[Stan Weiss]

Originally Posted by weedburner

If you are looking for your best ET, more launch rpm is better as you will have more stored energy available to get the car moving. Problem is the traditional model of adjustable static+CW clutch tuning won't get you there,

Don't confuse more launch rpm with a heavier flywheel as they are not the same with regards to storing energy prior to the start. While a heavier flywheel/clutch will also make more stored energy available for launch, the problem with a heavier flywheel is that the energy you draw from it during launch then has to be paid back before you cross the stripe. The car can launch harder with more stored energy available, but it will then accelerate slower as the rpm's climb back up. Gain some here, lose some there, it basically balances out when the outflow of stored energy is efficient. The problem you see with a heavier flywheel is that it increases the percentage of stored energy that is lost when the tires spin. That is the basic reason drag racers see benefits when switching to lighter clutch/flywheels, an overall reduction of stored energy means less stored energy ends up wasted during wheelspeed spikes after the shifts.

It might help to think of the engine's rotating assy as a torque storage device similar to a battery. You lose engine torque output as you charge it up with rpm, then gain engine torque output as you draw rpm out of it. When you cross the stripe, the engine's rotating assy will be fully charged with rpm. Sure there will be give and take as the engine loses and regains rpm as the car works its way thru the gears, but those gains and losses basically cancel each other out. The thing to look at is the net difference between engine rpm on the starting line vs engine rpm crossing the finish line.

...If the rpm on the starting line is the same as crossing the finish line, then all the engine's torque production made it to the transmission's input shaft to accelerate the car. Start with a full battery, end with a full battery.
...If the rpm on the starting line is lower vs crossing the finish line, that's like starting out with less than a fully charged battery. Some of the engine's overall net torque production gets absorbed charging its rotating assy, reducing the overall net torque available to accelerate the car between the start/finish lines.
...If the rpm on the starting line is higher vs crossing the finish line, then that extra start rpm becomes energy available for launch that doesn't have to be paid back before crossing the stripe. You end up with more torque available to accelerate the car between the start and finish lines than the engine actually produced.

The key to taking advantage of high rpm starts is the ability to control the stored energy discharge rate, which is in-turn controlled by how fast the clutch pulls the engine down against WOT. For that you need a 2-stage clutch. A 1st stage that maximizes efficiency during high rpm launch, then a 2nd stage rate that maximizes efficiency after the shifts.

There is another case where the car is geared for the best 60' time and et which has it on the rev limiter before the finish line.


Stan