How relevant is BHP per tonne

Born_2_Kill_83

Born_2_Kill_83

Born_2_Kill_83

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I've been thinking about this recently, you would think that a car that has best BHP per tonne is the quickest. But does it go deeper than this?

For example a CLS 55AMG has around 476BHP and 516Torq's (one nice quick car) that gives it 251BHP per tonne (EST)

Now lets take the Jaguar XJR X350 that has 400BHP but weighs less therefore has around 245BHP per tonne.

Which would you have your money on being the quickest car? mine would be on the merc everytime, but with only 6BHP per tonne difference should this be so simple.
How much of a factor does momentum play when a car is moving? If a lot then BHP per tonne is irrelevant in a mid-range speed race.

What im trying to say for example is that 2 cars have the same stats but one a lot more weight and power surely the one with more power will still come out on top?

Your thoughts?
 
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My thoughts are immediately - and as usual with these types of thread - define "quicker". On the way to Tesco? Round Silverstone?
 
Interesting, though I would imagine there is a lot more to it than just the BHP/ton figure. You would need a way to rationalise all the factors which affect the car's acceleration/speed.

For example, I would say the speed you are traveling at will have a big impact on this in the form of air resistance.

What if the car with a higher BHP/ton ratio was less aero dynamic than the car with the lower BHP/ton? Which would be the quicker one then?
 
From a 0-60 i would have said the Jag becuase of its lower weight. The rest would depend on gearing and as other have said, what type of race are we talking here?

Ignoring traction and ech cars indivdual power band.
 
BHP per ton isn't solid evidence, even in a straight line, although its can be a good indication.

A few other factors;

1. The BHP part is peak BHP, far more important is how it delivers the power before this point.

2. Gearing - Longer gearing will mean a car with the same bhp/ton will be slower to accelerate

3. Unsprung mass can have a huge effect. 2 cars with identical engines, gearboxes and overall weights but one with 20kg less unsprung weight is going to be much faster.

4. Transmission loss - some cars lose more power in noise and heat through the drivetrain than others.

Not to mention smaller factors like traction, tyre compound, wheel size (although falls under gearing technically, I guess), aerodynamics etc.
 
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Loowi_ashley said:
o.k lets talk about and round off both, from a standing start and lets say a rolling 50mph. Not round a track as more things come into play on this.
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i think situation i expect the car with simply more power will have the advantage. at higher speed you have to overcome increasing wind resistance so bigger bhp *** etc
 
Surely at the higher speeds BHP/tonne plays less of a role in combating resistance than overall power would be?

Might have the BHP/Tonne to get to 60 fast but total overall power not managing more than 140.
 
BHP per ton isn't solid evidence, even in a straight line, although its can be a good indication.
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its more relevant than saying

this car has 200bhp and this car has 400bhp.

at least it quantifies it against the most limiting factor, which is weight?
 
Sin_Chase said:
I think it is fair to say BHP/Tonne is the most telling indicator of a vehicles speed if you take it in context.
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In terms of a vehicles maximum speed, given suitable gearing and simmilar aerodynamics, then yes big BHP wins.

In terms of acceleration at lower speeds, bhp/tonne is very relevant.
 
Sitting in my vehicle with a power to weight ratio of 9.54 BHP / Ton (with its current load) I'd say its relevant, but only as relevant as torque & gearing.....
 
BHP/Tonne is very relevant IMHO.

Obviously Aero and Gearing is going to affect the top speed, but if I was only using one number in a comparison it would definitely be the BHP/Tonne.


In a track scenario, a high BHP/Tonne car is going to accelerate faster. Might not have the best top speed (but how long is the straight anyway?). Then turns up a corner and the high BHP/Tonne car weighs less so stops a lot quicker and corners faster, then accelerates out faster.

I was passenger in Dad's Westfield at Castle Combe the week before last (it was only a public day so no timing and stupid amounts of traffic). Yes the high power cars start creeping up on the straights, but the 500BHP/Tonne Westfield just laps so much quicker than anything that was there. Still being foot flat-to-the-floor for an extra second or two coming into the corners compared with the lardy Scoobs and Evos is just hilarious! :D
 
Loowi_ashley said:
I've been thinking about this recently, you would think that a car that has best BHP per tonne is the quickest. But does it go deeper than this?
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Yes

BHP/Tonne is just one of many other factors that can influence this including gearing / grip / transmission loss etc..

Its a big factor, but not the only one

/thread.
 
BHP/tonne is highly relevant at road speeds. Once you start going proper fast, air resistance becomes far more of an issue than weight and so outright BHP becomes the winner.
 
In terms of acceleration power to weight is a dominant factor in assessing perfomance. The only other real factors are the ability to put the power down at low speeds and drag at higher speeds.

For any other measure of performance power to weight is utterly meaningless.
 
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