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Old 03-11-2003, 10:33 PM   #1 (permalink)
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BHP vs RWHP

So what is the difference in the old rating of BHP (Brake HP) versus RWHP? Is the BHP rating simply of the engine with no drivetrain loss or belt driven accessories?

Probably a really silly question.

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Old 03-12-2003, 12:19 AM   #2 (permalink)
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hmm not that silly. Im interested in an answer too.
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Old 03-12-2003, 01:22 AM   #3 (permalink)
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BHP and RWHP are two very different things, but a lot of people are confused as to their true meaning. Let me start with RWHP and then we will get around to BHP.

In the case of an automobile people are generally interested in knowing the power made by the engine and the power available at the drive wheels. Engine power is measured at the crankshaft/flywheel and this is often refered to as flywheel hp. Drive wheel power is measured at the rear wheels for a rear-wheel-drive car (RWHP), at the front wheels for a front-wheel-drive car (FWHP) and at all four wheels in the case of an all-wheel-drive car (AWHP). Because of the losses involved in transfering power from the engine to the drive wheels, flywheel hp is always higher than the drive wheel hp. This loss is categorized as driveline loss. It generally ranges from 15-20% depending on whether the car is a 2 wheel drive (FWD or RWD) or a 4 wheel drive (AWD).

HP measurement for cars is normally carried out by first measuring torque and then computing the associated power. Torque measurement can be performed using either a "brake dynamometer" or an "acceleration dynamometer." In the first case a force (torque) is applicated by means of a brake to keep the engine speed a constant. This force is exactly equal to the torque produced by the engine at that speed. Measurements are carried out a intervals of 250rpm to obtain a torque graph for the entire rpm scale. Using this torque curve power can be computed and this power is known as brake-horsepower or BHP. The second type of dyno, measures the rate at which a drum of fixed mass is accelerated by the engine. From this data the system computes the acceleration torque and then power. This way of measuring power should be known as acceleration HP. While either type of dyno can be used to measure flywheel HP or drive-wheel HP, it is generally found that brake dynos are employed to measure flywheel HP and acceleration dynos are used to measure drive wheel HP.

Both brake HP and acceleration HP have their own place. Brake HP gives a true indication of the ability of an engine to perform constant work (say keep the car at 175mph for the entire afternoon as is the case with the NASCAR cruisers). An acceleration dyno, on the other hand, is more useful if your intention is to determine a car's ability to accelerate - i.e. fly down a drag-strip. Note that unlike BHP, acceleration HP takes into account the effect of reciprocating mass and rotational intertia. However, I will not get into a discussion on that unless folks are interested.

Hope this discussion was of some help.

Best regards.
-Anil
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Old 03-12-2003, 02:18 AM   #4 (permalink)
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I would be willing to bet that Anil played with slide rulers when he was a kid. :D
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Old 03-12-2003, 04:31 AM   #5 (permalink)
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I would be willing to bet that Anil played with slide rulers when he was a kid. :D


Excellent response Anil!

Part of my work at Detroit Diesel Corp involves dyno testing our engines. BHP is the standard measurement used in the industry...no accessories, no fan blades, as measured at the flywheel. Formula: Torque X RPM divided by 5,250.

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Old 03-12-2003, 06:20 AM   #6 (permalink)
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Lets modify Anil's response a bit. the 15-20% loss through transmissions is a good number. The 15% fits the 6 speed manual loss and the 20% fits the auto lossed as I have seen for the Camaro.
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Old 03-12-2003, 10:43 AM   #7 (permalink)
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Haha, slide rulers were a little before my time. However, my dad did own a beautifully crafted one that caught my attention at an early age. Seriously though, it was my fascination for math and physics that eventually got me interested in cars. When I was working on my previous car (http://www.geocities.com/arao99gsx) I would spend a hell of a lot of time trying to understand the theory behind engines and modifications. Sometimes I understood the logic, sometimes I didn't. But it was a fun excercise.

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I would be willing to bet that Anil played with slide rulers when he was a kid. :D
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Old 03-12-2003, 11:00 AM   #8 (permalink)
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I would spend a hell of a lot of time trying to understand the theory behind engines and modifications. Sometimes I understood the logic
Hehe...the words 'theory' and 'logic' used in the same paragraph as engine modifications. Only missing the word 'addiction' to put it into perspective....as in.....'My theory in modifying my car was to use no common sense or logic but rather to let my addiction prevail":D :D
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Old 03-12-2003, 11:06 AM   #9 (permalink)
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I hear you Anil, I love math and physics to, and its neat how ultimately a high performance car runs on math and physics.

-Austin
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Old 03-12-2003, 04:29 PM   #10 (permalink)
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Awesome answer!!!!

Thanks !!!
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Old 03-12-2003, 05:45 PM   #11 (permalink)
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John Lingenfelter told a group of us gear heads at the ZR1 Gathering that the drive train loss on at C5 was a lot less than a C4 , 15 to 16% on a C5 6 speed compared to 18% on C4 6 speed

Rick:D
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