Can any engineers here provide a link or a formula that gives a theoretical horsepower calculation taking into account Displacement, Pressure Ratio, and RPM? The formula I have used in the past is shown below, but does not take into account RPM, which is suspect.
Est HP = ((Displacement in cu/in) * (Pressure Ratio * 14.7) * .0755)
I am open to formulas that are based upon Airflow (cfm) also.
much obliged,
Peakhp
seeking hp calculation formula...
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Q45tech
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Using cfm is so ambiguous because it varies in density with temperature and barometric pressure and other factors.Modern OBD2 ecu generate an accurate air flow in grams per second.
"with air flow increased to 320 grams per second [405 HP at 6,000 rpm] in the '02 LS6, up from 304 grams per second of air flow in the '01 LS6."
One might conclude that 0.79 grams per second of air is needed per horsepower.
http://www.superflow.com/suppo...s.htmh ... amtest.htm
"""170 Gm/sec works out to 300 cfm (and about 205 HP) (at sea level), which works out to 84% or so VE--454 grams = 1 lb. (conversion constant)0.075 lbs./cubic foot = typical sea level density of air at a reasonable temperature
(1) therefore 170 grams per second /454 grams/ lb. = .374 lbs./sec
(2) therefore, .374 lbs./sec. x 60 seconds/minute = 22.47 lbs./min. (of air, number used in (3) & (4) below)
(3) therefore 22.47 lb/min. / .075 lbs./cubic ft. = 299.6 cubic feet/min.(cfm) of air.
(4) therefore, 22.47 lbs./ min. x 60 min/hour = 1348 lbs/hour of air.
(5) therefore 1348 lbs./ hour / 13.5 (typical WOT a/f ratio for an NA engine) = 99.87 lbs./hour of fuel
(6) therefore 99.87 lbs./ hour of fuel/ .48 lbs. fuel/ brake horsepower hour ( a pretty typical brake specific fuel consumption figure for an NA engine) = 208 Horsepower.
35 lb/hr or 370 cc/min injectors = 4.4 grams fuel per second at 43.4 psi at 4,020 rpm [67 rps] x 4 =268 injector pulses of 11.0 milliseconds each but only 9.5 millisec of full flow and 1.5 millisecs of ramp up and shut down.....call it 10 millisecs of flow= 118 grams per second x 60 x 60=.....................
"with air flow increased to 320 grams per second [405 HP at 6,000 rpm] in the '02 LS6, up from 304 grams per second of air flow in the '01 LS6."
One might conclude that 0.79 grams per second of air is needed per horsepower.
http://www.superflow.com/suppo...s.htmh ... amtest.htm
"""170 Gm/sec works out to 300 cfm (and about 205 HP) (at sea level), which works out to 84% or so VE--454 grams = 1 lb. (conversion constant)0.075 lbs./cubic foot = typical sea level density of air at a reasonable temperature
(1) therefore 170 grams per second /454 grams/ lb. = .374 lbs./sec
(2) therefore, .374 lbs./sec. x 60 seconds/minute = 22.47 lbs./min. (of air, number used in (3) & (4) below)
(3) therefore 22.47 lb/min. / .075 lbs./cubic ft. = 299.6 cubic feet/min.(cfm) of air.
(4) therefore, 22.47 lbs./ min. x 60 min/hour = 1348 lbs/hour of air.
(5) therefore 1348 lbs./ hour / 13.5 (typical WOT a/f ratio for an NA engine) = 99.87 lbs./hour of fuel
(6) therefore 99.87 lbs./ hour of fuel/ .48 lbs. fuel/ brake horsepower hour ( a pretty typical brake specific fuel consumption figure for an NA engine) = 208 Horsepower.
35 lb/hr or 370 cc/min injectors = 4.4 grams fuel per second at 43.4 psi at 4,020 rpm [67 rps] x 4 =268 injector pulses of 11.0 milliseconds each but only 9.5 millisec of full flow and 1.5 millisecs of ramp up and shut down.....call it 10 millisecs of flow= 118 grams per second x 60 x 60=.....................
Agree with you on air flow in CFM is dependant on temperature and pressure, and lbs/hour is more accurate.
The formula works great! Two variables that affect the outcome that are vary from engine to engine are the BSFC and AFR at WOT. For na engines you used .48, and I've read that .45 should be used for NA, and .55 should be used for forced induction engines, but it was in the context of fuel injector choice. These are pretty close guesses and .48 might be the most accurate, I don't know. I was under the impression the extra .1 BSFC was to provide additional fuel for a cooling effect and protection from predetonation. Its my perception that this extra fuel isn't necessarily turned into energy, by the same token that incrementally richening an air/fuel ratio doesn't incrementally increase power. So I'm unsure whether I should plug in .55 for a forced induction engine in this hp formula or leave it at .48 even if it is forced induction.
With regard to the AFR would it be more accurate to leave it 13.5 or plug in what actual target AFR we are tuning for.
You know, after thinking a moment about it, its always going to be a theoretical calculation anyhow and just getting close (+-5%) is good enough. Actually the difference between my formula and yours is only about 2-3%, which doesn't mean anything, but they do both estimate consistantly and correlate well.
Just reiterating your formula for everyone.
HP = CFM * 4.4934 / AFR / BSFC
Where AFR=13.5 and BSFC=.48
thanks,
peakhp
The formula works great! Two variables that affect the outcome that are vary from engine to engine are the BSFC and AFR at WOT. For na engines you used .48, and I've read that .45 should be used for NA, and .55 should be used for forced induction engines, but it was in the context of fuel injector choice. These are pretty close guesses and .48 might be the most accurate, I don't know. I was under the impression the extra .1 BSFC was to provide additional fuel for a cooling effect and protection from predetonation. Its my perception that this extra fuel isn't necessarily turned into energy, by the same token that incrementally richening an air/fuel ratio doesn't incrementally increase power. So I'm unsure whether I should plug in .55 for a forced induction engine in this hp formula or leave it at .48 even if it is forced induction.
With regard to the AFR would it be more accurate to leave it 13.5 or plug in what actual target AFR we are tuning for.
You know, after thinking a moment about it, its always going to be a theoretical calculation anyhow and just getting close (+-5%) is good enough. Actually the difference between my formula and yours is only about 2-3%, which doesn't mean anything, but they do both estimate consistantly and correlate well.
Just reiterating your formula for everyone.
HP = CFM * 4.4934 / AFR / BSFC
Where AFR=13.5 and BSFC=.48
thanks,
peakhp