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Tool Talk Discussion Forum

Re: Sparkle question

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Indiana Ken

10-06-2013 06:45:44

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Regarding the use of "factors" to compute engine horsepower:

1) There is the number "5252" which is the constant used to make the units work out.

2) There are "correction factors" to allow horsepower measurements to be compared at standard day conditions. This allows horsepower measurements taken at different weather conditions to be compared. This is called "corrected HP".
Brake horsepower is determined by testing the engine on a dynamometer. All testing is conducted at full throttle with engine speed (RPM) controlled by the dyno. The dyno does not measure HP directly; the measurements taken are, torque and RPM.

HP is then computed from: HP = [Torque (ft-lbs) x RPM / 5252. Since torque is measured at RPM values through out the engine's operating range there are multiple values for torque and HP. Generally a curve (graph) is constructed and the peak torque and peak HP values are reported.
Typically the HP is corrected to standard conditions, also called SAE correction factors (J codes). A current standard is J1349 which corrects to; 29.23 in hg, 77 degrees F and 0 % humidity. Also, in common use is correction to; 29.92 in hg, 60 degrees F and 0% humidity. The difference between the two is approximately 4%, with the later being the greater.


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John T

10-06-2013 15:16:45

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 Re: Sparkle question in reply to Indiana Ken, 10-06-2013 06:45:44  
Thanks, gotta love those laws of physics, thanks for that missing number, now if we can get Rich on board were there

John T

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10-06-2013 12:53:59

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 Re: Sparkle question in reply to Indiana Ken, 10-06-2013 06:45:44  
Hello Indiana Ken,

Pretty close from what I was thought in engine school. The 60 degrees and 29.92 was called then, sea level H.P. While the SAE H.P. was @500 feet 85 degrees and 28.38 In/Hg,


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