Is BHP/Tonne a good performance indicator

cymatty

cymatty

cymatty

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As above really, with differing 0-60 times and in gear times on different types of cars, is the above a better measure of performance.

Thanks. :)
 
The power to weight ratio is a good indicator of "all-out" performance between about 30-100 MPH, which is where most people are most of the time. As speed picks up past a ton, the actual power output becomes more important and the weight less important, thus making a 1000kg 100hp car slower than a 2000kg 200hp car.

Important to remember when calculating this ratio is to use accurate and standardised power outputs and accurate and standardised weights! A car that BMW say is 1500kg is actually lighter than a car Volkswagen call 1450kg :p

For example, my car's EU DIN kerbweight is 1505kg and my car's DIN HP is 231 as standard. EU DIN Power-to-weight is therefore 153 HP/TON. Volkswagen Golf GTI has a DIN kerbweight of 1336kg. To make that EU DIN, add 75 to make 1411kg, 200 DIN HP, 142 HP/TON.
 
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yeah i think BHP per tonne give a good indication of a cars speed.

I also think 0-100 times are quite good for comparison.
 
Acolyte said:
yeah i think BHP per tonne give a good indication of a cars speed.

I also think 0-100 times are quite good for comparison.
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I think 30 upwards is a measure really of performance. As the first 30mph is all down to the launch and the grip available
 
ScoobyDoo69 said:
I think 30 upwards is a measure really of performance. As the first 30mph is all down to the launch and the grip available
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Yeah and then the rest is down to sheer power, or gearing.
 
As already mentioned, power to weight defines the acceleration of a car.
Above legal speeds, the important factors are power and aerodynamic drag. That's why a Caterfield is stonking on acceleration, but has a rubbish top end, as they have mediocre power and a rubbish drag factor, but weight a similar amount to a packet of Walkers.
 
Mr_Sukebe said:
As already mentioned, power to weight defines the acceleration of a car.
Above legal speeds, the important factors are power and aerodynamic drag. That's why a Caterfield is stonking on acceleration, but has a rubbish top end, as they have mediocre power and a rubbish drag factor, but weight a similar amount to a packet of Walkers.
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Ah OK so are you saying for normal day to day country road blasts and the good ol' traffic light grand prix the higher power to weight the better. However for Autobahn races you need brute force and a good drag factor.
 
Basically bhp/ton is acceleration

Outright Bhp is top speed, weight isn't really a factor in top speed.
 
cymatty said:
Ah OK so are you saying for normal day to day country road blasts and the good ol' traffic light grand prix the higher power to weight the better. However for Autobahn races you need brute force and a good drag factor.
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Yes.

As a good example, think of the old Calibra, not that powerful in today's terms, but very fast in a straightline due to it's excellent aerodynamics. However, from a standing start, it was only OK.

The above is also a very good reason for why today's cars are so fast at the top end (excluding diesels). Most of them have much higher power ratings to overcome their weight, and good drag factors to reduce fuel consumption. The result are cars that accelerate fairly well, but having fairly amazing top speeds.
 
Mr_Sukebe said:
Yes.

As a good example, think of the old Calibra, not that powerful in today's terms, but very fast in a straightline due to it's excellent aerodynamics. However, from a standing start, it was only OK.
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A little over 6 seconds to 60 isnt too shabby for a big car! The nasty 4X4 was its biggest downfall IMO.
 
I agree that power is more important than weight as speed increases, but never really understood why. It's obvious that some force terms don't depend so much on mass, but I never know what...

What are the forces that act upon a car?

Internal mechanical friction
directly (?) proportional to speed, but not mass

Air resistance
proportional to some power of v, but not sure what. no dependance on mass. web says v^2 or v^3 dependant on where you look.

Acceleration
directly proportional to mass and speed

Any others?

edited having searched google..!
 
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To accerate a car, the energy required is related to mass.
To maintain a speed, the required energy needs to overcome frictional losses.
At high speeds the energy to accelerate is overshadowed by frictional losses.

That's why some of the new and very heavy Merc AMGs are so fast. The mass is far less relevant than their power and drag coefficient.
 
Mr_Sukebe said:
Yes.

As a good example, think of the old Calibra, not that powerful in today's terms, but very fast in a straightline due to it's excellent aerodynamics. However, from a standing start, it was only OK.
Click to expand...
What cars in the same class with the same power/weight ratio accelerate quicker?
 
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