Definition of Horsepower

[jetmech]

Long story short, the two car clubs at my university had a joint dyno day and the dyno we went to was very......questionable/sketchy. The power and torque curves were NOT crossing at 5300 rpm ( just approximating here ) and there were MANY different vehicles on the dyno that day. To my understanding, if a dyno does not show the power and torque curves crossing at about 5300 rpm, you should question the dyno. BTW, the dyno looked like it was purchased at WalMart. I'm too curious here guys, a little help? And I was hoping someone had that very detailed definiton of horsepower which explains why power and torque curves should cross at about 5300 rpm.

[Fasglas]

Yes, every dyno chart should have the torque and horsepower lines cross at 5252 rpms. That's because horsepower is a product of torque and rpm.

The formula to find horsepower is...

Torque x RPM / 5252 = Horsepower

[Atomic]

the classic definition of horsepower from the late 19th century works out to 33,000 ft-lb/min. Basically the amount of work a horse was thought to be able to do in a given time.

In an engine, power = torque x angular velocity

to convert from an angular velocity to RPM you need to multiply by 2 pi

so we have 33000 hp = 550ftlb/s * 2pi and we need to rearrange this:

hp = (Torque*RPM* 2pi)/33000ftlb/min

simplifying we can divide 33000/(2*pi) and get 5252.117

rewriting we now have 1hp = (Torque*RPM)/5252
and as you can see, when rpm=5252 it cancels with the denominator and hp=tq


I think any dyno that doesnt cross at 5252 has something funky with atmospheric corrections going on.

[Fasglas]

I think any dyno that doesnt cross at 5252 has something funky with atmospheric corrections going on.

Any dyno chart that doesn't cross at 5252 has something wrong going on. Torque and rpm is how you find horsepower. To be correct every dyno has to use the standard formula to find the horsepower being produced. There is no other way of measuring the horsepower of an engine. Finding the horsepower any other way would stray from the standard and be meaningless.

Even dyno charts that are corrected for the weather still cross at 5252 because the torque number is what is corrected, then the corrected horsepower is figured from the corrected torque.

[jetmech]

Thanks guys!