - and much higher than most forced induction engines - it takes more energy to turn the engine over if its not generating power. The higher the compression ratio - the quicker the car will slow down if your not adding power as compressing the air highly during each cycle is taking more energy from the car.

That plus low gearing means quick deceleration when you lift off. Certainly mine seems to slow down pretty quickly when you lift off compared to anything other than a diesel :-)

Cheers,

Tricky
Nice explanation, I appreciate the 'nerd content'.
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      03-04-2010, 02:15 PM   #8
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Engine braking is one of the sexiest sounds a V8 can make
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      03-04-2010, 03:50 PM   #9
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The E90 definitely engine brakes more than the E46. I had noticed this as well.
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      03-04-2010, 04:58 PM   #10
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Quote:
Originally Posted by drburton View Post
Nice explanation, I appreciate the 'nerd content'.
Yeap good info! My motorcycle, F800 BMW has amazing engine braking...I think it has high compression ratio too.
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      03-04-2010, 05:02 PM   #11
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Quote:
Originally Posted by immiketoo View Post
Engine braking is one of the sexiest sounds a V8 can make
Really? The throttle blip (if done right) sounds sweet on a downshift. But my standard engine braking sounds more like a whine, like an electric motor. Maybe the transmission?
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      03-04-2010, 06:57 PM   #12
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Quote:
Originally Posted by drburton View Post
Really? The throttle blip (if done right) sounds sweet on a downshift. But my standard engine braking sounds more like a whine, like an electric motor. Maybe the transmission?
Throttle blips are cool, but a V8 with an exhaust on decel is amazing.
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      03-04-2010, 07:33 PM   #13
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Compression ratio and engine size, I think. I like the strong engine braking of our car.
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      03-04-2010, 07:52 PM   #14
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I especially notice it when engine breaking from 3rd to 2nd. I have noticed I have to be very careful to rev match when doing so. It must have something to do with the gear ratio going from 3 to 2.

Correct me if I am wrong, but I don't think it is bad for the engine. It is just more noticeable with a high compression V8.

P.S. I have to be very careful when engine breaking and approaching a red light. I nearly got rear ended by somebody in a SUV talking on their cell even though she was coming up to a red.
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      03-04-2010, 08:05 PM   #15
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Quote:
Originally Posted by JohnnyRoaster View Post

P.S. I have to be very careful when engine breaking and approaching a red light. I nearly got rear ended by somebody in a SUV talking on their cell even though she was coming up to a red.
That's a constant threat whether engine braking or not. I always look forward and in the rearview when braking--too many people yacking or texting on cells these days. It's simply a required defensive driving technique given that people are doing lots of things--focusing on driving not being one of them.
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      03-05-2010, 06:16 AM   #16
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Quote:
Originally Posted by JohnnyRoaster View Post
P.S. I have to be very careful when engine breaking and approaching a red light. I nearly got rear ended by somebody in a SUV talking on their cell even though she was coming up to a red.
What I do in situations like this (someone close behind me) is rest my foot lightly on the brake so that the brake lights come on. I think it's sort of good ettiquette too. Kinda rude to slow down so quickly without some visual indicator.
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      03-05-2010, 06:54 AM   #17
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On a track or when driving at the limits engine braking is not used, It is a technique from the days of drum brakes that faded. I personally only use it when going down a slong and steep grade.

It is a good idea to get in the habit of rev matching every down shift. If you are near the limits of the car and do not rev match on a down shift there is a very good possibility that you will spin out (trailing clutch oversteer). It is also much easier on the clutch.

The brakes act on all 4 wheels, engine braking only acts on the rear wheels. At the limits the brakes are as much for balancing the car, "rotating" the car and managing the contact patches as they are for slowing the car.

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      03-05-2010, 09:30 AM   #18
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I've always been told that some EB is good for the motor, almost like a reverse Italian tuneup.
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      03-05-2010, 09:32 AM   #19
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I've noticed the engine braking too and I kinda like it. Although I typically don't downshift until later in deceleration when I'm in S3 mode, which lessens the effect. And of course when I use S4 it rev-matches, which also lessens the effect. Rev matching does seem better for the car though (easier on the clutch and tranny).

One thing I came across a few weeks ago when searching the forum for the cause of my DSC/engine malfunctions (turned out to be idle control valve and has already been fixed) was that the intake hose can come loose on our cars, which is a pretty serious problem. It seems much more rare than the idle valve issue but if you search, you'll see that it's happened to plenty of members already. Apparently, the intake hose issues have to do with stress/pressure put on it when you let off the gas. Makes me wonder if this significant engine braking that is characteristic of our cars is essentially the cause of this problem...??
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      03-05-2010, 12:41 PM   #20
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Quote:
Originally Posted by captainaudio View Post
...when going down a slong ...
...
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      03-05-2010, 12:48 PM   #21
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Exclamation Warning, hair splitting content

Quote:
Originally Posted by BCA View Post
I've noticed the engine braking too and I kinda like it. Although I typically don't downshift until later in deceleration when I'm in S3 mode, which lessens the effect. And of course when I use S4 it rev-matches, which also lessens the effect. Rev matching does seem better for the car though (easier on the clutch and tranny)...
Engine braking is when the engine is retarding the wheels when the clutch is engaged and the throttle is lifted, because there is less power developed than required by the wheel speed.

That transitional effect of the clutch engaging and having to spin the engine up to speed is called inertia braking. That will cause miniscule clutch wear, but might cause you to spin out when the clutch is let out too suddenly and you where close to the adhesion limit.

This is removed with re-matching downshifts.
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      03-05-2010, 02:18 PM   #22
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I remember the engine braking on my 1995 5.7l Trans-am was pretty substantial too, maybe the more cylinders the better
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