Michael K writes:
I have 2008 Corolla with manual shifter. There is almost no engine braking power when I downshift. I know it’s not a malfunction (it’s how it’s designed), but I would like to understand why.
For example, when I’m approaching a red light and I downshift, the car barely slows down. Even if I downshift to even a lower gear to keep the RPMs high, there is still very little braking power provided by the engine. Or, going downhill and downshifting to keep the car’s speed stable without touching the brakes works only on small hills. On a steeper downhill the car continues accelerating. Of course, it’s not as bad as if the car was in neutral, but still…
Before this Corolla I had a 97 Suzuki Swift with manual. One of the cheapest cars on the road, but the braking power of the engine was good.
Sajeev replies:
The mechanics of engine braking are simple: instead of stopping the wheels at the source, you drag on them with the engine’s rotating mass. But there’s much, much more going on: factors like engine vacuum under negligible load and compression ratio determine the “plushness” of the piston’s pillow during the compression cycle. The softer the pillow, less engine braking. And I suspect the Corolla’s piston pillow is Hyatt-worthy.
But I reckon vehicle weight is the real problem. A low compression engine in a 2600lb sedan will not engine brake like an 1800lb hatchback with the rolling inertia of tumbleweed. Just an educated guess . . .
Bonus! A Piston Slap Nugget of Wisdom:
I may live in tabletop-flat Houston, but engine braking is only for killing speed over long periods of downhill travel. Approaching a red light on a city street (even the Streets of San Francisco) should involve coasting in gear until you can pop the shifter in neutral. This allows for maximum control and minimizes engine and clutch wear.
And if that sounds like a load of trash, consider the fuel economy benefit of low engine speeds. Much like a hybrid’s regenerative brakes, you can use the stop pedal to save Mother Earth. How great is that?
Or consider your wallet: brakes are one of the cheapest subsystems to fix. Powertrains are obviously not. The most brake wear is on the front axle: my last front brake job set me back $60 for (upgraded) Performance Friction pads and another $60 in rotors. Even if parts and labor is $250 for a front brake job every 30-40k, that’s but a chunk of the fuel and powertrain costs of unnecessary engine braking.
[Please send your technical queries to mehta@ttac.com]
69 Comments on “Piston Slap: Swift action on Engine Braking...”
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I don’t know the explanation for loss of engine-braking effectiveness. But we lost it some time ago (80’s) during the adding of air pollution controls. It used to be much more effective.
One of the driveline components that could take a lot of extra wear would be the clutch (unless you carefully match the speed of engine with wheels when shifting down). A clutch job is probably more expensive than a brake job.
The other thing I would worry about would be fuel consumption. You run the potential of using more fuel using the engine as a brake. Although engine controls could sense non-acceleration and lower fuel delivery.
That Corolla’s designed to have very little drivetrain drag and low rotating mass in the engine in the name of efficiency. Acceleration is more efficient, cruising/coasting requires less gas, but there’s very little to bring the car’s speed down when decelerating. It’s all about fuel mileage.
I don’t recommend aggressive downshifting for lights, etc., but I don’t recommend putting it into neutral for the coast either. I have an instantaneous mpg readout on my xB, and like all modern engines, they shut off fuel totally above idle speed. That means you’re using zero fuel, if you coast down in gear, rather than letting it idle in neutral.
I do use engine braking on longer grades, since I drive in the mountains a lot. My engine gives quite ample braking effect, but then it is geared lower than a Corolla.
I’ve always wondered about this. On both my manual transmission cars, a Miata and a TSX, I downshift to slow myself down when exiting the freeway or coming to a stop. It was what my parents taught me, I’m not matching engine speeds either, just smoothly releasing the clutch as I go into the lower gear for a second then, selecting the next lower gear, or popping neutral.
Should I stop doing this for the good of my clutch? My parents Jeep, Mustang, and Escort all had this method applied to them and all the clutches went well over 100,000 miles and outlived the vehicles. I especially like to do it with the Acura because the brakes are warped and the car shimmies a bit when I brake coming off the freeway.
Any additional advice would be appreciated.
I really don’t see how putting it in neutral on a downward incline would significantly wear on the clutch. It actually provides less wear than downshifting four or five gears and let’s you proceed on a faster level of speed.
Unless you’re using a hybrid, it usually makes more sense to simply coast in neutral if you know that you’re going to a stop or near stop. Then you should shift to to the appropriate gear when you want to proceed.
Carguy622:
My advice is fix the brakes and stop using engine braking for off ramps and stop signs in general. For long downhills, you want to help your brakes by using a lower gear and allowing the engine to do some of the work. This is to prevent the brakes from overheating at which point you lose them entirely. This assistance is not needed on short hills.
P.S. I usually do leave the car in neutral and coast up to a stop until and unless (sometimes the light turns green before I get there) I need to use the brakes. This saves gas and potentially brake wear. This, of course, is all dependent on distance, speed, and to a lesser extent traffic.
Y’all are CRAZY!!!!!
While you should be in the right gear in case a sudden avoidance maneuver is needed why wouldn’t you let Mr. Bosch do his thing through the ABS while you slow down. Traction and control will be better for most of us that way. Not to mention the lack of wear and tear on everything from the clutch out to the CV joints.
That way you can leave the rev-matching Andretti impersonations to the open road, where they are more rewarding and safer.
Another wonderful benefit of the Diesel engine in a small car: the engine braking power is prodigious!
I live in a VERY hilly/mountainous area but rarely downshift, unless it is required or desirable in the specific circumstances. For all the reasons explained above. Brakes are cheap, drivetrains are expensive.
–chuck
Never saw the point in downshifting to a stop- basic logic tells me that the clutch is absorbing the energy required to slow the car down. For a while, i tried coasting in gear when coming to a stop, but I never had an appreciable difference in fuel economy. Still seems like it’s adding wear to the clutch plate by doing this, but I’m not certain about that. My habit was always to coast to a stop in neutral and just brake, I’m okay with that.
Cool– while we’re on the subject- i just got a car with an automatic with a manual shift feature.
Since i’ve never had one before, i was curious as to whether downshifting on these is advisable to do on a regular basis, or if it is not worth the extra wear and tear.
There’s no clutch to wear out, but i’m wondering if it is adding additional wear to the torque converter. i am not too familiar with the operation of automatic transmissions..
carguy622: Uh, yes. Dropping into 2nd and releasing the clutch is almost like how brake pads reduce wheel speed via friction, except, y’know, that brakes were designed to be used that way.
Ideally, you’d rev-match to reduce the difference between rotation speeds of the clutch disc and the flywheel. Any difference between the two speeds leads to wear of the clutch disc.
I think this one is Hyatt worthy.
BTW My 19 year old Suzuki has excellent engine braking, and use very aggessive down shifting especially when being tailgated.
The amount of engine braking available has to do with gear ratios, dynamic engine compression ratios, and a host of other things (friction really isn’t one of them). But what is probably causing this problem is the idle air control follower routine in the PCM. What’s happening is that the PCM adds additional idle air as the vehicle speed increases; this raise the effective idle speed and helps to decrease powertrain lash for the average driver (you know – the folks who haven’t yet figured out the analog nature of a throttle pedal). Engine braking is decreases, but the vehicle becomes less sensitive to ham-fisted attempts at shifting.
I’m skeptical about the perceived need for engine braking in mountainous areas going downhill. I think this is a hangover from the old days, when you really risked overheating the brakes. My guess is that you’d need really long, steep downhills before you’d have a problem with today’s brakes. Of course, if you do leave it in the normal gear for the speed you are going, and take your foot off the gas, you will get that fuel shutoff that Paul Niedermeyer refers to, and so you will use less fuel than if you were coasting.
But downshifting to slow down on normal terrain is definitely nuts.
Engine braking should never be used except for when its done for safety reasons when going down extremely steep hills and/or to prevent the rotors from warping, again when going down on a steeply descending highways where you can easily end up going over 90mph without even trying hard..
My guess is that you’d need really long, steep downhills before you’d have a problem with today’s brakes.
And well, there are plenty of places with really long descents in some states. I am not talking about some short hills. Consider driving on I-17, from Flagstaff to Phoenix, which descends about 6000 feet in about 80 miles, with many curves I have driven there, and I always felt that shifting down one gear does help to stay in control. Otherwise, it’s easy to end up going dangerously (or at least illegally) fast.
Another possibility, going down a steep mountain in a snowstorm. I have been once caught on a top of a mountain (was skiing there) before I realized that none of the snow was cleared. The optimal speed for going down in those conditions would be 15mph, but if you don’t go down in say the first gear, then you need to constantly brake. One uneven move, and the car may end up in a ditch..
But if you don’t drive in mountainous areas under conditions I described, you certainly never have to think about engine braking.
My 2000 Corolla is much the same. It has the same motor. This motor was engineered to minimalize internal friction as much as possible. It has a 10:1 compression ratio which I don’t think qualifies it as a low compression ratio. My ‘90 Eclipse turbo with a 7.8:1 CR provides better engine braking.
For all you guys talking about clutch wear.. do you not rev match every downshift?
I always downshift to at least 2nd gear (in my 6spd Miata) when slowing at a light, offramp, etc. Did this with my last car too, and when I replaced the clutch at 76k miles it looked like new.. could easily have lasted another 100k miles IF I wasn’t turbocharging the car (hence the need for a new clutch).
Engine braking is just bad. What would you rather pay for, a set of brake pads or a new transmission?
You use your engine to brake if you are worried about brake failure or lockup. Otherwise, your best ways to slow down your car are to take your foot off of the gas, or put your foot on the brake.
The way I see it, if you always rev match when downshifting, then there is virtually no argument against engine braking in majority of stopping situations. Clutch wear doesn’t happen and engine is not consuming gas maintaining idle RPM. Keeping the drivetrain in gear is better for control. I would wager that the extra drivetrain wear from reasonable engine braking (<3000 RPM) is inconsequential compared to the wear from other normal and spirited driving. I would never, ever use the clutch’s friction to help burn off speed and I would not force the engine to high RPMs. That said, if there’s a fast stopping situation or if my brain is too whacked to coordinate things right, then by default I’ll mash the clutch down and use the regular brakes.
Well, the issue with the weak engine braking effect is compression, weight and gear ratio as has been elaborated on above, but I still wouldn’t recomend an increased reliance on the brakes.
You are barely using any fuel at all while engine braking (I thought this was just manuals, but I read an interview with a Toyota mechanic who claimed that autos use this trick too (true?)), while that neutral glide leaves you at idle sucking gas. Not all cars will do this in top gear, so 4th may be required to see fuel savings. Obviously this only applies as a fuel saving strategy when you need to stop at the end of the hill or control speed due to cops or traffic.
“consider the fuel economy benefit of low engine speeds.”
this is just wrong when talking about decceleration.
Rev-match every downshift, if you want to save on clutch (and other) wear, just don’t overshoot the target too often or you’ll just be transferring that wear to other parts. If you can’t heel toe due to pedal placement use third as your final braking gear instead of 2nd. A clutch will last a looong time unless you shift like an ogre, so I don’t really see the utility of wasting fuel milage to avoid shifts. I have a friend who swears up and down that his gf’s refusal to rev-match resulted in severe drivetrain lash on their second car.
The clincher is safety and control. It’s never really safe to coast for long periods of time, especially in a situation where a forced stop is possible. Keeping the car in lower gears keeps weight over the front axle and the throttle responsive.
Pch 101: Engine braking is just bad.
That’s like saying all sugar is bad. It’s a matter of how much, when, where, and why. The engine braking issue is not so simple. I live and drive in a mountainous area, often on steep, winding gravel roads. I assure you that having my car in the right gear descending these roads is quite important. It provides for a steady, smooth controlled descent, and has me in the right gear for when I need to accelerate, as in curves or brief uphills. It also keeps the brakes cool and ready. To drive in these conditions in neutral, or a high gear would be plain stupid.
The wear on a transmission and clutch from reasonable downshifting is negligable; think of how much shifting you’re doing when you’re accelerating, stop and go driving, etc. Appropriate engine braking is good. A simple yes-no, like so many things in life, won’t do.
If you want engine braking, try a V-twin motorcycle. My new-to-me TL1000S all too often has me tapping the brakes only to light up the brake lamp in order to warn those behind me that I’m slowing considerably. Even at 3,000 RPM the effect is pronounced (redline: 10,500). It’s so strong, in fact, that the bike came equipped from the factory with a slipper clutch, which allows the clutch to slip on trailing throttle to help avoid locking up the wheel on botched downshifts. However, shifting into neutral and rolling is obviously not an option on a bike.
In the car, I brake in gear until the engine speed approaches idle, at which point I’ll clutch it. If slowing for a curve, I’ll heel-toe it to the appropriate gear. If anticipating a stop, it’s neutral time. My main goal is to keep things smooth. Smooth is in control. Smooth minimizes wear on everything.
As has been said, the computer cuts fuel during in-gear deceleration. Unless you’re carburated, it actually uses more gas to idle in neutral than it does to engine brake. On the other hand, keeping the engine spinning does mean more wear, but how many people wear out the bearings in their engines before scrapping the car, anyway?
There are no absolutes.
iganpo
+1
Although I do use the clutch only in emergencies (while hard braking engaging the clutch in 2nd, no rev match), even if I would never ever do that without cause. I might not if I could heel-toe in my car, but honestly I kind of like having a sacrificial part I can extract extra drag from.
I posted the question to Sajeev. I match RPMs when I downshift (though it’s not as easy to do well with this 08 Corolla as it was with the Suzuki). I used engine braking with that Suzuki and the clutch started dying at 210 thousand KM (about 170 thou miles). When done properly (matching RPMs), engine braking doesn’t stress the clutch much, and it helps to extend brake pad life. On that Suzuki I put at least 120 thousand km on the front brake pads and they were still good when I was getting rid of the car (the rear pads were never replaced). Plus it’s cool to almost-stop without touching the brakes. That’s one of the main advantages of manual trannies. If I knew that Corolla’s engine braking is almost non-existent, I’d get the auto tranny (I couldn’t test drive manual). Thanks for the answers.
I live in a VERY hilly/mountainous area but rarely downshift, unless it is required or desirable in the specific circumstances. For all the reasons explained above. Brakes are cheap, drivetrains are expensive….
You nailed this in a nutshell. Brakes are made for easy replacement, a clutch, by its inherent design, is not. Not to mention the wasted gas.
“Unless you’re using a hybrid, it usually makes more sense to simply coast in neutral if you know that you’re going to a stop or near stop.”
Not really. Modern vehicles actually shut off the fuel injectors under deceleration conditions. You burn less fuel by leaving the engine in gear until the last possible moment if slowing down with your foot off the accelerator pedal.
Whether to engine brake or not also depends on the car. My normal car has a decently responsive 200+HP engine and a grippy clutch. Rev matching is easy and the engine drag slows the car down well enough. I recently drove around a 2008 Mini Cooper, non S. The engine had so little power that rev matching required flooring the throttle for what seemed like a full second, and engine drag didn’t seem to do much. The experience in the Cooper made engine braking seem somewhat pointless. Still, I prefered keeping the car in gear and knowing what gear was appropriate for my current speed. And rev matching is fun =)
John Horner : Not really. Modern vehicles actually shut off the fuel injectors under deceleration conditions. You burn less fuel by leaving the engine in gear until the last possible moment if slowing down with your foot off the accelerator pedal.
That’s been mentioned several times, but the instant fuel economy meter in both my Fords says otherwise. Coasting off a freeway, one gets 2-5mpg better in neutral and the other gets well over 10mpg more. (these systems measure economy with fuel injector size, on/off duration and vehicle speed) Hence why we have a comments section, to discuss my answers and find the truth!
Sajeev Mehta
“the instant fuel economy meter in both my Fords says otherwise”
From what I’ve read this is likely due to the fuel milage computer reading total # of pulses as opposed to the duration of those pulses.
The answer is simple. Unlike his previous car, a Suzuki, but like an appliance, Corolla engines have no compression. This is so nothing can disturb the living room sofa experience. The feel of engine braking (ewwww!) would be most upsetting to the Corolla crowd.
Another wonderful benefit of the Diesel engine in a small car: the engine braking power is prodigious!
I found that out in a diesel Golf in winter. Shee-yit, did I ever break traction on that first downshift.
I assure you that having my car in the right gear descending these roads is quite important.
In my view, driving in a gear that is appropriate for acceleration or maintaining speed, and engine braking, are two different things. Michael K is talking about using his motor to stop at red lights, not about driving in the correct gear on a mountain road.
I think this is on topic:
There is a leg at the beginning of my morning commute where I can let off the gas and coast for about a mile. It consists of some back roads in the city, so there are two turns to make with no stop signs, then a long downhill sweeper. The road before the first right turn is barely downhill. After the turn, the road gets much steeper, I have to brake for the left turn, then it’s just as steep all the way down the sweeper, where I actually speed up while coasting, then I gently brake to a stop when it flattens out.
Is it better for fuel economy to have the car in neutral while speeding up during coasting, or to keep the car in top gear? I’ve tried both, and the difference is negligible according to my mpg meter. But I guess I want to know in general terms. Thanks B&B!
Say What?
I disagree with the majority here. I drive by throttle. I speed up/slow down with throttle seldom touching the brake, unless I am going to stop dead.
If you push the clutch and hit the brakes every time you want to slow down, you are still cycling the clutch just as much as someone who downshifts to engine brake.
Engine braking on its own has nothing to do with the clutch. 98% of my engine braking is done without changing gears.
If you gear down because speed has changed, or specifically to engine brake, you should be rev matching to the best of your ability to minimize clutch wear. If you can’t rev match while downshifting, well that is a separate issue.
You are not braking with the clutch!!! You brake with the engine. There is a reason they call it engine braking instead of clutch braking.
For best safety and best car control it is a good idea to have the gear engaged as much as possible.
The thought that there is something wrong with engine braking is utter nonsense. Propagated by the lazy.
My BMW’s instant MPG gauge says the opposite. Different cars, different computers…?
In any case, the fuel savings of one driving style over another when it comes to coasting to a halt is darn near immeasurable IMHO.
As mentioned repeatedly above, EFI motors shut off fuel injectors on decel (0% throttle, above idle). I’d suspect some of the modern EFI systems wanting to inject a little fuel once in a while on Decel for some emissions reason, but I can’t be certain.
Engine compression plays a big factor in the amount of engine braking, but so does overall vehicle weight compared to the size of motor. My 3.0L 8.8 CR in a 2700lb car had much better engine braking than my 3300lb 2.4L with 9.x compression and my 2600lb 2.0L 8.0CR car.
Also, we can’t overlook the EGR effect (external or internal). The Corolla above probably utilizes a large amount of internal EGR at low throttle positions to reduce the pumping losses. In this case, it has a negative effect on engine braking.
That’s been mentioned several times, but the instant fuel economy meter in both my Fords says otherwise. Coasting off a freeway, one gets 2-5mpg better in neutral and the other gets well over 10mpg more. (these systems measure economy with fuel injector size, on/off duration and vehicle speed)
Ford engines (and most engines that I have driven) will go into a progression of modes when the throttle is lifted while the vehicle is under travel. For the first five seconds (for Fords) the computer continues to supply fuel to the engine. Any engine braking during this time will be slight. At around the 5 to 6 second mark, the engine will shift into engine braking mode where fuel is stopped and the engine becomes a large air pump.
So for your mileage computer test, you would have to make sure the engine had reached the engine brake mode before any comparison was accurate. Despite the instantaneous labeling, the mpg is still an average only over a smaller time. Since your vehicle would be decelerating faster with engine braking over that same period of time I think it would be difficult to use the mileage computer result with any confidence.
The last point I would like to make is that modern computer controlled engines are better balanced, more precisely controlled and will assist drivers to prevent stalling. One of the side affects of these improved engine controls has allowed for a reduction in fly wheel weight. For people who don’t rev match while down shifting, much of the initial braking force came from the energy needed to speed up the fly wheel. With a lighter fly wheel, that initial braking force in much gentler then some of us may remember from older vehicles.
Two words:
electronic throttle
In other words, the PCM is commanding some throttle opening to “smooth” things out. The nice thing about electronic throttle in most MT cars is that you can creep along in 1st gear with your foot off the clutch as the electronic throttle will try to keep the engine from stalling (”stall saver”), and if you give it a little throttle, or back off while in 1st, you don’t get the herkey-jerkey driveline oscillations. Makes stop-and-go a bit easier if you can develop the technique, and your clutch will thank you.
David Holzman :
There are no absolutes.
Is that absolutely true?
eamiller, it doesn’t take electronic throttle control (ETC) to perform idle air follower functions. All one needs is an idle air control valve, which is standard equipment for electronically-controlled engines.
On the subject of fuel cutoff during overrun as discussed by others – this is a very complex function! Most cars won’t shut off the fuel completely, as this would lead to excess drivetrain lash and potential issues with tip-up upon resuming acceleration. It’s desirable to keep the engine running with a bit of fuel – too much or too little, and you get some popping from the exhaust due to incomplete combustion (sounds awesome in a high-perf application, and terrible in some other cases). There are some pretty wicked algorithms that go into getting this “just right”, since obtaining the right air-fuel ratio during higher-RPM no-load conditions is rather tricky.
I too am positively confounded by the prevalence of engine-braking hatred. On an “enthusiast” site! I mean, I understand why hypermiling elicits derision but engine-braking?
I drive a full-size V8 pickup with a 5spd. I have been engine braking since it was new (once I figured out how as it was my first clutch vehicle) and I now have 117.5k miles on the factory clutch and factory brakes. For most of that time I rev-matched as well as possible but lately (past 15k or so) I haven’t bothered to heel-toe as aggressively to 100% match speeds.
I can confirm due to the instant MPG readout that after 2-3 seconds of high idle/no load operation the ECU does cut fuel and allow engine braking to keep the engine turning. This will yield better fuel economy than coasting/braking in neutral or coasting/braking in high gear *IF* you plan on coming to a stop. If you are maintaining speed or encounter a hill which you will not run away down you drop to neutral for best economy.
I have heard about the Ford Focus having a rather large air bleed hole in its throttle body plate. I suspect that with the very low rotational inertia of some modern engines that large air bleeds or aggressive idle air solenoid profiles are required to prevent stalling on slow shifts or on sudden throttle lifts after dropping to neutral. Large air bleeds will also prevent significant portions of engine braking by reducing the magnitude of manifold vacuum acting on the pistons during their intake stroke. My truck feels as if it has a rather large rotational inertia and therefore must not need this air bleed; it has very good engine braking.
Good point Jacob, I forgot to mention using engine braking in slippery conditions. Of course , that doesn’t mean slamming it into a lower gear to slow down it means staying in a low gear to avoid speeding up. Under snowy/icy conditions, brakes don’t always produce the desired result.
@iganpo: My BMW’s instant MPG gauge says the opposite
+1! 1995 M3 instant gas usage indicates 0.0 litres/hr under deceleration. Under idle conditions (e.g. neutral) it shows something more like 1.4 litres/hr.
As to the issue of fuel consumption, some modern manual cars turn off fuel flow when engine braking. My ‘07 Accord (I4, 5MT) does this. You can feel when the fuel injection system resumes fuel flow. This is done to improve fuel economy. The automatics might do it too – I have not driven one to find out, and don’t understand torque conversion enough to know if it would be useful if it were implemented.
A long, long time ago, when I took my drivers license test for the first time, the examiner failed me for, among other things, taking the car out of gear and coasting on the way to a stop (as taught by my father). He insisted that I gear down all the way to 2nd and not go to neutral until the car was almost to the point of lugging, so that I would be able to accelerate if need be in an emergency. Possibly a Canadian-only perversion, but as a result I rev-match downshift all the time as a habit. No clutch has ever died before my car in all my years of driving, so I must be doing it fairly well.
Many newer cars have camshafts that automaticaly advance or retard. Could this make the engine less efficient as a braking air pump? My old 49 Mercury, when engaged in overdrive , had a freewheeling feature, no engine braking at all.
I thinks are a bit off-track here. A lack of engine braking may be attributed to emission related pcm programming. On my old 03 Focus with Zetec & 5 speed, engine braking is fine until the A/C is on, then it is practically nil. When questioned, the factory techs at the dealer I worked for indicated it was a common concern, and was entirely emission related, at least on that particular model. Apparently automatics are easier to run “clean”, hence the tradeoffs with the manual programming.
Hope this helps.
Dave
“will not engine brake like an 1800lb hatchback with the rolling inertia of tumbleweed.”
What exactly is a 1,800 lb hatchback? Even a 1990 Miata was 2,100 lbs. And cars have only gotten heavier since then.
The original Geo Metro was under 1600lbs.
I’ve noticed the same thing in my Mazda3. Engine braking doesn’t do much of anything below 4000 RPM, which is almost 40 mph in 2nd gear. I use the ‘coast to a stop in gear’ method.