Power to Weight ratio..... A Fools economy??

[Grant]

Not the clearest answers...

Im talking about two cars with the SAME power to weight ratio but one car has 300kw and the other has 150kw...

Im asking as speeds rise, does this power to weight thing mean less and less and the more powerful car will come over the top..

forgetting about diff ratios, to simplify things...
1st: weight has no effect on top speed. power, aerodynamics and friction do.
so a 300kw car has a much higher top speed than our 150kw polos

2nd: as you get closer to your top speed, your rate of acceleration slows down. this is because at higher speed there is more friction (both internal and from the road) and more wind resistance (aerodynamic forces).

3rd: since the polo has a lower top speed than a 300kw car, its rate of acceleration is going to decrease sooner than the 300kw car.

this is why, if given enough road, the 300kw car will always eventually take the lead.

but chances are you have stopped racing by this time, so the polo is declared the winner.

[Grant]

Not the clearest answers...

Im talking about two cars with the SAME power to weight ratio but one car has 300kw and the other has 150kw...

Im asking as speeds rise, does this power to weight thing mean less and less and the more powerful car will come over the top..

so to summarise..

generally, power-to-weight means less as you approach higher speeds

and yes, more powerful cars will always come over the top if given enough road.

[mazgtr]

Two cars having a race.

1st car: 1000kg / 200bhp

2nd car: 2000kg / 400bhp

If you ingnore gear ratios, wheel diameters, torque curves and just treat them as identical.

Both cars line up for a drag.

Initially as both cars accelerate, they will be the same.

The 2nd car although weighing twice as much, has twice the power to overcome its inertia and gravity.

They both have the same ratio of power to weight. So initially they are even.


The force that seperates them eventually though is WIND RESISTANCE.

Now for simplicity sake lets say they both have the same Coefficient of Drag.

This means they both the same aerodynamic shape.

As speed rises the force of the air against the vehicle rises.

As both cars lets say hit 150 kmph side by side, the air is pushing on both cars at 150kmph.

Its this extra resistance on the cars that tips the balance in the favour of the heavier car, with more BHP.

As the speeds rise, air resistance starts to absorb engine power from both cars.

Vehicle weight starts to become less of a factor than outright BHP.

Each extra KMPH is costing both engines more and more power. This is when the additional 200bhp of the 2nd car starts to win out.

The extra proof is in the top speeds of listed performance cars. I cant think of any cars with 200bhp that can reach 300km/ph redardless of weight, cars simply absorb to much wind resistance, but there are lots of cars with 400bhp that can reach 300km/ph, many of which would weigh in around 2000kg.

So in finishing, given a long enough road and high enough speeds.

The car with the greater amount of BHP, but the same power to weight ratio will pull ahead once air resistance is factored in.

Damn two new posts since i started typeing this response.!! LOL!!

[GTI POLO]

Thats a bloody good response... Sorry to ask such questions... I have a VW (non watercooled) of a mind..

Do we think this would be a factor from say between 100km/h and 200km/h???

It doesnt seem to effect 0-100km/h... As our cars accelerate 0-100 similarly to cars with same power/weight ratios...

[pologti18t]

Wind resistance builds rapidly over 80km/h.

The important figure is not Cd but CdA (co-efficient of drag by the cross sectional area).

Its the reason bikes have such a high topspeed for their power output. The over Cd is nothing flash but the cross sectional area is small.