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BHP and Torque?!?


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One for the techies I think.

 

Yesterday on the rolling road, I was the only person to have had a higher torque reading than bhp!

 

What is the actual relationship between torque and BHP?

And why do people want more bhp when it is torque that actually provides the accelaration?

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bhp is calculated from the torque figure and rpm. If your peak torque figure was higher than your bhp, it just means your power band is lower down the rev range, or maybe you need finer tuning to keep the torque higher up the rev range and thus give you more bhp

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Also have to think about weight. Your car might be very powerful and put loads of torque on the rolling road but if it weighs 5 tonnes its not going to accelerate very fast is it? Or do they put resistance on the rolling road to simulate the cars weight? Or am i talking rubbish?

 

example - diesels have loads of torque so they can pull heavy loads.

But they dont accerate very quickly?

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Also have to think about weight. Your car might be very powerful and put loads of torque on the rolling road but if it weighs 5 tonnes its not going to accelerate very fast is it? Or do they put resistance on the rolling road to simulate the cars weight? Or am i talking rubbish?

 

example - diesels have loads of torque so they can pull heavy loads.

But they dont accerate very quickly?

 

No, a rolling road doesn't consider weight or wind resistance, or a number of other things.

 

Diesels do generally have heaps of torque, but due to the low speed of the flame path they don't tend to rev fast, so with the exception of some of the very new ones, they don't tend to accelerate fast either.

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bhp is calculated from the torque figure and rpm. If your peak torque figure was higher than your bhp, it just means your power band is lower down the rev range, or maybe you need finer tuning to keep the torque higher up the rev range and thus give you more bhp
That makes sense as my peak torque was at 3500 RPM.

 

and also in the real world bhp means nothing!! torque is acceleration ;)
Just what I thought...
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The rolling road measures torque only.

The bhp is calculated from the torque and rpm.

The torque is measured at the wheels and then at full rpm the throttle is lifted and the rollers measure the engine/transmission drag as the car slows down.

These 2 figures are added together to give the flywheel hp.

 

Most 'normal' cars have a higher torque figure than bhp figure.

Cars with higher bhp than torque tend to be high revving, clutch slipping beasts.

 

Of course 'higher' is just a coincedence of the units, when n/m becomes more popular for torque then everyone will have a higher bhp figure than torque.

 

For max acceleration you need lots of torque all thro the rpm band you are using between gears.

It's no good having a big torque figure if the slope of the graph is very steep before and after the peak, unless you've got 6,7 or 8 gears to play with.

 

It's difficult to fine tune the position of the peak torque point. This is usually a characteristic of the camshaft. subsequent tuning then alters the amount of torque, but not the peak torque point.

 

One last thing, it's easy to spot a bogus dyno readout cos due to the formula mentioned earlier, the torque and hp lines on the graph will allways cross at 5252rpm.

If they don't then it's crap. I've seen lots which don't.

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The rolling road measures torque only.

The bhp is calculated from the torque and rpm.

The torque is measured at the wheels and then at full rpm the throttle is lifted and the rollers measure the engine/transmission drag as the car slows down.

These 2 figures are added together to give the flywheel hp.

 

Most 'normal' cars have a higher torque figure than bhp figure.

Cars with higher bhp than torque tend to be high revving, clutch slipping beasts.

 

Of course 'higher' is just a coincedence of the units, when n/m becomes more popular for torque then everyone will have a higher bhp figure than torque.

 

For max acceleration you need lots of torque all thro the rpm band you are using between gears.

It's no good having a big torque figure if the slope of the graph is very steep before and after the peak, unless you've got 6,7 or 8 gears to play with.

 

It's difficult to fine tune the position of the peak torque point. This is usually a characteristic of the camshaft. subsequent tuning then alters the amount of torque, but not the peak torque point.

 

One last thing, it's easy to spot a bogus dyno readout cos due to the formula mentioned earlier, the torque and hp lines on the graph will allways cross at 5252rpm.

If they don't then it's crap. I've seen lots which don't.

That is exactly what I wanted to understand.

So BHP is basically a manipulation of the torque figure and it means very little.

 

Keith maybe you could shed some light on my dyno graph?

If my peak torque is at 3700 RPM and Krsmayo's peak torque is 5000 RPM does that mean my engine may have some different cams in it?

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Also, what is the best way to get my torque readings to stay level after they peak?

 

Thanks again.

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