Engine braking - any negatives?

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shiftmate
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Engine braking - any negatives?

Post by shiftmate »

I'm new to manual transmissions, and have always heard that engine braking is bad for the engine. But from what I understand about MTs, engine braking:

1. saves you gas - because in neutral, you'd be using gas to maintain idle revs, whereas with a downshift and no throttle, the wheels drive the engine and the gas is cut off (at least in modern engines)
2. reduces wear on the brakes

So are there any negatives to counteract those positives, assuming you're not engine braking by downshifting into redline revs?
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Re: Engine braking - any negatives?

Post by Tups »

You shouldn't use engine for braking - the brakes are there for that. However, if you are approaching e.g. red lights, slowing down by coasting on gear is a good way to improve your gas mileage because not only do you save gas due to DFCO, but also you might avoid stopping and launching at the lights.
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Re: Engine braking - any negatives?

Post by theholycow »

Engine braking is harmless but only really worthwhile in certain situations. Brakes are made for stopping the vehicle and they are very good at it.

FYI, engine braking can be done with an automatic even more easily than with a manual.

Addressing your specific concerns:

1. This effect is called Deceleration Fuel Cut Off (DFCO) and is not as simple as you described it. There are plenty of variables that the computer considers when it decides whether to DFCO or not. Connecting a meter to a fuel injector wire can show you exactly when you get DFCO so you can learn your car's DFCO behavior patterns and, optionally, try to leverage them.

2. Yes, but at what cost? Either you execute a flawless double clutch rev match to get into a lower gear (which spends fuel for a blip and reduces the chances of DFCO engaging), or you put wear on your clutch and synchronizers. Brake pads can be replaced easily at home for $20 and an hour of work.
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Re: Engine braking - any negatives?

Post by FDSpirit »

I tend to use engine braking in the situation Tups described. I coast to lights on roads I normally drive on to avoid having to launch again. I've never really been one to ride my brakes. My pads are still in like new condition after almost 3 years of driving on them.
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Re: Engine braking - any negatives?

Post by theholycow »

^Brake avoidance with coasting is a good practice for fuel economy as well as brake pad life. It applies especially to red lights, as described, and also to turns...any time you'd spend fuel maintaining speed then brake and have to re-accelerate, if you can coast and then safely keep some of your speed you'll have to re-accelerate less.

So, a few more thoughts...

Fuel usage during non-DFCO engine braking is not merely the same as at idle. It is almost 100% proportional to RPM (measured with the previously mentioned meter connected to a fuel injector wire). It is certainly far less than it would be if the throttle was open, but more than idle.

For most of the issues I've discussed, we are splitting hairs. Regardless of which way you brake, the amount of wear on brakes or on clutch and synchronizers is tiny. Nothing is free, you rob Peter to pay Paul, but it's a tiny amount. Here's a couple more important issues...

Brake lights: Your brake lights do not come on while engine braking. If there is nobody around then it doesn't matter. If you are trying not to attract attention (slowing down because you see a cop and you were speeding) then it is helpful. If you are trying to stop your vehicle and someone is behind you then it is a Bad Thing...if the cop believes that your brake lights didn't come on then you may be partially responsible when you get rear-ended.

Traction: In low-traction conditions, your brake pedal engages all four wheels. Engine braking only engages one wheel (in a 2WD vehicle with a normal differential), making loss of traction more likely. On the other hand, if you're using your brake pedal and your drive wheels have extra traction when your non-drive wheels are almost skidding, engine braking becomes your best friend (this doesn't actually happen unless your brake proportioning valve is failing).
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Re: Engine braking - any negatives?

Post by Shadow »

We all use engine braking whether we realize it or not. If you're in gear and coasting, then you're using engine braking to some degree, more so in lower gears than higher gears. It's not harmful to the vehicle at all. And yes, the fuel pulse often does go to zero during engine braking.

And yes, there are situations where I use engine braking to avoid using my brakes. A perfect example is a long, straight downhill road near my house. I'm normally in 4th gear as I approach the hill. I'll shift down to 3rd gear and coast down the hill. This allows me to maintain my speed without having to brake-coast-brake again-coast etc... until I get to the bottom of the hill.
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Re: Engine braking - any negatives?

Post by 94Corolla5Speed »

I will say that engine braking might not be the best thing to do if you're at high RPM in say 1st, and you instantly take your foot off the gas. Fine for your engine, but it might not be the best for your mounts (in a FWD car)

Then again, the amount of wear is minimal, so disregard everything I just said and engine-brake away!
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Re: Engine braking - any negatives?

Post by shiftmate »

theholycow wrote:2. Yes, but at what cost? Either you execute a flawless double clutch rev match to get into a lower gear (which spends fuel for a blip and reduces the chances of DFCO engaging), or you put wear on your clutch and synchronizers. Brake pads can be replaced easily at home for $20 and an hour of work.
Excellent point.
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Re: Engine braking - any negatives?

Post by tankinbeans »

Since I've gotten to be okay at rev-match downshifting, I use downshift engine braking (instead of just leaving the car in car while coasting) in a coupe situations.

1) If I'm behind a guy who is tail-gating the guy in front of him and hits his brakes every 2 seconds to maintain a "safe" distance, I will use downshift engine braking (a term I just made up and I hope is clear) in conjunction with the regular brakes to scrub a ton of speed at once. With this I won't apply the accelerater, but I will ease of the clutch slower than normal like a "regular downshift". I know people are going to tell me that I should be paying attention, and I am, but there will be those times when you're trying to leave a buffer and the guy in front will nearly come to a complete stop in no time. You get used to how much he has to back off and you try to back off accordingly, and then he throws a wrench in the whole process. (I hope this is clear enough without me sounding like an irresponsible driver, because I try to play well with others in traffic.)

2) The second is when I am approaching a stop light and trying to leave the car in gear to coast through, but am following somebody who doesn't know whether to speed up or slow down. I know that you can ride pretty much all the way up to the light in high gear, but if you can't really be changing speed too quickly or your car, at least mine, will shudder and stall. In these cases I'll shift into whichever gear will allow my car to be spinning at 2,000 RPM so I don't get that gut-wrenching shudder.

I could probably be doing things slightly differently, but I'm still learning what works best for which situations. As you can see, the only time I really use downshift engine braking is when I'm following somebody who doesn't know if he wants to poo or get out of the outhouse.
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Re: Engine braking - any negatives?

Post by scionkid »

1) if you are already using the btake to slow down, there is no point in down shifting. You down shift as you brake because you are expecting to accelerate immediately afterward (eg slowing down for a turn) or the hill is steeper than you thought and you need more engine braking. And even when you have your foot on the brake, you can rev match by heel & toe.
Downshift engine braking as you described uses the clutch as a brake, an expensive one at that. IMO, downshifts should be rev matched. Engine brake occurs when you ease off the gas from the rev match.

2) Down shifting in that situation is normal
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Re: Engine braking - any negatives?

Post by tankinbeans »

scionkid wrote:1) if you are already using the btake to slow down, there is no point in down shifting. You down shift as you brake because you are expecting to accelerate immediately afterward (eg slowing down for a turn) or the hill is steeper than you thought and you need more engine braking. And even when you have your foot on the brake, you can rev match by heel & toe.
Downshift engine braking as you described uses the clutch as a brake, an expensive one at that. IMO, downshifts should be rev matched. Engine brake occurs when you ease off the gas from the rev match.

2) Down shifting in that situation is normal
Like I said, I'm sure there are things I need to be doing differently. I'm still not good, or even fair, at heel-toe. Usually when I downshift and use the brake is when the people in front come to an almost complete stop unexpectedly. I do pay attention, but situational maneuvers always apply. I do take your point and don't use engine braking that much, for the above-mentioned situation. Otherwise, my downshifts are always rev-matched, if a little poorly, for turns and such.
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Re: Engine braking - any negatives?

Post by scionkid »

I get your point. There are times when l couldn't downshift in time. In those situations, I would just brake and floor the clutch if necessary. Then downshift and rev match whenever I can.
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Re: Engine braking - any negatives?

Post by Rope-Pusher »

http://www.designnews.com/document.asp?doc_id=235313
Chevy Volt Tech Watch: Regenerative Braking

GM tuned the Volt's control software so that the car drives like a snappy gas-powered car with an automatic transmission. If you want your Volt to drive more like a sports car with a manual transmission, do the following:

Take off and get up to a reasonable speed.
Shift the transmission selector to the Low setting. This engages electric motor braking when you’re coasting.
Press the dashboard Sport mode button. This will give you a bit more zip at higher RPMs.
Now take your foot off the gas pedal.

The Low setting makes the Volt's regenerative braking considerably more aggressive, like driving a manual transmission car in second or third gear. You'll slow down quickly without needing to tap the brake pedal, though you'll still need to do that to come to a complete stop. Doesn't that feel more like you're driving a sports car?

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Re: Engine braking - any negatives?

Post by Charles421 »

Eh I don't have anything to add except I downshift going towards a light if I know it's going to turn green. I would sooner downshift on my own time at my preferred RPM than do it at some unknown speed closer to the light.
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Re: Engine braking - any negatives?

Post by theholycow »

Charles421 wrote:I would sooner downshift on my own time at my preferred RPM than do it at some unknown speed closer to the light.
At speeds closer to the light, your own time becomes relative.
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