Compression Braking
Compression braking or as it is commonly know Engine braking is an often used and just as often misunderstood term. I hope in this post to clarify exactly what it is and how it functions as well as discuss its use, miss-use and disadvantages for track use.
In a future post I will also be looking at how Compression braking has been implemented in GT4.
How does Compression Braking work?
In the simplest terms Compression Braking is using the engine to slow the car. It occurs because unless the clutch is engaged and/or the brakes are applied the speed of the driven wheels is determined by the engine; remove your foot from the throttle and the engine will slow and with it the driven wheels. However if you remove your foot from the throttle and engage the clutch, the engine and driven wheels are not connected and the only thing slowing the car will be friction from the tyres and the resistance of the air, as a result the car will take much longer to stop.
This is all fairly simple stuff and it is possible to judge approximately what speed a car should be doing at any engine speed in a particular gear, using the ‘MPH per 1,000 rpm’ figure, which is derived from the cars gear ratios, axle ratio and tyre diameter.
If for example we take the Ford GT, with the following MPH per 1,000 rpm figures as an example.
1 = 9.8
2 = 15
3 = 20.1
4 = 27.2
5 = 33.2
6 = 40.6
Now using these figures we can examine Compression braking both in gear and through the gears.
If the car is in 3rd gear at 5,000rpm the car speed would be approx 100.5 mph, if you lifted off the throttle completely and relied on the drop in engine speed to slow the car the speed would drop according to the engine speed (if you remain in 3rd gear) as follows.
5,000 rpm = 100.5 mph
4,000 rpm = 80.4 mph
3,000 rpm = 60.3 mph
etc
This clearly shows the relationship between allowing engine speed to drop and it slowing the car; the amount of time this would take depends on a number of factors, such as engine compression ratio (generally the high this is the quicker the drop in engine speed) and flywheel and driveshaft weight. Using this method to slow the car down will always be far, far slower that using the brakes.
Compression braking when changing down through the gears can have a far more dramatic effect as illustrated below (using the Ford GT figures).
The car is travelling at approx 100 mph in 3rd gear (approx 5,000 rpm) when the driver begins to brake slowing the car to 50mph, the speed required to take our imaginary corner.
At this point the gear is changed to 2nd with the revs at approx 2,000 rpm, when the clutch is disengaged the engine speed dictates a speed of 30mph (2 * 15mph per 1,000 rpm).
The car is travelling at 50mph, but the engine speed and gearing dictate that it should be travelling at 30mph. This 20mph difference in the speed the car is travelling at compared with the speed the engine wants to travel at is going to be transferred to the driven wheels and slow the car in a dramatic and un-controlled manner. In turn the wheels will also try to resist this rapid deceleration and speed up the engine.
If however the driver had ‘blipped’ the throttle to approx 3,300 rpm before the clutch is disengaged then the engine speed would match the car speed and no additional braking would be experienced, keeping the car stable.
Compression braking on the road
Many driver use compression braking on the road when changing gear to help slow the car and with older cars it can help if the brakes are poor or fading. With modern cars it is generally considered unnecessary as the braking system is more that sufficient.
Many people would argue that using compression braking saves on wear and tear on the brakes, however just as many people would say that what you save in pads and discs you lose in wear and tear to the engine and drive-train. Its hard to say who is 100% right on this one as it would vary from car to car and driver to driver.
On a personal level I don’t using compression braking when changing down as I consider the sudden braking force it generates too sharp and prefer the smoother experience you get with heel and toe downshift that match engine speed to road speed.
One useful aspect of in-gear compression braking is often used in off road driving when descending a steep slope, here the rev limiter and knowledge of mph per 1,000 rpm can be used to control your descent speed without touching any of the pedals.
Compression braking on the track
I would be surprised if many professional drivers use compression braking (even on endurance events) and I am yet to come across a race school that recommends its use. While it does help conserve the pads, it does no good to a race tuned engine; they are not designed to slow the car, but to power it.
With Compression braking it is very difficult to judge how much additional deceleration you will get, as a result if you are already at the threshold of braking (or near it) you could well overload the tyres grip level and lose control, flat spots on the tyres will result which will hammer your lap times.
Changing the pads on an endurance race spec car is relatively quick and easy, and a task regularly carried out in the pits during endurance racing. It is also far less time consuming to have to change the pads, than it is replace an engine component (or retire through engine failure) or lose time over a number of laps because you've just flat spotted a tyre.
I have watched numerous endurance races, and have session reviews of Le Mans dating back to the 1960's, also a documentary on the Morgan team at Le Mans and Bathurst. I can't remember ever hearing a driver talk of using compression braking out of choice. Now if the brakes have failed for some reason, you may not have a choice; but that’s a different thing.
You should also remember that compression braking will only directly effect the driven wheels, shifting the brake bias in that direction. With the majority of race cars being rear wheel drive, this would mean a major brake bias to the rear wheels, if the car is anything other than straight you may now be in a situation of just trying to control the car rather than brake and setup the car for the corner entrance. Even if the car is in a straight line, if the car is running a high compression engine (which increases the effect of engine braking) it can be enough to lock the rear wheels and get the back of the car twitching. Neither of these scenarios is worth conserving a little bit of brake pad material. You would get more of a saving on the brakes through good brake control and technique than you ever will through engine braking.
A quick quote from Danny Sulivan (of the Skip Barber racing school) illustrates this point
"To put it into perspective, at Laguna Seca, which is hard on brakes, Rick Mears and I were team mates at Penske and Rick finished the race with only 70 thousandths of an inch of brake pad material left. I only used 70 thousandths of the pad in winning the race. People brake differently but can still run the same lap time, especially in a race"
The following is an extract from the Russ Bentley books "Speed Secrets - Profressional Race Driving Techniques"
"Again, the reason for downshifting is not to slow the car. I can't emphasise this enough. That's what brakes are for. Too many drivers try to use the engine compresion braking effect to slow the car. All they really achieve is upsetting the balance of the car and hindering braking effectiveness (if the brakes are right at the limit before locking up and you then engine braking to the rear wheels, you will probably lock up the rear brakes), and more wear and tear on the engine. Brake first, then downshift."
This is from Skip Barber's "Going Faster"
"What downshifting is really for.
We ask this basic question of every racing school class. The most frequent (and incorrect) answer is, "to help slow the car down." In a racecar with good, durable brakes (the majority of modern racecars), downshifting to help slow the car down is unnecessary. The brakes slow the car down. You downshift to get the car in the proper gear to exit the corner."
Now Russ Bentley has raced Indy cars, World Sports cars (including endurance) and is now a race instructor, Skip Barber should need no introduction, but the book I refer to has been written with the assistance of ten instructors from the Skip Barber Racing School.
Brakes slow the car, not the engine; unless your brakes are shot in which case you do not have a lot of choice, but unless you're Moss or Fangio you're not going to win like this.
Problems with Compression braking on the track
1. Its less effective than normal braking.
As Compression braking effects only the driven wheels it will have a major effect on the brake bias of the car; incorrect brake bias (or brake balance) can increase braking distances significantly. Even with 4WD cars the effect of engine braking is limited to the front/rear split of power distribution and will normally affect the brake bias negatively.
It is also not possible to accurately predict the level of Compression braking or modulate it once it has been applied, making accurate and controlled braking almost impossible.
2. It does not give ‘more’ braking force if you are already at the limit
This one is a common myth of Compression braking, that it will allow you to get more braking force for free. While in the distant past braking systems were not powerful enough to exceed the grip limit of tyres (and this is a long way in the past), modern braking systems are more than capable of exceeding the grip levels (measured in straight line braking as the slip percentage) of the tyres.
If you are already at the braking ‘threshold’ adding more braking force through compression braking is just going to exceed the slip percentage and lock the tyres.
Using the Ford GT example from above and assuming that the car was already braking at the tyres threshold (assume max 10% slip percentage for this example). If when the clutch is released (with the car at 50mph) the tyres are already using the full 10% slip in braking, the additional braking caused by compression braking (20mph vs. 50mph) will exceed the 10% max by a large margin, the rear wheels will lock, the tyres will flat spot, braking distance will actually increase and unless the car is totally straight the major rear bias will possibly cause a loss of control.
When to use Compression braking?
Obviously with road driving compression braking and its use is very much a case of personal preference; however on the track most instructors and drivers share the belief that the disadvantages more than outweigh the advantages (and some would argue that it has no advantages).
However should you find yourself in an older car (historic racing), suffering from brake fade or even brake failure then you have little choice but to use what ever method you can to slow the car.
As Skip Barber’s book says:
”In this case you certainly do use the downshift to slow the car down – but it’s a last resort”
