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Monday, July 11, 2005

Do You Use Engine Braking?

Autoguy has finally joined civilization, and taken up the art of manual gear selection.

Ben Kraal argues that "gears are for going, brakes are for stopping", and advises against engine braking.

A question for my fellow gear rowers:

Do you use engine braking to help you slow down, by keeping the car in gear and just stepping on the brake, waiting to clutch-in until the RPMs drop to near the stall point?

Or do you always clutch-in at the start of braking, and use only the brakes to stop?

Or, do you drive like a "race car driver", and downshift before braking, to maximize engine braking? And, I believe, maximize clutch wear!

In my experience, there is less braking effort required if you use technique #1 above, and I suspect less brake wear. If you rely only on "foot" braking when driving downhill, you will boil your fluid, and have little braking power at all.

Some people claim that keeping the engine in gear will put additional stress on the drivetrain. I don't see how this is so, but then again, I'm an electrical engineer.

25 comments:

Ben Kraal said...

*ahem* Ben advises against engine braking in emergency situations.

Of your 3 options, in normal driving, I drive the 1st way.

The Angry Engineer said...

It's usually Option #3 for me, grabbing as low of gear as practical to maximum engine braking and optimize acceleration if traffic starts moving again - but I know how to blip the throttle to match revs while downshifting (something I picked up from riding sportbikes), and thus there's a minimal increase in clutch wear. Trust me, if there's going to be a wear issue with that part, it's going to come from trying to get my car moving, not slow it down.

A lot of compression + a lot of cubes + short gearing = good engine braking, and my car makes great sounds while doing it.

Big Ford Fan said...

I've been driving stick shift cars for about 20 years and I always "engine brake" leaving the car in gear until revs have dropped and car is almost stopped.

In situations that require, I also downshift as I'm slowing, to take advantage of "engine braking"

When I first learned to drive a stick, I had a panic stop and hit both the brakes and clutch. This tossed the car into a spin, which I was able to get out of by engaging the clutch and applying power to the rear wheels again.

You can throw the car out of gear approaching a stop light, but in a panic stop, you should leave it in gear. More control of the car.

Anonymous said...

I vary between #1 and #3, #2 is usually a bad practice, except perhaps at low speeds on ice.

Most of the time I use clutchless shifting, using clutch only from a dead stop. It requires appropriate matching of gear speeds and engine revs. It is quite simple to brake and assist with engine braking once you get the hang of it. The requisite smooth downshift means the braking assist is appropriate, not too wimpy and not too brutal.

My cars have generally been small displacement fours, sixes, and a five-cylinder, with a broad rev range and obviously sturdy gearboxes. Clutches generally last in excess of 200k miles.

Anonymous said...

I *used* to use engine braking, but now I don't. Coming to a stop, I generally pop the transmission into neutral and coast. Brakes are cheaper than clutches.

sh said...

Coasting is illegal in many states, especially as anonymous #2 (shifting into neutral) advocates - the biggest problem there is you're hanging your a$$ in the wind if a situation arises where you have to speed up or if your master cylinder fails.

Engine braking is the way to go. The most advantagous position is to keep the traffic flow in mind and judge the best time to make a gradual slowdown if your main goal is to limit wear on both the engine and the brakes.

Or, you can do as the angry engineer and expect race-level mechanical bills while impressing the neighbors. ;-)

The Angry Engineer said...

SH,

I will say that if I were to coast up to a stop, it'd be very quick for me to put the vehicle back into gear again - certainly no slower than stomping the gas and waiting for the average slushbox to find the correct gear.

And as far as "race-level mechanical bills" are concerned, I've got that anyways - it's simply a consequence of building a very powerful engine, backing it with a six-speed transmission that was never factory-installed into this particular vehicle (building your own manual-transmission vehicle is somewhat more hardcore than simply buying one), and driving the piss out of it. My clutch assembly costs nearly $1000, so trust me, I'm not real anxious to wear it out faster. If I'm properly matching revs (easy to do when the throttle response makes it seem like the engine is reading my mind), then there's no clutch slip, and therefore no clutch wear. But hey, what do I know? I only inspect the clutch every few thousand miles or so.

For what it's worth, engine braking is a bit tougher on the engine itself (I believe this concern was raised in AP's original post). Due to the geometry of the crank and connecting rod, the largest force exerted on the rod is at the top of the stroke, and it's a tensile load on the rod. The high vacuum (or more accurately, the low manifold absolute pressure) created by closing the throttle at high revs adds to the tensile load on the rod by virture of the corresponding vacuum in the combustion chamber (primarily during the transision between exhaust and intake strokes), and denies the piston the cushioning effect of the combustion chamber charge during both the compression/power and exhaust/intake stroke transisions.

johnnie said...

I coast in gear until it's no longer practical. I used to shift minus the clutch, but it's slow and pointless in a street car with a syncro box. When I can get a dog engagement gearset for my street car, then i'll return to clutchless shifting.

Dublin Saab said...

SH,

If your master cyl fails? What if an automatic blows a hydraulic line? eh? Perhaps we should walk everywhere to maximize safety. Also downshifting will cause MORE wear on the MS and lead to an earlier failure.

Personally I go into neutral for braking and then slip into 1st as I approach a complete stop. Why? Stopping a moving car requires the removal of a lot of kinetic energy, energy that will be bled off as heat and wear. So the question is what do you want to put excess heat and wear? For me the answer is easy, brakes are cheap, transmissions and clutches aren't.

sh said...

dublin saab -
an automatic blows a hydraulic line

Huh?
First off, who cares in a manual braking thread. Secondly, I'm no auto tranny expert, but I believe that you still have the option to jam an automatic into low which uses a different set of rules.

The reason is you have a backup. The point is: it's illegal in many states to coast. I didn't make the law, I just pointed it out and looked for the reasons.

If YOU wanna walk, why even bother to post in an auto blog?

I'm with angry engineer up to the "But hey, what do I know?" part. But, hey, he's the angry engineer, ya know. :-)

Dublin Saab said...

SH,

Oh, illegal? Well shiver me timbers matey. I suppose you never ever speed then Mr. Perfect Law Abidance. I bow to your text book honoring of the letter of some idiot bureaucrats idea of a safety law.

And as for blowing a line on an automatic you are wrong. You lose pressure and it will slip into neutral and coast. Trying to put it into low will only cause a loud grinding noise.

sh said...

So, basically what you're saying:

1) You're not talented enough to match revs.
2) But you still like your speed
3) Yet you want to do everything on the cheap.

That could very well be the reason that bureaucrats make idiot laws on safety. Or is that safety laws for idiots?

Anonymous said...

Engine breaking is a damn nuance when done in front of homes; especially when speeding and going up a steep hill coming to a stop light when brakes and/or using the natural hill resistance to stop with should be used. This is why I’m pushing for a strong law in Michigan to outlaw engine braking except in emergencies going down hill.

Anonymous said...

It's too bad that no one here realizes that rev-matching actually reduces clutch wear. The racecar-driver method is the way to go if you want LOWER maintenance bills. Sheesh.

Vitor said...

"It's too bad that no one here realizes that rev-matching actually reduces clutch wear. The racecar-driver method is the way to go if you want LOWER maintenance bills. Sheesh."

Completely agree (if the throttle gets blipped for revs matching)! Driven a stick since ever and it's basically obvious if you really think about it. Although it is not easy to do (in some cars almost impossible because of the pedals position) and most people just don't see the point.

Anyway, if anyone wants to learn how to drive, watch motorsports. Things are done for a reason! ;)

Enobong said...

Shifting into neutral before braking is OK if you are already in low speed. At high speed, taking the load off the engine suddenly, by shifting into neutral, will over rev it. You may not burn out your engine, but doing it all the time will wear the engine prematurely. Downshifting before braking is good practice, and can even save you in an emergency situation. Combining downshifting with brakes, you bring the vehicle to a stop in less distance.

Micaila said...

Ok, so, I am a fairly new 5-speed driver. How does one go about matching revs? Is it practical for use in heavy-traffic city driving? I am constantly in stop and go traffic, if I do get up into 4th then I am quickly stopping again. I always pop it into neutral and break to a stop, but I was basically self-taught to drive a stick, so I guess I never really thought about it....please advise.
I occasionally down-shift to slow myself in lighter traffic approaching a light for example, but do I have time to do this every time?

Daeden said...

what are the implications on a car's MPG if u break using your engine?

Sven said...

Good Job! :)

sexy said...
This comment has been removed by a blog administrator.
sdh said...

leaving the clutch engaged as you slow gives you better mileage as it isn't sending fuel, so if you just want mileage its better to leave the clutch engaged as long as possible and downshift when practical rather than releasing the clutch and and using just your brakes

Anonymous said...

Yah, that's what I want to do... replace a transmission / clutch / ujoints, etc, instead of a $40 set of brake pads. You're not driving a damn Peterbilt with a jake brake. Gasoline engines have very little on the way of compression braking. Idiots.

chris said...

On the hydro line on an auto trans when was the last time you heard of that happening? Also I have downshifted in my pickup when towing a load down a grade more than once. Its a bit better than smoking the breaks trying to control my speed using those. Even with trailer breaks its still easier to downshit and there is less wear and tear.

Anonymous said...

I see that this conversation will never be won. The people that want to downshift will do so, the people that don't won't. I drive an 04' Kia which offers very little engine breaking. I however still match revs and downshift just to make that crappy car more fun to drive. Plus it makes for smoother gear changes than just chunking it in with the syncro. A part of me thinks that I need this talent for when I hit the powerball and then will buy a Ferrari F-40. Mainly though I just do it for my own amusement. For the people that don't see or understand this theory, I guess I'll just keep seeing their brake lights all the way through town.

George said...

I like to use engine braking, actually I kinda can't brake without using it. It is very helpful when you speed up so that when you brake you don't need to step on the brake pedal so agressive and it doesn't block your wheels. When approaching to the stop sign or traffic lights I pop the transmission into N to save up clutch...