Engine Braking
 

I have questions about engine braking. I know there may be threads about it already, but I kind of have specific questions so I felt making a new thread would be the best course of action.

I've done some googling on the subject but I keep seeing conflicting information, and I want to understand it from an engine + transmission wear perspective (and clutch wear if applicable), as well as mechanically understanding what exactly is going on. When I google for information I keep seeing things like "I heard its bad for the transmission (or clutch), so why risk it just use the brakes they are cheaper to fix". I am not satisfied with such answers, I want to know what will happen - I don't want to speculate.

As far as I currently understand it; it is fine for the motor and could possibly cause wear on the transmission and/or clutch. Mechanically what is happening is the motor is spinning, drawing in air but no (or minimal?) fuel, acting as a compressor and this is the source of the braking. One thing I am confused about is how much air is being admitted to the engine during this compressor mode operation - is it the same as the amount admitted during idle conditions, or is it some higher amount corresponding to RPM or something?

When I try to picture it, I don't understand how engine braking would cause increased wear on the clutch. Doesn't the clutch only have excessive wear when it is slipping (i.e. partially engaged)? Assuming I do a proper rev-matched downshift and the clutch is fully engaged (foot off clutch pedal), how would engine braking cause any more clutch wear than normal driving? Same for the transmission - how would engine braking cause any more wear than normal driving? The engine (through compressor action) is where the energy is being transferred, so when I see arguments saying engine braking causes increased friction in the transmission and thus faster transmission wear I can see where the idea is coming from but I'm not certain.

What I am doing now with my car is using engine braking quite a bit. I am mostly engine braking using downshifts from 4500 RPM down. I rev-match whenever I downshift. I'm not excessively downshifting, but lets say I'm getting off the highway - I'll be driving in 6th or 5th gear, but when I get to the off-ramp I will engine brake in 4th gear and then maybe 3rd, and when I get to ~15kph I will shift to neutral and brake to a stop. During engine braking if I need to brake even faster, then I will use my brakes but without disengaging the clutch unless my speed gets so low that I will come to a stop.

I just want some certainty in what the results of my actions are. I'm not overly worried or fearful in how I am driving now, but my car is relatively new and I am also relatively new to standard transmissions, I just don't want to cause excessive wear over the life of the car. If I understand the whole process mechanically then I can be sure of what the results will be.

Thanks in advance!

I was curious about all of what was mentioned above myself. Hopefully we can get some logical answers. Subscribed.

There are a lot of theories our there and there really isn't a 'right' answer. The fact is that driving inherently causes wear so it's really where your bar is. I personally do something in the middle, engine breaking sometimes because it's kinda cool. I do rev match almost every down shift though, not necessarily engine breaking.

Sent from my Galaxy Nexus

NPR has a radio show called "Car Talk", which is hosted by two mechanics--both have degrees from MIT and own their own repair facility near Boston. A person phoned in (on one show a few years ago) and asked if their husband was right in saying you should save your brake pads by using engine braking. They said he was essentaily nuts. Engine and tranny make the car go. Brakes make it stop. Use the parts as they were designed. I"ll stick with their advice, although a little engine braking now and then is just fine. But you know . . . Don't use it in place of the brakes!

utilizing the engine to help brake the car is beneficial in that it lessens wear on the brakes (particularly) the brake pads,so subsequently,they last longer over time,but,of course they will still wear out,just takes longer to do so,and there is real value in that.

Let me see if i can break dpwn a few of these.

The engine pulls in an amount of air corresponding to the volume of the cylinders and the speed of rotation. The ecu simply stops adding fuel.

It will increase the wear on the clutch because you cannot engage a clutch without SOME slippage. Just dropping into neutral causes less because you are not re engaging.

It also increases transmission wear because the transmission is being operated under load, when it would not be if you brake in neutral.

Clutch wear will increase, brake wear will decrease. Churches are hard to change, brakes are not. That said, I usually downshift and use the brake pedal. These cars are made for this and frankly unless you are a bonehead with your rev matching I think you will do just fine for tens of thousands of miles. You cause more clutch wear slipping it in reverse than this will do in a week. Don't worry about it.

Source: engineer.

Engine braking becomes very important and necessary on the other side of the mountain. Brake fade is real.

First of all, thanks to all of you who have responded so far!

I want to clear something up here - my goal isn't necessarily to save the brakes. I could care less about getting new brake pads now and then. I really just want to drive in the most proper way.

A question about braking using regular brakes - would you depress the clutch and/or shift to neutral during this? Personally I try to avoid holding the clutch in at all or shifting to neutral if I am moving. I want the car to be in gear so I have maximum control - its a safety thing.

See, this seems like overly simplified reasoning to me. While I do not doubt the wisdom, knowledge and qualifications of the Car Talk guys, I need more of an explanation to fully understand it. The engine has two modes of operation - working as an engine or working as a compressor. I would argue that it is designed to work either way. But then again, I am not an expert.

this is correct,however it has been MY experience that the wear associated with this
procedure to slow the car is negligible over time,and again,in MY experience has never caused ANY durability component issues with any of the cars I have driven.

My 03 wrx has 150,xxx miles on it. Cluth was swapped at 80,xxx. I've geared down and used engine braking every time I drive. I have seen no adverse effect on anything.

According to black stone labs my engine is doing great. I have the "glass" 5 speed too that is known for being weak.

yes descending steep inclines is exactly the situation that lends itself to
utilizing the compression of the engine to "slow" the vehicle,without over stressing the brakes,
and no sarcasm intended,however it is NOT a requirement to be an "engineer" to understand this,
just many years of driving stick.

As the engine works as a positive displacement piston compressor during this type of operation, that makes sense to me. I have some mechanical inclination (I work as a nuclear power plant operator - but I am still in training and thus a noob). Since it is naturally aspirated, the engine will draw whatever volume the piston displacement will demand, correct?

So as far as engine braking goes - clutch wear is entirely from changing gears and not from the act of engine braking itself. This is what I was thinking, it makes the most sense. I'm also not worried at all about slightly increased wear from changing gears a couple more times than normal.

As far as transmission wear - i.e. "transmission operated under load" - is this significantly different than wear + load experienced during normal driving? Because if it is pretty much the same, again I don't think I would be that worried. There will ALWAYS be wear, it is EXCESSIVE wear that I want to avoid. These transmissions were designed for a certain amount of regular loading, if engine braking doesn't exceed this loading then I don't see why I should be worried about it.

I had my evo for 3 years with 107k miles (clutch was changed at 98k) and engine braked all day, infact i never changed the brakes since they were almost brand new.

After experimenting with different driving methods i found the following.
1/ Excessive downshifting causes more clutch wear , particularly more wear on the thrust bearing which spins fast on rev matched downshifts.
2/ No downshifting causes the syncro to sludge up and causes stiff and crunchy shifting.
3/There is plenty of "fun factor" in heel/toe rev matched downshifts.
The new nissan 370Z has an auto blip downshift function which does the same as heel/toe , but without the "fun factor".
I have experienced brake failure on a commodore and a torana and managed to drive a long distance back home using engine braking + handbrake.

I engine brake all the time. I always rev match, and frequently heel toe. If you're rev matching clutch wear should be relatively minimal as you can get it from fully depressed to no slippage very quickly since you're not at all worried about stalling the car and everything is spinning at approximately the same speed.

My Impreza had over 200k on it when I sold it and I drove it the same way.

The only thing engine braking has the potential to do that is negative that I've heard about is slightly increased oil consumption because of the way the pressure differential is on the rings while running in pump mode. Not an issue with an engine that's been taken care of.

I bought this car to have fun. I find rev matching and good smooth heel toe shifts very satisfying. The car will easily do them for lots and lots and lots of miles, so I'm going to do them. :)

Nathan

Brake pads are easier and cheaper to replace than engines and clutches. But wear and tear is the natural course of the life of a car so a little here and there doesn't do anything in the grand scheme of things.

I can't help but have my BS meter go off when someone uses the term "Save your brake pads". The fuck? Should i not open my door and crawl through the window so i don't wear out the hinges? :bs:

The shit people say...

It should be stated that some automatics are designed to utilize engine braking and will do so automatically. For example my moms Yaris and my companies Honda Element will downshift when you are driving downhill while braking (conditions such as speed and gradient must be just so). For the Yaris at least, this is noted in the owners manual and described as being normal and a way to lengthen brake life. Often, in the Element, it is actually a bit of a jerky experience and is a little unsettling on snow. So from an anecdotal view I would say some engine braking is just fine.

Not to thread jack but does anyone know what happens to the PCV/oil flow during hard engine braking?

Negligible wear and tear if you're not slipping the clutch.

Did you read this?
http://auto.howstuffworks.com/auto-p...ine-brakes.htm

Exactly right.

It will wear less than it would under powered acceleration, but more than it would in neutral. While the load is not very high, it is higher than neutral. Le gasp, your transmission might only last 400000 miles instead of 500. Really, don't worry, this won't break the car.

love you too :) I posted it that way because I spend too much time on reddit where you need credentials for knowing how to breathe.

there could be some other things going on but i think its worth noting that your car is engine braking all the time. if the wearing of the engine that you are concerned with is purely mechanical, the drag of the engine is always happening. i dont see how it can be bad for the trans, clutch or engine considering that the car probably has 40 hp worth of braking but nobody is worried about the damage caused by the 160 hp for acceleration

I think it depends on how you are defining "engine braking". Sliding into a lower gear on a steep grade? Not an issue. Charging a corner at 9/10 and dropping two gears and using the driveline to slow the car? Probably gonna wheel hop and that's never good.

Ingen....eer. WVU fan are ya?

Actually with the throttle closed you will not pull in the same amount of air as when the throttle is open. You are throttling the inlet with a closed throttle body. The motor goes into a high vacuum situation, and the effective compression is way less than if the motor were accelerating. Actually, even under acceleration the cylinder won't pull in the maximum amount of air it could because very few naturally aspirated engines achieve 100% volumetric efficiency due to intake manifold design, throttle body restrictions, etc.

interesting tidbit: in NASCAR, on the restrictor plate motors they were forced to run at the super speedways, teams were building the motors with 17+ to 1 compression ratios in order to maximize the cylinder filling while doing the equivalent of trying to suck through a straw.

Man, i havent thought about this in ages, I always rev match, heel toe, left foot brake, and ALWAYS engine brake as well as downshift and use hard brakes. I do this because im accustomed to maximizing brake force and minimizing brake distance.

It drives me crazy when I'm riding with someone who always coasts in neutral instead of letting it engine brake. There is nothing wrong with engine braking as every pro driver I've seen for years has done so. In fact, in some states you aren't even allowed to coast in neutral. So don't even worry about it. You're not gonna blow your engine or clutch out much sooner simply for engine braking. On the contrary, having to bring the engine back up to speed every time you want to accelerate means you'll be slipping the clutch more and sending more vibrations thru the engine and transmission/driveline.

Y'all first time mt drivers?

Learning how to use engine braking is part of drivin manual.
No harm
No foul

I am, yes.

I have found a lot of FRS owners are first time manual drivers.

Ive recomended to most I meet to take an advanced driving course at their nearest circuit, go to auto-x, drift events, and track days. Most of the people their can help you learn to master the manual, but the most invaluable lessons will come from the advanced driving course. Ive helped teach a few select candidates, but its a rare to be inspired enough to do so.

A guy at my work is an instructor at a track local to my area - he's been bugging me to take a driving course with his racing group in the spring.

I'm leaning towards doing it.

Well, don't ride the clutch, no hurky jerky, k?

But no don't be afraid to engine brake, motors are made to do this

Learning to heel toe is fun, you can use engine braking to corner by adding load to the driveline

Rev matching is important

Gem

Semi-trucks do this all the time.

Engine braking has the potential to wear things out faster, depending on how it's done, and to what extent.

In normal driving, simply taking your foot off the gas is 'engine braking'. Gearing down first will increase engine braking, but it can slightly increase clutch / syncro wear depending on how much of a downshift, and how the downshift is done.

Let's say you go from 5th gear to 3rd. If you simply jam the transmission into 3rd, the syncros have to work quite a bit to match the large gear ratio difference. There's very slight wear here. Then letting out the clutch, and letting the engine come up to speed on it's own puts slight wear on the clutch, as it has to slip slightly to speed up the engine.

You can pretty much completely avoid this wear by double clutching on the shift, and rev matching while you let out the clutch. And on modern fuel injected cars, the increased rev's don't use any more gas because the injectors completely turn off. (aside from the gas needed to double clutch in the first place, but that's fun, so worth it right?)

There's really not a huge need to engine brake to this degree. Most people gear down one gear at a time, and not at the higher rpm range. For example, I'll let the gear I'm in slow me down to 1,500 rpm or so, then gear down one gear. This will bring the rpm's back to 2,500 or so. Then I'll slow down to 1,500 rpm again. For this kind of engine braking, I don't bother double clutching / rev matching. It's not worth the hassle. I just let the syncros and the clutch to their job. There's not really a lot of wear in this example. I simply use the brakes combined with this slight amount of built in engine braking.

You could say it's a safety issue too. Say you're on the brakes decelerating when suddenly you need to accelerate to avoid something. If you're engine braking in addition to regular brakes, all you need to do is get off brakes onto gas and you're going. If you're coasting in neutral you need to get back into gear, which is why it's dangerous to use "Mexican overdrive" (who coined that term?) with auto trans.
ANyway, take the driving class! I've been auto-xing for years, but am finally taking my first class next week! Being where we are, we don't have much opportunity.

man's correct! learning how to use the engine as a break
is very useful to know.

As you disengage the clutch, the flywheel will be spinning at engine RPM, and the clutch will be spinning proportional to the wheel speed (depending on what gear you're in). Going from a higher gear (fewer rotations of the clutch for every rotation of the tires) to a lower gear (more rotations of the clutch for every rotation of the tires) can put stress on the clutch because it's going from a slower speed to a much faster speed very suddenly. This can build up friction heat, scar the flywheel, strain the springs in the clutch, etc....just puts excess stress on the whole system. If you rev match it's a bit better because you're basically increasing the revolutions of the engine to match the revolutions that the clutch will be spinning at once that lower gear is engaged (clutch released).

this is correct,you should NOT be worried about it,as an engine that utilizes a manual transmission is constantly in a state of load,along with the transmission.the car is designed to withstand these stresses,and last,without breaking.

I see what you are talking about - the distinction between volumetric and mass flow rates. I guess during engine braking the suction pressure of the piston moving down causes the air to expand, resulting in lower mass flow rates. The determining factor here must be what kind of pressures we are talking about - like you said, vacuum pressures (below atmospheric). I can see how developing that vacuum aids in engine braking.

Volumetric efficiency is something I didn't consider before, I'll have to look more into it. I'm curious what kind of numbers this car has.

Of course this isn't something that I need to think about while on the road driving - but I like learning about this kinda stuff.

Technically, every shift shortens the lifespan of the clutch (and anytime you engine brake, you're putting strain on the drivetrain) because any set of mechanical components has a limited number of cycles. But in this case, the typical number of cycles before failure is so high that it's not worth worrying about (assuming one doesn't dump or slip the clutch a lot).

I pretty much always engine brake, doing a quick double-clutch and rev match with almost every downshift (the exceptions being low RPM downshifts, for the reasons titan_rw mentioned). This is anecdotal, but in over 20 years of driving I've never needed a clutch job (though I replaced the master and slave cylinders on my 13-year-old MX5... a common issue with the NB).

Doing some ballpark estimates (counting upshifts and downshifts), I came up with 90,000 shifts/year in my BRZ. If I keep her for 12 years, I'll hit 1 million shifts, which underscores why I won't settle for a crappy shifter (WRX, MS3, Focus ST, MCS, GTI, etc) or even a mediocre shifter (STI).

Putting it another way:

• Does it reduce brake wear? Yes.
• Is that why I do it? No.
• Does it increase wear on the clutch? Yes.
• By enough to worry about it? No.
• Is it safe? Yes (in fact it can increase safety).
• Is it fun? Yes (this is mostly why I do it).

Holy crap do it! I have been driving stick for a while now, and would still jump on that. I bet it will shorten the learning curve a lot.

Wow, so much misinformation in this thread, with only a bare few bright nuggets of truth!

In the interests of having complete control of the car and maintaining ability to accelerate if it becomes unexpectedly necessary *at all times*, the car should *always* be in gear with the clutch engaged while driving, except when upshifting, downshifting, or while stopped.

While slowing down, you will get engine braking, which is NOT GOING TO HURT ANYTHING. The slight drag loads on the drivetrain is MUCH smaller than the loads it was designed to handle.

If you are slowing for a stop sign or traffic light, you don't have to downshift through all the gears, but it is good form. To downshift through the gears while braking with a 3-pedal car without automatic rev-matching requires heel/toe, a topic worthy of its own thread so I won't get into it here. Other than to say that if you have a modern synchro gearbox, double-clutching is NOT REQUIRED.

If you are slowing for a lower-speed curve that requires you to be in a lower gear on exit, the RIGHT thing to do is get into that lower gear *before* the curve. If braking is involved, this requires heel/toe downshifting.


Anyway, you use "engine braking" NOT as a primary way to slow the car (though that is a somewhat beneficial side-effect). You end up using engine braking because if you are driving properly, maintaining maximum control and ability to accelerate instantly at any time, you get engine braking when slowing down.


Regarding how much air goes into the engine, if it was the same whether the throttle was open or closed, then what's the throttle for?!