Originally posted by Christian:
Originally posted by Big Jim:
Originally posted by Christian:
Originally posted by Big Jim:
On decel, the engine is not driving the wheels. It is adding resistance and as such is actually assisting the brakes. Depending on conditions, this may not be significant, or it can be very significant.



But surely, unless you a braking extremely softly, when applying braking power you are working against power sent from the engine to the transmission, even though that power is dropping it is still there. I can understand it assisting in slow braking but in fast braking, especially emergency braking, clutch in is best as there is no power being sent from the engine.
At least this is the way I see it in my mind, the physics of this makes sense to me.



Once you lift your foot off the gas pedal, the power generated from the engine turns from propelling the car (positive power) to slowing the car (negative power). Surely you can feel that as you lift the throttle. While this is happening, the engine is assisting braking all the way down to engine idle.



Okay, I may be a little slow here on this one

If the engine is still producing power then even if that is less than the power required to propel the car at the same speed (hence engine braking), would that power still not be put to the wheels (even if it is less than required for propulsion) meaning that when you brake ... you are braking not only the velocity/momentum of the vehicle but also the revolutions of the engine ... whereas if you brake clutch in/in neutral you are braking against purely the velocity/momentum of the vehicle ... my understanding of physics, back from A-levels 15+ years ago, so it is a little shoddy!

I can picture this scenario ....
Say at 50mph you need 10% disc braking to stop in distance x, if you apply engine braking you would need only 5% (making up numbers here) as the engine braking allows for 5% braking power ... which is why we brake with engine engaged.

But I don't see this ...

If I want to perform an emergency stop (slam on brakes as kid jumped in road) ... my aim is to stop as quickly as possible. At this point I imagine that removing engine power from the scenario would reduce braking, as the engine is rotating it is producing power, which goes to the wheels which causes forwards momentum (even if it is less), which means I have to apply more braking force to the wheels ...


Sorry to turn this into a school lesson, just I don't quite get it.

Cheers,
Christian./



You are almost there. If it is producing less power than required to propel the car it is engine braking and that engine braking is assisting in stopping the car.

Jim Johnson 98 SVT 03 Escape Limited