Compression braking - good? bad? ugly?
 

I'm curious to hear opinions regarding the advisability of using compression braking. I'm not talking about during racing, but rather for everyday driving, i.e., when slowing down (instead of just regular braking).

I tend to downshift as a matter of course to slow down on freeway off-ramps, going down hills, and other such times...instead of riding the brakes. That is undoubtedly part of the reason I've got almost 40K miles on my X5 and still haven't had my first brake job yet. Obviously, this saves money on brakes, but how much extra wear and tear does it put on the transmission, etc.?

The first rule of thumb is that brakes are cheaper than engines. Using the transmission instead of the brakes all the time is false economy, IMO.

If you are not over-revving the engine, then it doesn't particularly hurt the engine. Higher piston speed results in more wear, but you are likely within a normal rev range. Holding it in a lower gear doesn't hurt the transmission, but the shift itself does cause wear on the clutch packs. BMW engineers the transmission controls to reduce the shock when the transmission upshifts (by backing off the throttle via the ECM), but that effect isn't there when you downshift manually. Downshifting at every stop sign and exit ramp will likely shorten transmission life, but it isn't a linear relationship, and you may never see the effects in the real world.

Personally, I don't downshift coming to a stop or on an exit ramp, but I do drop it down to 3rd coming down one particularly steep 3 km hill every morning. I just prefer not to ride the brakes all the way down, and I don't want to speed through the neighbourhood.

I do use the transmission more when descending hills when hauling a trailer. I trade off the increased wear to gain more control.

you shouldn't ride the breaks
but you should use them properly, when descending on hills, break harder, let go, break harder let go, give them time to breath, cool off a little
not like typical minivans you see break lights on all the waaaay the f00kin hill ;p
as far as using the gears to slow down, yes it's perfect but on manual cars, on automatics i'm not so sure about the extra wear

I have observed that on my 04 4.4 if I am in sport mode, the transmission downshifts agressively under braking - just as I do in my manual transmission Z4.

Very helpful post, JCL. From what you're saying, it sounds like it would be better to keep the X5 in S mode rather than downshifting manually, since presumably BMW's engineers have made the shock to the transmission less traumatic to the engine when it downshifts itself (which, in S mode, is at higher RPMs, therefore providing at least SOME compression braking, whereas in standard auto mode, there is virtually NO compression braking), right?

I don't think there is a traumatic shock to the engine in either mode. I think that there is more wear on the transmission, but unless you are are taking it to extremes I wouldn't worry about the engine wear. A guide would be that if you are around the rev range where the transmission would shift anyway (ie fairly close to the S mode shift point) there isn't much wear of any type to worry about.

The advantage to the manual mode is that you can modulate the throttle to try and reduce the driveline shock. Driving well is about smoothness.

I am not a fan of S mode, but that is just me. I think it is an operating mode dictated by the marketing department, not the engineering department. I do find the manual mode helpful, but only in certain situations, ie a long hill at a certain grade, or towing a trailer. The modes are there for people to use, however. We don't have sufficient empirical evidence to be able to link driving styles to transmission wear. I would just keep in mind that at any mileage from 100k up, an X5 transmission could go, based on anecdotal evidence on this board and elsewhere. I wouldn't try and speed up that process, myself. That's my quarter done.

Oops - I didn't mean to type "engine" - I meant the transmission. But your additional comments make sense.

The 4.6is (and 4.8is) supposedly have more heavy-duty transmissions than 3.0 and 4.4 X5's, but I think all of your comments still apply. I'm just finding that since I got the Dinan S3 upgrade, I have an urge to keep RPMs higher (on average) than I did before (perhaps to hear the throatier growl of the Dinan exhaust):o , but I suppose as long as I don't get carried away, I shouldn't cause TOO much damage...

Interesting Thread and I was/am curious to see some
responses from the handful of us mannytranny drivers...

I must admit I downshift in the X, (5 spd manual), not
to the point of high revs, but frequently snick down
through the gears rolling up to a stop. And, back at
our Mtn house area, I use a lower gear frequently
in the steep descents.

I do the same on my MN6 Vette and I've done it
for years on all my M'Cycles: I find that my courage
is less than it used to be, so in very sprited runs
on our constantly curvy 2 lane blacktops, I go into
corners hotter than I should/like, but ease off the
throttle in that lower gear for some very firm but
controlled, (to my mind), deaccelleration to apex and
then, nail it coming out.

Not lecturing on the best line, as it is really different
for hot 4 wheelers vs 2 wheels usually, and I understand
trailing throttle, trailing brake, late braking, apexing, etc.

But, at nearly 60, my reflexes and courage are slightly
eroded, so I find going in hot, corner-wise, can often
be saved smoothly, by staying in a lower than normal
gear and letting engine decomp. be my initial/added
braking force for just a second or two.

I never did much lower gear braking on the automatics
I've driven over the years, other than to slow gently
in the snow and/or on hilly, slippery terrain. I don't
see a lot of benefit in an auto: the sumbitches are
pretty smart, programming-wise.

My Qtr's up...
BR,mD

Even though it is a beemer, I agree with JCL. Tranny life is more critical than brake life.
That said, shift for fun not habit is my credo.
Esp with 6 spd auto tranny, believe me, it
is shifting enough already.jmho.