curious about engine breaking 1

[rwelk]

I don't know if anyone can answer this question. But I was just curious if engine braking a car (manual transmission) has any negative effect on the motor, or gearbox?

 

[ewh]

I do know that it's cheaper to replace your brakes than it is your engine.

 

[GregLocock]

I agree with the second poster, but so long as you do it correctly the loads are well within the capabilities of the components. Unless I am on a slippery surface I brake in neutral.



Cheers

Greg Locock

 

[patprimmer]

I agree with both previous posts except that it is may be cheaper to replace motor and gearbox than to repair damage caused by loss of brakes due to overheating. It depends on why you want to use the engine as a brake.

If you need to use the brakes to the extent that fade becomes an issue, also use engine braking, otherwise, Greg's advice is sound, although it is mostly more convenient to just brake in the gear you are in , or the one you will need after braking with the downshift occurring during the braking.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[rmw]

Brake fluid is hydroscopic (I hope I spelled that right) meaning that it has a tendency to absorb moisture over time. If you have ever had your brakes get really hot, like on a long mountain down grade, and had the moisture in the brake fluid boil, and all the brakes just go away, (I have, more than once) you will use all the engine you can, including turning on accessories to absorb power, and use the brakes as little as possible.

That is the way my old daddy taught me to drive, anyway.

Now, in an area where engine braking is enhanced through the use of devices that modify the valve timing, and convert the engine into a compressor during engine braking, on long haul diesel rigs, the engine wear factor due to engine braking is documented at about a 15% penalty on engine life. So, while probably not so severe for your case, it is nonetheless real.

rmw

 

[Tmoose]

If I down shift by e-a-s-i-n-g out the clutch and dragging the engine up to speed, rather than matching revs a bit before simply enagaging the clutch, I suppose each down shift adds a tiny bit of extra clutch/flywheel wear. Starting-from-rest techniques and Shifting like that, up or down, might be the difference between a clutch lasting 3,000 miles (young man with 427 Camaro) and 240,000 miles (my old Volvo 240).

I think my current car has PLENTY of brake capacity even for coming down Mt Washington, so the overheated brake defense would be like wearing elephant repellant.

A greater issue for me is the smooth-ness upsetting the chassis or my passengers.

 

[rmw]

Tmoose,

If you ever loose your brakes on a long (and usually curvy mountain road), due to overheating or whatever, post back, and I will recommend a good brand of elephant repellant. Actually your laundry man will wash a real good brand of elephant repellant out of your britches.

rmw

 

[automatic2]

No adverse affect whatsoever, assuming you use good shifting/clutching techniques, ie speed matching, no lugging. Gives good braking quality without excessive nose dive, very important for cornering.

 

[notnats]

Constant negative pressure in the cylinders with the throttle closed may suck some oil past the rings and valve guides, but that will only result in a puff of smoke when you accelerate again.

Jeff

 

[jetta87]

This is a bit off subject, but does anyone know if a car stops using fuel when engaged in engine braking? I heard this a while back, and it makes sense to me, but started quite a discussion (arguement, really) on a roadtrip this weekend.

Thanks.

 

[GregLocock]

Depends on the car.

Many (most? virtually all?) EFI or electronically controlled carburetter cars do shut the fuel off, most of the time, when the engine is in overrun, ie throttle is closed but revs are higher than idle.

However, in order to prevent excessive contamination of the cat by drawn over oil, or maybe just to keep the cat hot, several engines seem to give a quick burst of fuel at around 1800 rpm on decel. (VW Jetta 1988, Nissan funny little sports car from 1995 I particularly remember). This can be quite annoying.





Cheers

Greg Locock

 

[evelrod]

I guess I'll add my two cents worth---having raced drum braked cars in the 50's and been left "wanting" on more than one occasion---a one hour race ment that you did the last 45 minutes sans brakes!
My dad tought me to drive in cars that required engine braking if you wanted to consistantly make it down from Cloudcroft, NM to Alamagordo---steep down hill mountain road in the 50's (still steep). Times have changed. Brakes no longer "fade" like they once did. In fact, my wifes car has better brakes than many of the vintage racers I compete against today. I still race, but no longer use the engine for anything but acceleration. Braking is solely by the quite capable brakes. Many of my friends drive modern sports cars and have never learned to use the engine for deceleration. So far as I can tell, there is just no need, under normal conditions, to use the engine for braking.

Question---does engine braking decrease the life of trans or engines? In the "old days" I did commonly replace "U" joints and axels rather often. A good lifespan for an engine was 100k miles. A clutch was good for half that and valve jobs often came as early as 20k miles!
Was the cause the common practice of using the engine as a brake? Was it inferior materials or construction design/tecnique? Can we really compare yesterdays automobiles with todays? I think NOT.
To assume that most people driving today can master the art of heel/toe would, IMO, be a stretch. Indeed, most drivers nowadays can't even DRIVE a standard shift in the U.S. and the minority that can wouldn't know a 'double clutch' from a wing nut!

Rod

 

[MikeHalloran]

Whichever side of this old argument you come down on, I think it's important that you learn _how_ to decelerate a car with just the engine, because every once in a while, for a variety of reasons, you will be driving a vehicle with no useful brakes.

With luck and practice, it could happen to you more than once in a lifetime.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[MintJulep]

Evel,

My first car was a BMW 2002, the Getrag transmission was infamous for its poor synchromesh. The gas and brake pedals were however perfectly set up for heel/toe. So, I learned out of necessity to do both, either with or without use of the clutch pedal.

While the car I currently own is an automatic, on the rare occasions when I do drive a manual I still double-clutch my downshifts as a matter of habit. Passengers often ask what I am doing.

 

[GregLocock]

Yeah, my double declutching's like that!

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SolderingGunslinger]

AS long as you do not exceed the ratings of the drive components, engine braking shouldn't be a problem.

I do a fair amount of mountain driving, to and from mountaintop transmitter sites. On the way down the mountain I ride in a lower gear letting the engine do the braking. I apply brakes only when necessary for traffic or unexpected road conditions.

The "Sky Islands" of the Desert South West are quite a tourist attraction during the heat of the Arizona Summer or when snow covers the mountains above warm and sunny Tucson. When traveling the Mount Lemmon Highway, the smell of burning brake linings permiates the air. Flat Landers haven't learned the old mountain driving trick, Go down the mountain in the same gear you needed to get up.

Some truth can be found in most of the other posts in this thread. Remember, everytime you make a service application of your brakes, you are wearing off brake lining and drums/rotors. Often, after a long downhill run, you'll need them at or before the bottom. Save the linings, rely upon manifold Vacuum (or air compression with a Jacobson style engine brake in a Diesel) to slow the vehicle and a service application of the brakes to stop.

I learned to drive in a '52 Chevy 1/4-ton pick-up and graduated to an Austin Healey and Shelby Mustangs.

I remain,

The Ol' Soldering Gunslinger

 

[evelrod]

The front side of Mt Lemon is a piece of cake. Try going down the back side without brakes!

Rod

 

[ajack1]

Anyone who does any serious off-roading or rallying brakes with the engine, for general road use I would not say it is necessary but still a good tool to have in your box.

 

[NickE]

"Anyone who does any serious off-roading or rallying brakes with the engine,"-ajack1

yep, i prefer the engine braking when i dont need weight transfer, to make the front bite up and the back swing around in the infamous swedish flick a little LFB (Left Foot Braking) usually will do.

I find it interesting that no one here mentioned the use of engine braking in performance driving.

 

[ivymike]

evelrod - I still race, but no longer use the engine for anything but acceleration. Braking is solely by the quite capable brakes.

NickE - I find it interesting that no one here mentioned the use of engine braking in performance driving.

Actually, someone specifically mentioned NOT using engine braking in performance driving. Judging by the video footage, I'd say he's got a bit of experience too...