bhp/tonne a physics question

[funbobby]

so if you had 2 identical cars but one 400kgs heavier but with power upped so that they both had same bhp/tonne how would a straight line drag go all the way say up to 150? im thinking the weight becomes less of an issue when in their stride but im curious?

[harryowl]

Given a long enoguh straight, weight is not a limiting factor of top speed.

[RB5]

Does the weight figure include the driver?

[blueg33]

Not sure the op means top speed, more like acceleration. I suspect (but don't know) that gearing and aerodynamics will also be a factor.

[anonymous-user]

assuming both cars had the same aerodynamic drag (see note), then the more powerful car would win by some margin

Acceleration = (force at wheels - drag) / mass

at zero mph acceleration is the same for both vehicles as aero drag is also zero
at 150 mph, acceleration is less for the lower powered car, as it has lost a larger percentage to aero drag


of course, the actual result depends upon the relative magnitude of those factors.


(note: the cars would need to have identical shaped power curves (other than one being a multiple higher than for the lower powered car, and identical gearing, traction etc etc)

[funbobby]

i mean acceleration wise. so if the more powerfull car wins i wonder how big the margin would be say over a mile.

[irf]

with the same bhp per tonne the lighter car should get away a bit better, middle of the 'race' there wouldn't be much in it, over a ton or thereabouts the higher bhp car would start catching up and eventually win. thats obviously theory, there would in reality be many many variables.

[Turbocharger]

The area under the torque curve is more important - it could make 1000bhp from 13k rpm to 13200rpm, but if its not livable you'd need a million gears to get the best out of it, and a LandRover engine would be preferable if you only had one or two gears.

[K87]

Assumin bhp per tonne was constant throughout rev range the higher powered car would be quicker at high speed as aero drag becomes the significant force not mass