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Author Topic:   Convert PSI to Horsepower

Perry posted 01-07-2012 06:58 PM ET (US)
My mechanic just rigged up a dynamometer at his shop to help him diagnose problems with outboards under load. It reads in PSI.
Since my boat was in his shop for some routine service, I asked him to hook up my motor to his dynamometer. At 6000 RPM and WOT the dynamometer read 310 PSI and the fuel flow meter was at 13.4 GPH.
Any idea how to convert these figures to obtain HP?
contender posted 01-07-2012 07:29 PM ET (US)
What is the cylinder Bore Diameter?
How Fast does it extend in Inches Per Minute?
GPM= Gallons Per Inch (GPI) X Inches Per Minute (IPM) piston speed.
Gallons Per Inch = Bore Area divided by 231
HP= GPM X PSI X .000583
Peter posted 01-07-2012 08:03 PM ET (US)
You can't . You don't have enough information. You need to know the flow rate in GPM of the fluid being driven by the motor on the dynometer. Psi is simply pressure which is force per unit area. That doesn't tell you the amount of work being done per time. The GPM tells you the mass being moved per unit time which is the work product.
Peter posted 01-07-2012 08:06 PM ET (US)
You can't . You don't have enough information. You need to know the flow rate in GPM of the fluid being driven by the motor on the dynometer. Psi is simply pressure which is force per unit area. That doesn't tell you the amount of work being done per time. The GPM tells you the mass being moved per unit time which is the work product.
Perry posted 01-07-2012 08:13 PM ET (US)
Bummer, I was hoping there would be some easy formula to figure it out.
outragesteve posted 01-07-2012 10:18 PM ET (US)
Howzit Perry: I have used an outboard dyno a number of times. As your mechanic runs more engines, he can simply build a data base on comparable hp engines for comparison. All things being equal (not!) different brand engines of the same hp rating should pull close/similar readings. I used the dyno (is this the unit that attaches to the prop shaft/lower unit?) to pull the engines under load, which really helped in trying to find misses and other problems that happen "at speed" in the boat. Aloha, steve
(how's this weather we are having???? Great sunsets from my Outrage, with full moon in the background!)
Perry posted 01-07-2012 10:50 PM ET (US)
Hey Steve, the dynamometer is the type that attaches to the prop shaft. My mechanic used to be the head mechanic at Windward Boats before he left to set up his own business. My motor was the second one he used the new dynamometer on. He noticed that under load my fuel pressure dropped a bit and found that the fuel line was being restriced where it goes into the motor. I guess the clamp was too tight holding the control cables and other stuff. He was able to diagnose the problem real quick. The dynamometer is a real nifty tool.
Now that my motor is running great, I'm ready to go out to the buoys and catch some fish.
Aloha...Perry
jimh posted 01-08-2012 06:20 AM ET (US)
If you knew the brake specific fuel consumption of your engine you could convert its fuel flow into horsepower. Typical four-cycle engines have a brake specific fuel consumption of 0.45-lbs/HP-hour.
A flow rate of 13.4-GPH of gasoline at 6.25-lbs/gallon is a flow of
13.4-gallons/1-hour x 6.25-lbs/1-gallon = 83.75-lbs/hour
which would imply a horsepower of about
83.75-lbs/1-hour x 1-HP-hour/0.45-lbs = 186-HP
The problem is we don't have a really good figure for the brake specific horsepower for your particular engine at the engine speed and load that produced that fuel flow.
Peter posted 01-08-2012 06:30 AM ET (US)
It sounds like Perry's mechanic basically has a hydraulic load tester. They don't provide accurate, repeatable results from which you can develop a comparative HP database. It's unlikely to be calibrated. It's a bit better than a test wheel which they would put on a motor in a test tank.
Peter posted 01-08-2012 06:40 AM ET (US)
It sounds like Perry's mechanic basically has a hydraulic load tester. They don't provide accurate, repeatable results from which you can develop a comparative HP database. It's unlikely to be calibrated. It's a bit better than a test wheel which they would put on a motor in a test tank.
seahorse posted 01-08-2012 08:09 AM ET (US)
Chances are it was an International 2000 dyno that is made for load testing motors at dealerships. The booklet with it has the average psi readings for many models of outboards. They list a new motor psi, as well as a range for used motors. Barometric pressure, humidity, and air temperature play a big part in power output.
Years ago one of the engineers at the dyno company developed a formula for converting the psi to torque and from that info, hp is only a mathamatical equation. Of course it was not accurate enough due to a number of factors, but it would show a hp or torque curve indicating how the power was developed. The shape of the curve on a graph shows if the motor has a wide powerband or just a narrow range of rpm that has the highest output.
Jerry Townsend posted 01-08-2012 03:57 PM ET (US)
As Peter said - there isn't enough information. The dynamometer used is basically a water brake - and there is probably a table or plot correlating the pressure with the HP or torque.
The fuel flow only is indicative of the input energy - and as Jim states - is about 83.75 lbs/hr. And with gasoline giving about 20,500 Btu/lb, and there is about 2545 Btu/lb/HP (as I recall) - says there is about 674 "HP" going in - and then there is the efficiency of the engine and lower unit, et al - is what comes out. I don't have any information about the efficiency of a two stroke engines or of the lower unit - but an automobile engine's efficiency a few years ago was typically between 30 and 35 percent.
The calculation has nothing to do with the bore, stroke or piston speed.
I will check out the Btu - HP conversion mentioned above - I might have slipped a cog on that number. ----- Jerry/Idaho
Jerry Townsend posted 01-08-2012 11:17 PM ET (US)
The 2545 number is good - but I noticed a real slip - the units Btu/lb/HP should be Btu/hr/HP. Sorry for any confusion. ---- Jerry/Idaho
jimh posted 01-09-2012 04:59 PM ET (US)
Perry--What is the rated horsepower of the engine that was tested on the device?
Perry posted 01-09-2012 09:11 PM ET (US)
Jim, my motor is a BF135.
Honda claims it produces 140 HP at 5500 RPM:
http://img.photobucket.com/albums/v235/perrycl/hondaBF150.png

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Author	Topic: Convert PSI to Horsepower
Perry	posted 01-07-2012 06:58 PM ET (US) My mechanic just rigged up a dynamometer at his shop to help him diagnose problems with outboards under load. It reads in PSI. Since my boat was in his shop for some routine service, I asked him to hook up my motor to his dynamometer. At 6000 RPM and WOT the dynamometer read 310 PSI and the fuel flow meter was at 13.4 GPH. Any idea how to convert these figures to obtain HP?
contender	posted 01-07-2012 07:29 PM ET (US) What is the cylinder Bore Diameter? How Fast does it extend in Inches Per Minute? GPM= Gallons Per Inch (GPI) X Inches Per Minute (IPM) piston speed. Gallons Per Inch = Bore Area divided by 231 HP= GPM X PSI X .000583
Peter	posted 01-07-2012 08:03 PM ET (US) You can't . You don't have enough information. You need to know the flow rate in GPM of the fluid being driven by the motor on the dynometer. Psi is simply pressure which is force per unit area. That doesn't tell you the amount of work being done per time. The GPM tells you the mass being moved per unit time which is the work product.
Peter	posted 01-07-2012 08:06 PM ET (US) You can't . You don't have enough information. You need to know the flow rate in GPM of the fluid being driven by the motor on the dynometer. Psi is simply pressure which is force per unit area. That doesn't tell you the amount of work being done per time. The GPM tells you the mass being moved per unit time which is the work product.
Perry	posted 01-07-2012 08:13 PM ET (US) Bummer, I was hoping there would be some easy formula to figure it out.
outragesteve	posted 01-07-2012 10:18 PM ET (US) Howzit Perry: I have used an outboard dyno a number of times. As your mechanic runs more engines, he can simply build a data base on comparable hp engines for comparison. All things being equal (not!) different brand engines of the same hp rating should pull close/similar readings. I used the dyno (is this the unit that attaches to the prop shaft/lower unit?) to pull the engines under load, which really helped in trying to find misses and other problems that happen "at speed" in the boat. Aloha, steve (how's this weather we are having???? Great sunsets from my Outrage, with full moon in the background!)
Perry	posted 01-07-2012 10:50 PM ET (US) Hey Steve, the dynamometer is the type that attaches to the prop shaft. My mechanic used to be the head mechanic at Windward Boats before he left to set up his own business. My motor was the second one he used the new dynamometer on. He noticed that under load my fuel pressure dropped a bit and found that the fuel line was being restriced where it goes into the motor. I guess the clamp was too tight holding the control cables and other stuff. He was able to diagnose the problem real quick. The dynamometer is a real nifty tool. Now that my motor is running great, I'm ready to go out to the buoys and catch some fish. Aloha...Perry
jimh	posted 01-08-2012 06:20 AM ET (US) If you knew the brake specific fuel consumption of your engine you could convert its fuel flow into horsepower. Typical four-cycle engines have a brake specific fuel consumption of 0.45-lbs/HP-hour. A flow rate of 13.4-GPH of gasoline at 6.25-lbs/gallon is a flow of 13.4-gallons/1-hour x 6.25-lbs/1-gallon = 83.75-lbs/hour which would imply a horsepower of about 83.75-lbs/1-hour x 1-HP-hour/0.45-lbs = 186-HP The problem is we don't have a really good figure for the brake specific horsepower for your particular engine at the engine speed and load that produced that fuel flow.
Peter	posted 01-08-2012 06:30 AM ET (US) It sounds like Perry's mechanic basically has a hydraulic load tester. They don't provide accurate, repeatable results from which you can develop a comparative HP database. It's unlikely to be calibrated. It's a bit better than a test wheel which they would put on a motor in a test tank.
Peter	posted 01-08-2012 06:40 AM ET (US) It sounds like Perry's mechanic basically has a hydraulic load tester. They don't provide accurate, repeatable results from which you can develop a comparative HP database. It's unlikely to be calibrated. It's a bit better than a test wheel which they would put on a motor in a test tank.
seahorse	posted 01-08-2012 08:09 AM ET (US) Chances are it was an International 2000 dyno that is made for load testing motors at dealerships. The booklet with it has the average psi readings for many models of outboards. They list a new motor psi, as well as a range for used motors. Barometric pressure, humidity, and air temperature play a big part in power output. Years ago one of the engineers at the dyno company developed a formula for converting the psi to torque and from that info, hp is only a mathamatical equation. Of course it was not accurate enough due to a number of factors, but it would show a hp or torque curve indicating how the power was developed. The shape of the curve on a graph shows if the motor has a wide powerband or just a narrow range of rpm that has the highest output.
Jerry Townsend	posted 01-08-2012 03:57 PM ET (US) As Peter said - there isn't enough information. The dynamometer used is basically a water brake - and there is probably a table or plot correlating the pressure with the HP or torque. The fuel flow only is indicative of the input energy - and as Jim states - is about 83.75 lbs/hr. And with gasoline giving about 20,500 Btu/lb, and there is about 2545 Btu/lb/HP (as I recall) - says there is about 674 "HP" going in - and then there is the efficiency of the engine and lower unit, et al - is what comes out. I don't have any information about the efficiency of a two stroke engines or of the lower unit - but an automobile engine's efficiency a few years ago was typically between 30 and 35 percent. The calculation has nothing to do with the bore, stroke or piston speed. I will check out the Btu - HP conversion mentioned above - I might have slipped a cog on that number. ----- Jerry/Idaho
Jerry Townsend	posted 01-08-2012 11:17 PM ET (US) The 2545 number is good - but I noticed a real slip - the units Btu/lb/HP should be Btu/hr/HP. Sorry for any confusion. ---- Jerry/Idaho
jimh	posted 01-09-2012 04:59 PM ET (US) Perry--What is the rated horsepower of the engine that was tested on the device?
Perry	posted 01-09-2012 09:11 PM ET (US) Jim, my motor is a BF135. Honda claims it produces 140 HP at 5500 RPM: http://img.photobucket.com/albums/v235/perrycl/hondaBF150.png