Why on all dyno charts do the HP and TQ lines cross at 5250? - FFCars.com : Factory Five Racing Discussion Forum

 
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post #1 of 13 (permalink) Old 10-25-2003, 11:47 AM Thread Starter
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Why on all dyno charts do the HP and TQ lines cross at 5250, or right around that number?

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post #2 of 13 (permalink) Old 10-25-2003, 11:57 AM
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Torque is "real". It can be measured.
Horsepower is not "real", it cannot be measured directly. It's a number calculated with a mathmatical formula. 5250 is the constant used in the formula to calculate horsepower.

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post #3 of 13 (permalink) Old 10-25-2003, 01:06 PM
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Ed's right. Here's the formula: HP=(Torque * RPM)/5252

As for why 5252, I don't know. I believe it stemmed from measurements of literal "horse power" in the 18th century; there's more complicated equations with 33,000 as the denominator (the number of pounds a horse could lift in one minute), and somewhere along the line it boils down to the "short" formula above.

Can you tell I'm not a mathematician?

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post #4 of 13 (permalink) Old 10-25-2003, 01:47 PM
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From the Web...

Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per minute. When work is done by a source of torque (T) to produce (M) rotations about an axis, the work done is:

(Note: Π = pi)

radius x 2Π x rpm x lb. or 2Π TM

When rotation is at the rate N rpm, the HP delivered is:

HP = radius x 2Π x rpm x lb./33,000 = TN/5,250
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post #5 of 13 (permalink) Old 10-25-2003, 01:53 PM
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"Torque is "real". It can be measured.
Horsepower is not "real", it cannot be measured directly."

Actually you have it backwards dynos measure HP then calculate torque. I always thought it measured torque myself until I talked with a DJ tech rep & heres something I pulled off right off their website:

Quote:
Dynojet Research has created a Dynamometer that can duplicate the best "seat of the pants" road testing, right in the shop. Our method of measuring horsepower is relatively simple. Energy drawn from the rear tire is stored in a rotating mass sized to simulate driving a vehicle on the road. A computer connected to the Dynamometer displays horsepower by measuring drum acceleration. Runs with up to 1200 horsepower are processed into easy to interpret graphics and saved for future reference.
[ October 25, 2003, 10:13 AM: Message edited by: 331 Notch ]
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post #6 of 13 (permalink) Old 10-25-2003, 02:22 PM
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They intersect at 5250 RPM on the dyno charts when the scale for torque (usually on the right side of the chart) and HP (usually on the left side of the chart) are the same. Since HP=(TQ*RPM)/5250, at 5,250 RPM the formula becomes HP=(TQ*5250)/5250...( 5250/5250=1)...HP=(TQ*1)/1...HP=TQ at 5,250RPM! At 5,250 RPM since HP and TQ are the same number, if the scales are the same they will be at the same point on the dyno graph and thus the point where the curves intersect.

Phil
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post #7 of 13 (permalink) Old 10-26-2003, 12:36 AM
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post #8 of 13 (permalink) Old 10-26-2003, 12:52 AM
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Ed B.

Horsepower is "real" too. By that argument is speed (Distance/time) real either since one needs a calculation or if it's that constant thing, then Einstein may need to get real about E=MC<sup>2</sup>. [img]tongue.gif[/img]
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post #9 of 13 (permalink) Old 10-26-2003, 12:56 AM
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Mikee....only if you are an attractive single female between 18 and 28!

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post #10 of 13 (permalink) Old 10-26-2003, 12:58 PM
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331 - that's an inertia dyno. It still doesn't "measure" horsepower. It measures rotational acceleration. That number along w/ the known mass of the drum is then used to calculate horsepower and torque. A load type dyno (like a mustang dyno) DOES measure torque, then calculates horsepower.


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post #11 of 13 (permalink) Old 10-26-2003, 01:15 PM
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post #12 of 13 (permalink) Old 10-27-2003, 01:05 AM
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So just who is it that says-

"Those FFR guys are NOT rocket scientists. They're just some bunch of kit car builders anyway."

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post #13 of 13 (permalink) Old 10-28-2003, 05:12 PM
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Quote:
Originally posted by 331 Notch:
"Torque is "real". It can be measured.
Horsepower is not "real", it cannot be measured directly."

Actually you have it backwards dynos measure HP then calculate torque. I always thought it measured torque myself until I talked with a DJ tech rep & heres something I pulled off right off their website:

</font><blockquote>quote:</font><hr />Dynojet Research has created a Dynamometer that can duplicate the best "seat of the pants" road testing, right in the shop. Our method of measuring horsepower is relatively simple. Energy drawn from the rear tire is stored in a rotating mass sized to simulate driving a vehicle on the road. A computer connected to the Dynamometer displays horsepower by measuring drum acceleration. Runs with up to 1200 horsepower are processed into easy to interpret graphics and saved for future reference.
</font>[/QUOTE]Actually, only the inertia dynos (like the Dynojet) work that way. The more advanced dynos use a load brake and measure torque directly like Ed mentioned. An Inertia dyno does not measure horsepower, it measures acceleration. This measured acceleration calculated against a known mass, is calculated to equal a certain amount of power. They then back-calculate this against engine rpm to equal a certain amount of torque. It works in theory, but there is a lot in all of that back-calculating that can go wrong. This is also the exact same principle that an accelerometer based unit like a G-Tech Pro works on. If you can do direct "first principle" measurement, it is a lot more precise. An inertia dyno is not measuring the car, it will only tell you what the roller did.

See www.dynapackusa.com for an example of a direct measurement dyno.

John

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