[FMG_V6_btb]

The technical physics was explained in my last post.

When you are driving and push the gas pedal, the engine torque is used for two things:

1- accelerate the rotating masses -> T=I*A
2- accelerate the car -> F=m*a

The lower the rotating masses inertia (I) the lower the engine torque needed to rev-up these rotating masses, as a consequence of this, more torque is available to accelerate the car. And thats why with the same engine power (without modifications in the fluid dymamics neither in the thermodinamics of the engine) you feel the car accelerates better (and it does).

I remember that some years ago I drove a '75 Corvette, and whith the car stopped, when I pushed the gas pedal and the engine rev-up, the entire car roll in the opposite direction of the engine, that was because the high crank and flywheel inertia. The car roll was produced by the reaction torque of the engine in its mounts (the inertia torque above explained).

The same physics can be applied to the wheel rims, a friend of me own a 156 JTD (turbo Diesel), and he changed the original 15' steel rims with its 185/65/15 pneumatics, for a 17' aluminium rims with 215/45/17, that last rims weighted at least 2 kg more than the original steel ones (!!!), and I suppose the pneus were also heavier, as result of this the car acceleration was around 1 sec. worse in 0 to 100 km/h than with the original wheels.

Also, I must to point here, that for the above mentioned reasons, inertial test benches to measure the power are not valid, are only good for before-after comparison.

Paco.