Let's talk about engine braking. This is a very useful lesson for the more advanced drivers and we will talk about the effects of engine braking and braking and cornering.

Braking Without Engine Braking

Let's imagine we brake on a straight line from full speed to full stop with the clutch engaged on a flat track. This means the engine is not connected to the tires and there is no engine braking effect at all. The tendency to lock up the rear tires or the front tires depends only on the brake bias. With a high brake bias you will lock the front's first, with a low brake bias you will lock the rear's first. This tendency can be represented by a straight line in this graph.

Adding Engine Braking to the Equation

Now let's add the engine braking to the equation. This time, instead of braking with the clutch engaged, we will just lift the throttle in gear and start braking hard. The forces acting on the tires are now two: the braking plus the engine braking. The engine braking depends on the RPM of the engine.

If we brake without changing gears, say on 6th gear, the graph will look like this. This means at a higher RPM there is a little bit extra braking on the rear tires and then that little bit extra braking disappears as the RPM goes down.

Now imagine that we are down shifting consistently and keeping the RPMs high during the entire braking phase. The graph will look like this, with the engine braking always spiking with the downshifts.

Impact on Tire Locking

Now here's the thing, this effect can be strong enough to affect whether we will lock the front tires or the rear tires first in threshold braking situations. This means it cannot be ignored. This is so effective that it's possible to lock the rear tires first or the front tires first without changing the brake bias, just by changing your downshifting speeds.

Dynamic Brake Bias

Dynamic brake bias is the combination of the fixed brake bias plus the effects of engine braking with each downshift. This means if you downshift very slowly and late, the dynamic brake bias will be more towards the front and if you downshift very fast and early, the dynamic brake bias will be towards the rear. Of course I'm talking about a rear wheel drive car. If you're driving in a front wheel drive, the effects are reversed.

The Importance of Consistent Downshifting

A very common mistake is to ignore the rhythm of your downshifting when braking hard on a straight line. This means you will sometimes downshift fast, sometimes downshift slowly and the behavior of the car will be considerably different, which will make you an inconsistent driver. This does not only affect the chances of locking the front or rear, but also the balance of the car coming into the corner. This means even without locking the tires, you will feel drastically the effects, the differences of balance when downshifting more quickly versus downshifting more slowly into the corner.

Adjusting Dynamic Brake Bias

By changing the brake bias, we change this fixed line. By changing the speed of downshift, we change this dynamic line that adds up to that baseline of the brake bias line. It's possible to time your downshifts to compensate the behavior of a car. For example, if the car is locking the rear very easily and you cannot change the brake bias for some reason, the solution will be to down shift a little bit later to prevent even more engine braking to add rear bias to the dynamic brake bias.

Engine Braking Versus Rev Matching

Now I want to talk about engine braking versus rev matching. These are actually different things. So let's say the engine braking here, when you start downshifting, boom, it goes up and then as the speed goes down, the engine braking is going down and down and down and down and then you don't shift again. Boom, you don't shift, you get more engine braking and it goes down.

Cars Without Auto Blip

This is different to when cars don't have auto blip and you downshift. By the way, most modern cars already have auto blip. We can also call them electronic blip, which means you don't have to worry about rev matching at all, just about the engine braking. But if you're driving, say, an MX5 in iRacing, then you might want to blip or if you're driving the Ray FF 1600 or if you're driving the Formula V, all these like beginner cars, the older cars, these cars do not have auto blip. So you have to blip the throttle every time you downshift to do the rev matching.

The Rev Matching Problem

By the time the revs are going up so that the rotation of the engine matches the rotation of the tires, there is some extra, extra that's different. It's not engine braking. There's an extra stress that will cause the car to get a lot more locking power. You can actually lock the rear tires if you downshift. So your driving car does not have the blip that can make the car lock up the tires for a while. But that's already enough to kick the rears away and make you spin. As soon as the tires match the speed of the new gear, then the rev matching problem disappears. But we still have the engine braking.

Key Differences

So the engine braking is a continuous thing. The rev matching problem when you don't match the revs and you downshift like that is a spike that quickly disappears. So of course these are bad. I've never seen anyone trying to not blip the throttle in cars that don't have auto blip to make more rotation because this rotation is uncontrollable. It's spiky. It overheats the rear so it's not a good thing. You want to do a blip on the throttle real quick when you downshift in these cars.

The cars that do not have auto blip to neutralize this. But you're still going to have the engine braking. So the engine braking is an ongoing process that goes down and it's a controllable consistent amount of rotation of controllable stress that you can have on the rear tires. Not matching your revs is not. It's a spike that removes a lot of grip from the rear tires and it's not very useful. It's a good thing to blip the throttle, kill this and still have the engine braking at your disposal to make the car rotate in case it's under steering and you need that extra bit of rotation.

Practice Exercise

To make sure you really really understand this, try to without changing the brake bias, lock the front tires and then lock the rear tires only by changing how you downshift. Obviously you have to be right on the limit so that you don't lock both front and rear at the same time so that you can really feel that difference.

After you're really comfortable doing that, start feeling the difference of the engine braking as you turn into the corner. Remember that you're going to feel the effects more strongly when the RPMs are high on initial turn in. Mid corner it's not going to make that much of a difference because the RPMs are already going to be a little bit lower.