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TORQUE not BHP

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lambert:
Wot e sed. :D

L90OOK:

--- Quote from: "Bigbluemaverick" ---Wot e sed. :D
--- End quote ---


 :(biglaugh): ROFPMSL :(biglaugh):

thermidorthelobster:
They're actually two sides of the same thing, and they're directly related by a formula.

1 HP is 550 ft lb per second.

What does this mean?  It means that 1 HP has the ability to move something 550 ft per second against a force of 1lb.  So if you had a 1lb bag of sugar on the end of a string, and you had a 1HP winch, the winch would be able to move the bag 550 feet vertically (against the force of gravity) in one second.

Alternatively it could move 550lbs of sugar 1 foot, or 1100lbs of sugar 6 inches, etc.  (You can also substitute other things for sugar - flour, salt or other cooking ingredients all work :) )

In a rotating engine, the engine also turning a certain number of revolutions per second (or minute).  This means that power can also be related to torque and the rpm of the engine.

Specifically, power is torque x the engine speed in rpm, but we also have to multiply by a constant to convert from seconds to minutes, and allow for 1HP being 550 ft lb, not 1 ft lb.  If you do the math (this describes it quite well) you find that power is torque x rpm divided by 5,252.

So say your engine outputs 200 ft lb of torque at 2,000 rpm, you can say that the POWER of the engine at 1,000rpm is
(200 x 1000) / 5252 = 76hp.  But this only holds AT 2,000 rpm.  When you see HP quoted for an engine, it's always the MAXIMUM HP across the whole rev range.

Now, you'll notice that for the same torque, the higher the rpm, the higher the HP.  If you had 200 ft lb of torque at 4,000 rpm, ie the same torque, the power would be 152HP.  This is why peak power is high up the rev band.

This is also why people talk about torque as 'low-end grunt', and power as high-revving power.  Because power only becomes significant at higher engine revs.  If you had a totally flat torque curve, the max power would always be at the high end of the rev range.

If you're sufficiently bored, you can take a tuner's torque curve and do the maths, and work out what the power curve for the same engine is.  Do this and you'll actually invariably find that below 1,000rpm they exaggerate the power!  If you plot torque against (power / revs) you should get a straight line...  but you never do, because they fix the graphs.

You can also see that if you do something which changes the TORQUE of the engine, by definition you also change the POWER, but only the power in that part of the rev range where you've changed the torque - which isn't necessarily the max power.  If you change the torque at higher revs, then you're boosting the power much more strongly that if you change the torque at lower revs, so high-end torque boosts will mean the max power is likely to go up, whereas low end torque boosts won't unless the torque curve is affected quite noticeably.

Does this make any sense?

jjsaul:
As someone about to start automotive engineering at uni i find this very interesting!
Thank you for all the explanations so far  :D

rollazuki:
Good luck jjsaul, I wish Id done auto eng instead of electronic eng. Its pants!

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