Tell me about Downshifting (or Engine Braking)

I’m still new to manuals, and I have a couple questions about downshifting to slow the car. I know that it causes more wear and tear on the clutch, but that’s not what I’m concerned about.

Specifically, what’s the proper way of downshifting? Say your car is going 45 in 4th gear, but you you need to slow down to 30. From what i’d heard, you’d put in the clutch, shift to 3rd, then slowly release the clutch until 3rd gear is engaged. But from what I’ve read recently, it seems my understanding may be incorrect (because you’d want the clutch to be released as quickly as possible.

So how should one actually go about downshifting? Please explain to this mechanically uninclined idiot.

I’m sure the replies will be all over the board on this one. If I needed to slow down to 30mph in the situation you described, I generally wouldn’t downshift at all. I would leave the car in 4th, take my foot off the accelerator and use the brakes or just coast down to speed. I would not downshift until I needed to accelerate again. Most, if not all, autos are capable of cruising at 30mph in 4th over level ground. Obviously if you’re going uphill, carrying a heavy load, lugging the engine, etc. you’ll want to be in 3rd.

If I know I’m approaching a stop, I will usually just put the car in neutral and coast up. Doing so saves gasoline and wear and tear on the clutch.

In normal driving, you should not use engine braking to slow the car. You should also generally try to avoid shifting into a lower gear until the vehicle speed matches the engine RPMs in the new gear i.e., the engine will be turning 1000RPM at 25 mph in 3rd, as you make the shift, your foot is off the gas pedal and the RPMs fall so that when you engage third gear your engine speed and vehicle speed are roughly in sync. There is no need to let out the clutch slowly when downshifting, and downshifts should be as smooth as upshifts. If your car jerks, you are doing it incorrectly.

Keep in mind brake pads are inexpensive and easy to replace while clutches are expensive and more difficult to replace.

<hits submit and waits for the inevitable treatise on heel-and-toeing, powershifting, and double clutching which is sure to ensue>

Push in the clutch, rev the engine a bit while shifting so that the engine RPM matches the wheel RPM (or is at least a lot closer), then let the clutch in quickly. The closer you match the engine speed to the wheel speed, the less wear you’ll put on the clutch.

The extra wear and tear is going to affect more than just your clutch. You are going to be a bit unkind to your synchronizers and other parts of your transmission and drive train as well.

What Stan Doubt said.

Best way to slow from 45 to 30? Brake. Simple as that. No need to use the gears, so don’t - and the logic I was given when being taught is that some of the moments you’re most likely to need both hands on the wheel are while braking (any potential problems, from ice to worn tyres, are likely to make themselves known at this time). Once you’re down to 30, by all means shift down a gear.

Sorry, perhaps I was unclear. What I meant to ask originally was “If I had to downshift, how would I do it?” I’m not asking for opinions on whether I should or not. Also, the 45-30mph I listed was just a tool to help explain my point, might as well be 45-15.

Engineer_comp_geek seemed to answer my question pretty well, if anyone else wishes to add their input, that’d be great.

I’m sure you’re right, but for some reason I’m having a hard time wrapping my mind around this. It just seems so odd applying gas when you want to slow down. Is that correct? And what RPM should I be aiming for?

If you want your synchronizers to last, learn to double clutch, especially when down shifting.

From 4th to 3rd:

-Press in clutch. Move shift lever to neutral.

-Let cluch out, blip throttle to raise engine RPM to approximatly where it will need to be for 3rd gear at given speed.

-Immediatly depress clutch and move shift lever to third gear, then let clutch out.

If you match the RPM correctly, this whole process is faster (yes, really) and smoother (yes, really) than letting the sychronizer spin your clutch plate up, then using the clutch to spin up your engine. It took a lot of words to esplain it, but in practice it becomes just stomp-rev-stomp.

Upshifting is less stressful on the sychros and clutch, but can still benifit from double clutching. In this case, you omit the “blip the throttle” step, instead you pause in neutral for a fraction of a second (maybe a whole second in a big truck) to let the engine slow to the speed needed for the higher gear.

When you get really good at this you can do it without using the clutch.

This is exactly how I was taught to shift through 3rd-4th-5th. Basically, take your time over the shift, and you kick in with full power.

Mountain passes constitute “normal driving” for many folks. They should most certainly be using engine braking to limit speed on downgrades.

If you use engine braking to slow for a curve, you are then in the correct gear to accelerate through/beyond the curve.

The speed you are aiming for is what the engine would come up to if you just slowly let the clutch out slowly.

The idea is that you use a blip of throttle to spin the engine up to that speed instead of making the clutch do that work. Less wear on the clutch, and once you get good at matching the speed, you can accomplish your downshift much faster and/or smoother…you no longer have to trade fast for smooth.

In the double clutch version, you do this with the clutch out, and the transmission in neutral. This offloads the small clutches inside the transmission that are used to accelerate the clutch friction disk. (synchronizers)

That lets the sychros do thier thing. If you pause in neutral with the clutch engaged (up, out, whatever) and time it right, then the syncronizers have very little/nothing to do.

Using gear choice, i.e. limited power, to maintain a speed is one thing. Changing gear specifcally to decellerate is very different.

Steep grades and mountain passes are no more “normal driving” than the ambient temperature in a Kalahari kitchen is “room temperature”. You knew what was meant and the OP knew what was meant.

Seriously though, do you really have nothing better to do with your time than pore over posts for the most minute generalization or statement that might be construed as technically incorrect when interpreted literally?

Unless you’ve got insider knowledge to the contrary, the OP may well deal with steep gradients and mountain passes on a daily basis. On the other hand, he/she may well work in a Kalahari kitchen. Should we assume the generic average norm for all posters that don’t give specific information?

So, where’s the cite for avoiding engine braking during ‘normal driving’? Everything I’ve ever read or found online about downshifting says nothing about it being bad for the car, but actually recommends it for saving the brakes.

Just curious, but how does the average schmuck downshift? Do they use that method, or simply release the clutch slowly, as I mentioned in the OP?

I ask because I hadn’t really heard of the “matching RPMs” method until I researched it. Is it something most manuel drivers know?

I already gave a pretty damn good reason for using brakes as the standard method - if you hit a patch of ice (or oil, or gravel, or your tyre loses grip) you’re in a much better situation is your hands are both on the wheel, than if you’re busy downshifting just to save a miniscule layer of brake pad.

Safe driving isn’t just about ‘normal’ situations, it’s about being able to deal with unexpected ones.

Brakes are cheaper than clutches.

Is there any ‘harm’ in downshifting in a situation where engine braking is used, but then the gear you downshifted to is then used for acceleration.

i.e. you are comming up on a car going way under the enforcement tolerence of the speed limit. You know there is a passing zone comming up. You downshift using the engine braking to slow down (so you don’t run into this car driving too slow). When you get to that passing zone you can pass them in the gear you downshifted to. Is this somehow worse then braking to avoid hitting the slow moving car then downshifting into the same gear to pass them?

In addition to all that’s been said about wear to the clutch/transmission, you don’t have anything close to the same degree of control when engine braking as you do when using the brakes. If the car in front of you slows more than expected, you’ll need to get on the brakes anyway, so you might as well use them in the first place.

The situation you describe is a possible place to use “heel-toe” downshifting. See this thread (and in particular my post – #13) for a detailed description of the technique. (As a rule, however, it’s most useful on the racetrack, not on the street.)

See GD for response.