Posted by Indiana Ken on October 06, 2013 at 06:45:44 from (66.249.233.14):
In Reply to: Sparkle question posted by John T on October 05, 2013 at 18:56:36:
John,
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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