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

[GTI POLO]

Hi,

Ive read a few threads on an chipped Polo Gti Vs all sorts of other cars.. Mazda3 MPS is the most recent.

Now despite a chipped Polo having a similar Power to weight ratio as many of the cars considered...

Doesnt weight matter less and less as speeds rise?

So does a car with say 300kw and a Power to weight the same as a chipped Polo still have the advantage as speeds rise?? say from 100km/h to 200km/h

If weight matters less and less and then there is 300kw as opposed to say 152kw from our Polos...... Doesnt the 300kw car start to stretch its legs on us...

[GTI_jim]

are you talking about about economy (thread title) or performance???
I dare say a 300kw car will beat a chipped polo GTI most of the time anyway...

[Grant]

weight only effects acceleration, not top speed. generally acceleration tapers off when approaching the top speed of a car.
so correct me if i'm wrong...
a 300kw car will have a greater top speed than 150kw car. meaning that the faster your traveling, the slower the Polo's acceleration will be relative to the 300kw car.
does my jabbering make sense?

[GoLfMan]

it has a number of factors, those numbers are "ideal" numbers in that they are flywheel power.

so drivetrain loss comes into it so do gear ratios, power curves etc.

power to weight is a very misleading indication of a cars actualy speed in some senses, but its a good guide as to how quick your car may be..... ill stop my rambling

[JCM308]

From what I've read on various posts regarding chipped Polo's most people only talk about low accelleration (0-100, 80-120 etc) which makes sense. If you take on a powerful V8 you'll probably have it until about 140 and then you're gone (expirienced this many times with previous cars). Of course these sort of tests will only be done at the track

[Guy_H]

I Think Grant is spot on the money in this case!

[dubbed]

Well I'm no doctor of Physics so this could be completely wrong, but think about the following. (Just to make the numbers easy to understand I've chosen a motorbike with 1/3 of the power of a big V8 sedan). Again, I could be totally wrong as this is a wild hypothetical guess; but think about this:

You're going to have a very hard time catching a 110kw Ducati 999R in a 300kw VE GTS. The 999R has a power-to-weight ratio of approximately 100kw:160kg whilst the GTS has something like 100kw:400kg, these are rough estimates mind you.

A 999R will cover 0-400 metres in 10.3~ seconds going at over 215km/h whilst the GTS will be lucky to be hitting 175km/h and covering the same distance in (if it's lucky) less than 13.5 seconds. Now if you think about how much ground they will both cover after the 400 metre point one will have a clear advantage and already have a few seconds head start. Even if the Ducati maxes out at 260km/h (just to make a point) and the GTS could hit a top speed of 300+km/h (again all very hypothetical) the Ducati would be kilometres in front taking a hell of a long time for the GTS to gain that lost ground that the 999R has already coverd.

There's a lot more to power-to-weight to factor too - power curve and delivery, gearbox ratios, final drive ratio etc.

So what's my point? A Polo with a better power to weight ratio will get to one point quicker than another car with less power-to-weight even if it has more total power. Sure the other car will eventually catch up given enough road, but that all depends how far they're racing in a straight line...

Instead of all the rambling I could've just posted this:

http://www.youtube.com/watch?v=g2DBtNo2ipc

(Imagine the Polo is the bike and the 911 is a Mazda3 MPS lol!) So yes you're right, but the top speeds we can get on our roads means a chipped Polo has a very good advantage

[GTI POLO]

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..

[Timbo]

In summary, E=MC^2

[VW GTI]

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..

What gear ratios are they running?

The answer is not really simple.

[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.