Thoughts on Compression Braking

[31Hertz]

I was reading in the archives after searching "lugging engine" for ideas about shift points and economy and etc... when I came across a thread about Compression Braking. The were good arguments for both sides, but my conclusion is that it is a bad idea. One thing that came to mind while I was reading was that when coasting in gear (be it 3rd, 4th, 5th, ooor 6th<god i love my car>:D ) quite a bit of negative pressure would be created in the manifold. With an M60 and the issues with intake manifold gaskets and Oil Separators I am more than willing to avoid a situation like this. Now, I don't have my Bentley with me, and maybe there is some sort of vacuum or electronic measure against this very thing. (I remeber my '71 Beetle had a momentary mechanical throttle "back-off") My understanding was that fuel was cut off above 1500 RPM, but how is the extra vacuum handled? Can someone help me understand purely from a knowlege standpoint? Is this even an issue? BTW I try to keep my revs around 1750 or higher on flat ground and downshift for high acceleration.

[632 Regal]

it wont over vacuum, it will get to a point an not go more negitive.

[Jay 535i]

I'm a big fan of compression braking.

It hasn't hurt my Toyota in the 100,000km I've had it. I don't plan to treat my BMW any differently once the manual tranny is in it.

[Qube]

The bottom line is... would that wonderful sound of a matched downshift be worth a probably immeasurable decrease in overall engine life? It's a touchy question... can't really quantify it :(

[mholbrook]

I think the side that is "against" compression braking is that it is much cheaper to replace the brakes than the clutch. That said, I down shift all the time I don't usually downshift where the revs are going to be really high however. Usually at between 3-4k with a blip of throttle.

In an autocross situation, you might find yourself downshifting in a higher RPM range but it's been a while for me in that situation so my memory fades.

[Jay 535i]

I think the side that is "against" compression braking is that it is much cheaper to replace the brakes than the clutch.

If you match revs properly on the downshift you won't wear the clutch at all.

Case in point: with 200,000km on it my 1986 Toyota still has it's original clutch. I drive it hard, but properly.

You're going to hear both sides of the story. We've had this argument before. I remain unconvinced that properly-executed downshifts accelerate wear.

[31Hertz]

Thanks Regal. I had halfway guessed that BMW engineers would wrap their collective head around vacuum issues like this. I am no engineer. Jay, I think you are right about the clutch issue to an extent. Proper upward shifting/clutching will slow the amount of wear as opposed to a clutch-slipping personality. But I also think that if the pressure plate and flywheel come together at any point in time you will get some wear no matter how skillfully you match revs, roadspeed, and gear#. It is an art to be sure; useable for slower wear. I really enjoy trying to make a perfect shift every time. I love those magic moments where it feels like you made NO shift at all... oooh, buttah. I'm still getting aqcuainted with my 540/6. I guess you just have to choose your poison; because everything wears with use, right?

[Jon K]

Want to hear something ridiculous?

Here it comes.

I rev match perfectly - I can tell that I do because I am the stock clutch at 184,000 miles with a 9 psi supercharger and hard driving, but when i engine brake, i ALWAYS match dead on... clutches should not last this long :)

[Kalevera]

Sure they should!

My 325iX is creeping up on 250,000m. It had the original clutch until 246k, when I pulled the transmission for other reasons and figured I'd replace it while I was in there, anyway. The original clutch disk was in okay shape, but would've needed to be replaced within a year or so. The car was owned by a bunch of idiots before I got my hands on it (ha ha). Factoring in the additional drivetrain weight and corresponding stronger pressure plate, it could be argued that the iX clutch has a much harder life than others.

Pick up this month's Roundell. There's a story about a guy who just turned 300k on his '91 535/5. Completely original drivetrain, except for the head gasket, done at 227k.


But frankly, I still wonder how your clutch has lasted so long :) Especially after those videos posted a few months back.


best, whit

[Jon K]

Sure they should!

My 325iX is creeping up on 250,000m. It had the original clutch until 246k, when I pulled the transmission for other reasons and figured I'd replace it while I was in there, anyway. The original clutch disk was in okay shape, but would've needed to be replaced within a year or so. The car was owned by a bunch of idiots before I got my hands on it (ha ha). Factoring in the additional drivetrain weight and corresponding stronger pressure plate, it could be argued that the iX clutch has a much harder life than others.

Pick up this month's Roundell. There's a story about a guy who just turned 300k on his '91 535/5. Completely original drivetrain, except for the head gasket, done at 227k.


But frankly, I still wonder how your clutch has lasted so long :) Especially after those videos posted a few months back.


best, whit


I know, i wonder the same thing every time i drive it. It doesn't bite 2nd real hard if i throw it in fast, but it doesn't sleep either.