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#1
Old 05-18-2000, 03:43 PM
Charter Member
Join Date: May 2000
Location: SW Side, Chicago
Posts: 44,398
Could somebody please explain to me the difference between horsepower and brake horsepower? All the British car magazines I read refer to BHP. All the American ones I read refer simply to HP. A friend of mine told me they are not the same thing. He said BHP was the truer measure of horsepower. Another said it was the other way 'round. I've tried search engines, but I haven't gotten a satisfactory answer that I can understand. Anybody out there know?
#2
Old 05-18-2000, 04:53 PM
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Join Date: Jul 1999
Location: Suburban Seattle
Posts: 2,746
Brake horsepower just means they used an engine brake to measure the horsepower. It's a pretty accurate measure of the horsepower actually available to the drive train.

An alternative is to use indicated horsepower which is measured by recording the pressure and volume of a cylinder through a cycle (an indicator diagram) and then calculating the horsepower. Indicated horsepower tells you how much horsepower is available at the cylinder, but doesn't account for various losses. So brake horsepower is always less than indicated horsepower.

Neither is necessarily more accurate than the other. It just depends on what you want to measure. But unless you're an engine manufacturer, brake horsepower is the more useful quantity since you presumably want to know how much horsepower is available to the wheels (of course there are also transmission and gear train losses -- and you're probably spinning out your tires, aren't you?)

If the specs your reading just say HP they could be quoting either of the above or one of several other related ways of measuring HP. But they don't vary all that widely. IIRC, BHP is usually within 2% or so of IHP.
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#3
Old 05-18-2000, 05:44 PM
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Join Date: Apr 1999
Posts: 517
When an new-design engine is manufactured it is placed on a test stand,fitted with maximum intake and exhaust facilities,and provided with a pony brake to check the
hp delivered at the flywheel at a given rpm.

Having been brought up to speed the brake is exerted against the wheel to see how much effort is required,in Horsepower,to bring it to stall.

That's,basically,the story and the reason it's called 'brake' horsepower.

Actually it's a true measure of an engines power----and not the MadAv numbers the sales forces like to astound you with!
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