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Discussion in 'General Automotive Chat' started by Goubo1, Apr 8, 2013.
At what point does outright power over come power to weight ratio ??
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Ok I'm going to take a stab in the dark here and say when the power exceeds the weight per tonne. The problem is though, that the power is often measured from the flywheel and sometimes the drivers weight is excluded from the equation.
So for example:
Audi A3 2.0 T with gross vehicle weight of 1930kg
Say you had a Driver weight of 90kg
BHP at flywheel ?? but we know on a rolling road it is around 200 bhp stock from the wheels
Say the power at the flywheel was 200 though and gave us a Power to weight ration of 99.01
Basically the calculation shows that the power exceeds the weight of vehicle and driver per tonne.
So any power (BHP) higher than the weight (be it car and driver or car alone) is good. The further away you are from the figure per tonne then the better. For example, say we take a Mercedes SLR (2008) which has a gross weight of 1780kg... same driver who weights 90kg and our power to weight ration is 334.76 with the driver... you can see that figure is way out from 100 - the higher the number the better.
It would depend on the vehicle weight though. Now if I was a rap star and people wanted me dead but I loved my Mercedes SLR so much that it had become a 4000 kg car (bullet proof glass and armour plating adding all the weight).. the power to weight ratio would be 153... which is still good but not as good if the car weighed less. This would also mean that if I was go head to head with a Stage 1 Audi S3 then I would probably be neck and neck.
I hope that answers your question... someone correct me if I am wrong.
After you pass 500 Bhp it does not matter if the car is 1000 or 2000 Kg, it will just be fast ! The above example is a little bit better but if you have 300Bhp per ton or more then you're in a seriously quick car !
I know a guy with a Clio 172 and it has been turbo'd and is around 290/300bhp and it only weighs just a over a ton so has a very good power to weight ratio,but say I was in a fast car like a rs4 b7 414 bhp and around 250 bhp per ton.he has the better power to weight ratio,but at what speed would power to weight ratio no matter and the fact I have about 100 horsepower more come into play ??
For acceleration, never.
If you are comparing like for like cars, then it is easier, but to compare a Clio with an RS4, the science behind it all is more complex. 2wd vs 4wd for example, a 4wd car will always have more transmission losses to a 2wd, but can the Clio transmit all that power to the road with no wheelspin? Then there's the gearing of the gearbox, does a Clio sport gearbox really make the most of the available power with it's ratio's? I think not personally.
Power to weight ratio is just a gauge, reality is different. An RS4 with 414bhp will still be accellerating past 150mph, whilst a Clio with 300bhp will probably near it's rev limiter at the same speed?
So power to weight for acceleration and outright power for topend ??
It is complex. But simply put, for acceleration the power to weight ratio is the determining factor. At high speeds it is more of a power/air resistance battle. A 110bhp bike has no trouble getting to 165-170mph
As above really. Power to weight is quite relative to acceleration and overall power can determine top speed although other factors also come in to play such as aerodynamic efficiency, gearbox ratios etc.