It's been a couple of weeks since I wrote about our initial findings on horsepower and I do have an update.
I may of confused some of you with the initial report and findings, and after several emails I want to try to explain what I mean about continuous horsepower. Horsepower is simply a rating given and can be used and or read a couple different ways. The formula is very easy just multiply the torque (foot pounds) and RPM's divided by 5,252 and the result is horsepower.
Here's where it gets tricky. You have “Peak” horsepower and “Constant” horsepower. Peak HP is the point that an engine is ran unloaded to it's maximum potential and then the load is applied suddenly. This is the point that the engine is stalled and will make as much torque at the highest RPM's possible for the engines displacement capability.
Please UNDERSTAND, Peak HP is not usable HP, and in most cases a engine can operate 15-20% under it's peak HP. What I am trying to establish is the constant HP for our usage. For instance our M5 engine under dyno conditions without a clutch directly hooked up to the machine will be ran up to 18,000 rpm's and then the program initiates and abruptly loads the engine. The result is a Peak HP averaging: 4.6 HP @ 16,000 rpms. In reality this engine can not carry a load of this caliber , therefore the usable HP is somewhere less than 4.6.
Now if you were to do opposite, run the dyno up, rather than down, the same way we run our boats and start this engine and run the rpms up slowly by applying throttle (a load), HP will climb as speed is increased until you reach the maximum HP determined by the physical characteristics of the that engine. This will fall short of the peak HP, because an engine can not run constant at peak HP.
Try to imagine this… You are trying to break a large nut loose with a breaker bar. If you jerk with all your might you will subject the nut to a higher torque load than if you were to apply constant force. (pulling the breaker bar).
We all know that to jerk the bar provides more power but yet you could not sustain this power over a period of time. The jerking effect we will refer to as peak power. And the constant force we will refer to normal running conditions, or your full throttle.
Most manufactures rate their engines using peak HP not constant HP, therefore there is a misconception on what true HP ratings are for our gas model engines.
Peak HP is very useful information for engine builders like myself. When I make a change to a procedure and retest, if the change was for the better my HP should increase.
This is for example only (Basic analogy not considering, type, size, or weight of boat)
For instance; if I find that it takes 4.6HP to turn a 80 mm prop @ 15,000 rpm's and I know a 75mm by calculation spinning at 17,000 will make the particular model boat go faster and handle better, I can change the engines porting/modifications to increase rpm's and though this will effect HP (make it go down, power curve), as long as I have enough power to turn the prop that boat requires, I go faster, in effect we are eliminating the “waisted” torque.
Involving the example above, If we were to put a prop larger than the 80mm you would not have enough torque to turn that prop at 15,000 rpm's. The rpms will now drop due to added load thus loss of overall HP until the engine finds it's new normal operating range. To turn the new “bigger” prop faster more horsepower is required.
I hope this will help you better understand engine horsepower and it's usage. We are working on recording constant horsepower and when we understand our findings we will begin to rate all of our engines using “usable or constant” HP.
By: Tony Castronovo and Steve Arnesen
2-25-08