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Robert Noel Reddington
Grand Master
Username: bob_uk

Post Number: 896
Registered: 5-2015
Posted on Tuesday, 22 March, 2016 - 09:47 am:   Edit PostDelete PostView Post/Check IP

Recently in a posting about adequate and effortless in General discussion I touched on torque and BHP and came up with some educated guesses applied to my Shadow engine. For those who are not au fait with how the maths works.

BHP is a function of rpm and torque. Machines that measure power ( BHP) actually measure torque and rpm and then calculate the power automatically or leaves to the operator. Dynameters. Interestingly electric motors equate volts to rpm and amps to torque. Amps x volts = watts . 746 watts = 1 BHP. Neat ah.

The usual formula that is used is BHP = torque x rpm / 5252.

The 5252 is a constant. Image a 1 ft radius turning through 1 turn. The dia is 2 ft so the notional point at 1 ft radius will travel 2 x pi or 6.248 ft. 1 BHP is 33,000 ft lbs / minute.

33,000 divided by 6.248 = 5252.

If the engine is at 5252 rpm then the torque and BHP will be the same value. I call this all square.


200 BHP @ 4500 rpm. This means that because the engine is below 5252 rpm the torque must be more than the BHP. The maths shows 233.4 ft lbs. also it means that if the torque holds out to 5252 rpm the engine will produce 233.4 BHP. However generally engines like this tend to drop off at higher rpm which causes the BHP not to rise so pointless, use a higher gear.

Example Honda Blackbird bike 160 BHP @10,000 rpm. Because 10,000 is about double 5252 the BHP @ 5000 rpm would be about half. 80 BHP. But this engine at 5000 rpm will still be rising on the torque curve. Take another 30 BHP off and at an educated guess 50 BHP @ 5000 rpm. The capacity is 1 litre. 50 BHP @ 5000 rpm and 52 ft lbs is like a 1 Litre car engine.

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