Seneca Scan Gauge HP

[Crossingover]

Have you all noticed how little horsepower it takes to maintain flatland cruising at 60 mph? Coming home today I noticed it didn't take much over 100 HP indicated. Should I be surprised?


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[RyRoRyan]

This is not surprising at all. Actually I learned in high school that an average sedan take about 10-15 HP to cruise. Its all about physics...Force = Mass*Acceleration. Also Newtons law of inertia...a object in motion stays in motion with the same speed and direction unless acted upon by an unbalanced force.

You need force to accelerate, but not to maintain speed. Due to such, if we had no drag/friction, once you put an object at a certain speed, it would effectively maintain that speed forever unless you slowed it down some other way. Outer space is a great analysis of this. Since drag is basically 0 in space, you can put up satellites that spin with the earth, send other satellites to pluto and beyond all by just getting them going.

On earth, we have drag and friction with wind resistance, road to tire friction and mechanical friction in the powertrain. You also have to deal with gravity when climbing or falling, but lets just say flat earth for now to make it easy. That 100 HP is all you need to create an "equal and opposite force" in order to keep total force at 0 and therefor acceleration equal to 0 and speed constant. Think of drag and friction as a negative force and your engine as a positive force. If the total of those two forces is 0, then you will not change speed.

[Crossingover]

Good write-up there! I am more fascinated now by the situation where I would need all the available HP as a comparison. Have to do a 0-60 sprint while logging the HP to see. Familiar with the algorithm the gauge uses for HP?

[RyRoRyan]

There is a lot of math here and a lot of variables which we dont know like the real drag and powertrain losses and things like that so it is difficult to calculate without all this info. I dont know the exact math on the scan guage, but I will try to guess...

HP = Torque * RPM / 5252. We know our motors produce 660ft/lbs of torque pretty much at all RPM according to the cummins data sheet. As such, HP is just 660/5252 * RPM. But realize that this is at 100% load (full throttle). You remove some gas and air and the engine is not producing at 100% anymore. So what your scan gauge is doing is more than likely HP=660/5252*RPM*percent load.

Note that HP is DIRECTLY related to RPM's since the torque curve is flat. As such, the faster the engine is turning the more HP it will make. Like I said, I dont know the exact method they are using, but my guess is that if you have it floored at 1200 RPM and it shows 150HP, at 2400 RPM it will say 300 HP.