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The real deal on dyno... power measuring


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Here is a VERY good post by Chiplee. Thank you for your input Chip.

 

I will add this:

 

A dyno run that does not cross HP and TQ at 5252 it is not an accurate display of power output. Any knowledgable dyno tuner will confirm this.

 

Below is Chips post.

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You guys probably know this but just incase I'll add a little tidbit to the Torque/HP relationship. Â Believe me I'm not smart enough to figure all this out but I can cut and paste for the benefit of others with the best of 'em. Â Dynos do not measure HP they only measure torque. Â the computer multiplies torque by RPM and divides that number by 5252 to get HP. Â That's why Torque and HP will always be the same number at 5252rpm. Â Where does 5252 come from? Â That's actually quite interesting. (this is the cut and pasted part) Â "Awhile back, a gentleman by the name of Watt (the same gent who did all that neat stuff with steam engines) made some observations, and concluded that the average horse of the time could lift a 550 pound weight one foot in one second, thereby performing work at the rate of 550 foot pounds per second, or 33,000 foot pounds per minute, for an eight hour shift, more or less. He then published those observations, and stated that 33,000 foot pounds per minute of work was equivalent to the power of one horse, or, one horsepower. Â Everybody else said OK. Â

 

For purposes of this discussion, we need to measure units of force from rotating objects such as crankshafts, so we'll use terms which define a *twisting* force, such as foot pounds of torque. A foot pound of torque is the twisting force necessary to support a one pound weight on a weightless horizontal bar, one foot from the fulcrum. Â

 

Now, it's important to understand that nobody on the planet ever actually measures horsepower from a running engine. What we actually measure (on a dynomometer) is torque, expressed in foot pounds (in the U.S.), and then we *calculate* actual horsepower by converting the twisting force of torque into the work units of horsepower. Â

 

Visualize that one pound weight we mentioned, one foot from the fulcrum on its weightless bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two foot circle), and, incidently, we have done 6.2832 foot pounds of work. Â

 

OK. Remember Watt? He said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of that weight into 33,000 foot pounds, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 foot pounds per minute of work, and is the equivalent of one horsepower. If we only move that weight at the rate of 2626 rpm, it's the equivalent of 1/2 horsepower (16,500 foot pounds per minute), and so on. Therefore, the following formula applies for calculating horsepower from a torque measurement: Â

 

 

                  Torque * RPM

 

    Horsepower    =    ------------

 

                     5252

 

 

This is not a debatable item. It's the way it's done. Period."

 

I found that interesting and informative, and easy to understand which is a must for me.

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  • 4 weeks later...
Crap my dyno runs are very inacurit, mine crossed at about 3.4k, i wonder if the newer dyno mechines are more acurit or something. Or this is not completly true on a turboed car (or its outdated), i do the math on my 3 runs and it doesnt work out the hp doesnt come out right.
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Thats all tru.

 

dont confuse the lines crossing on the dyno sheet with the power and torque #s been the same at 5252rpm.

A tuner will change the scale on the torque and power to fit the print and to also get a better look at the curve so the lines could cross absolutly anywhere on the print.(still the same # at 5252rpm)

 

think of it this way.. some dyno prints have foot lbs and newton meters/ hp and kw on the same print..  im not sure at what # the torque and KW is the same with Newton meters but im pretty sure its diferent to ft lbs and hp.

 

Lizzord. have a look on your dyno print and check the # at 5252 rpm for power and torque. ignore where the lines cross on the page. they the same?

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not sure but I think that happens automatically when you fill out your profile and include the country you are from.

 

 

I've been meaning to add this for a while now just to make it painfully clear that I only wish I was smart enough to make the relationship between torque and hp that clear.  http://www.vettenet.org/torquehp.html and if anyone has any interest in more explaination I've gathered quite a few text files about the subject as well.  Let me know and I'll email them to you.

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  • 4 weeks later...

when doing dyno runs w/ the intercooled conquests/starions, i wonder if anyone ever thinks to bring a leaf blower w/ them to send air over the intercooler.  Sitting still on a dyno, i wouldn't think that intercooler would be all that effecient to really show max power.  

 

pen

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  • 1 year later...
just another tidbit.  if i'm incorrect then correct me, but i am pretty sure that torque stays the same at a certain rpm no matter what gear you.  Yet horsepower changes depending which gear your in.  or something like that ???   i forget.  someone correct me please  ;D    
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