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.

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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.