Heel and Toe versus Left Foot Braking with Sequential Gerboxes

[Brown, Jon Brow]

Can someone explain to me why racing drivers sometimes use the clutch with heel and toe downshifting in cars with sequential gearboxes?

I thought with a sequential gearbox you only need the clutch to move off the line. In the video linked Andrew Jordan is left foot braking and downshifting without any throttle blips.
https://www.youtube.com/watch?v=cOjpHRwgG_k

Compare this to the Carrera Cup, where the driver uses the clutch with massive blips on the downshifts
https://www.youtube.com/watch?v=Q3hrpkHoyrM

Is it that the BTCC gearbox is just better than the one in the Porsche, and can take more abuse, or is it just driver preference?

[Starter]

It may be that the engine management software provides for rev matching in Jordan's car. Even some street vehicles have that these days. A sequential gearbox alone doesn't eliminate the need for rev matching.

[Brown, Jon Brow]

But what about the use of the clutch on the Porsche video?

And would the fact the Vectra is front-wheel drive negate the need to blip?

[Rollo]

Can someone explain to me why racing drivers sometimes use the clutch with heel and toe downshifting in cars with sequential gearboxes?

I thought with a sequential gearbox you only need the clutch to move off the line. In the video linked Andrew Jordan is left foot braking and downshifting without any throttle blips.

On the upshift, it's far easier to match engine revs to roadspeed to enable you to change gears. On the downshift though, you're changing to a gear which uses more revs to achieve the same roadspeed.

The truth is that you're never not using the clutch. Even if you were able to change upwards and downwards without the pedal, the clutch is a series of plates which transfer from a driving shaft to driven shaft.

More than likely, Jordan is using a dog box without synchro and preloading tension through the stick itself. I suspect that he's able to do this in milliseconds due to monumental amounts of skill.

[Starter]

But what about the use of the clutch on the Porsche video?

And would the fact the Vectra is front-wheel drive negate the need to blip?
Front wheel or rear wheel has nothing to do with it. The gears could though. If the gears are straight cut (most racing trannys are), then not using the clutch is easy, you just bang it in. If the tranny is mostly stock, as in based on a street transmission, then you do need to declutch on shifts or risk damaging the gears.

[Rollo]

Front wheel or rear wheel has nothing to do with it. The gears could though. If the gears are straight cut (most racing trannys are), then not using the clutch is easy, you just bang it in. If the tranny is mostly stock, as in based on a street transmission, then you do need to declutch on shifts or risk damaging the gears.

There isn't really much of an issue with straight or helical cut gears.

Street transmissions have synchromesh which might be using any number of systems. You generally can't bang through the gears on car with synchromesh for various reasons depending on what sort of synchro system is used.

[Rollo]

Vaguely related - a lego gearbox - https://www.youtube.com/watch?v=qrzj9FAfPwM

[janvanvurpa]

Front wheel or rear wheel has nothing to do with it. The gears could though. If the gears are straight cut (most racing trannys are), then not using the clutch is easy, you just bang it in. If the tranny is mostly stock, as in based on a street transmission, then you do need to declutch on shifts or risk damaging the gears.

My god..Straight cut or helical is about how the TEETH mesh,

dog, synchro is how the gear selected is locked to the shaft...

Sequential or H is how the gear selection is done

Thus there are helical cut dog boxes, straight cut synchronized, and all kinds of permutations..

You really never looked inside a single gearbox in your whole life?

Jon some people do all the heel and toe thing because somebody told them its what real racers do..
maybe the Porsche box is a synchro and relatively wide ratio...

The need to lift depends on the angle of the undercut on the dogs.

I've used boxes where the undercut on the dogs was ZERO and those shifted beautifully at full throttle, just wiggle it barely even hear the change.
And I've used boxes that you must back off and de-clutch fully---slow synchros (small) and wide ratios (large % rotation difference between the gears)

Less the undercut, the tighter the service interval and the higher likelihood of the thing "rolling out of gear' when chopping throttle if the detent stuff ain't "vigorous"

[Zeakiwi]

The v8 supercars use a bit of clutch on the downshifts at times to make the garbox last a bit longer.
https://youtu.be/f7N3O-Qyivk

[Ari33]

We have a quaife 6 speed sequential in the mk2.
Clutchless downshifts are only possible with a 'flatshift' system for rev matching and we dont use the system at the moment because we feel that the gains are not greater than the reduced gearbox reliability it usually brings.
If we had the stronger tractive seq box we'd probably use it.

[MrJan]

Front wheel or rear wheel has nothing to do with it.

It does slightly, on a RWD car if you don't rev match on the downshift then you're going to lose the back end. On a FWD car it's still an issue but the car won't snap on you quite as badly.

[Starter]

It does slightly, on a RWD car if you don't rev match on the downshift then you're going to lose the back end. On a FWD car it's still an issue but the car won't snap on you quite as badly.
True, but it's very slightly.