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Old 12-09-2004, 08:09 AM   #12
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Re: more stupid questions!

Originally Posted by texan
Psman32@af- Actually, I have never understood why people think hp is what determines top speed. Now I'm not saying it's not, but I would like you to answer this hypothetical on top speed....

A vehicle has an engine producing a perfectly constant torque output. For this example, we'll say 200 lbs/ft at all RPM, with a redline of 10,000 RPM. So it's now easy to calculate hp from this, but what about top speed based upon that hp output?

Let's say this vehicle's top gear allows it to put down 640 lbs/ft of effective torque to the wheels (a 0.8:1 gear with a 4:1 final drive) throughout all of said top gear. Now we'll say this motor at redline, in this gear, can reach 200 mph. Now at 150 mph the total resistance to motion is 640 lbs/ft of pressure, while 200 mph is 700 lbs/ft of pressure. So since the horsepower curve is still climbing (to a total of 380hp @ 10,000 RPM), would the vehicle continue to accelerate or reach top speed at 150 mph?

All other things being equal hp DOES determine top speed.
IE. If the cars are drag equivelant (rolling and aero) the car with the greatest HP can go the fastest. (assuming final gearing is correct) Both cars will accelerate until all possible work energy is used. Please note that drag is not linear. eg. To double your speed takes 4 times the energy.

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