Engine Braking

[Wattage]

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.

[gmookher]

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

[mact]

Semi-trucks do this all the time.

[titan_rw]

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.

[root]

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.

[jmaryt]

Originally Posted by Wattage

I am, yes.

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

[CaptainSlow]

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).

[jmaryt]

Originally Posted by Wattage

I am, yes.

Originally Posted by Wattage

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.

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.

[Wattage]

Originally Posted by zooki

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.

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.

[Deslock]

Originally Posted by Wattage

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?

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).

[Ingen]

Originally Posted by Wattage

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.

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.

[ZDan]

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?!

[Ingen]

Originally Posted by zooki

Actually with the throttle closed you will not pull in the same amount of air as when the throttle is open.

Gah! I shouldn't drink and post.

You are correct and I was not - I somehow completely forgot about the idle air control system. My bad.

Cool tidbit about the nascar though!

Off topic: Does the compression braking / "jake brake" on a semi open the throttle and allow full cylinders of air to slow the engine? Just wondering, I know they can be very loud and intuitively it would make sense, but I am not familiar with the system beyond noise from a distance.

[jmaryt]

Originally Posted by ZDan

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?!

"double clutching is not required!" this is very true.with modern "helical" cut gears,double clutching is a thing of the past.it WAS required back in the day when truck transmissions gear boxes utilized "conical" cut gears,but with the advent of modern synchros,AND "helical" cut gears,double clutching now is something only people who have driven "stick" for a very long time do anymore.double clutching DOES increase wear on the "throw-out" bearing,however,the wear is negligible,and this part will STILL last a very long time,(hundreds of thousands of miles),so again,no cause for concern!