A discussion a few days ago with a friend revealed to me there is a wide range of understanding horsepower and torque, and how these are measured and applied to engines, especially for those of us who are Barth aficinados.

For those more schooled, yep, I know I'm simplifying, but that's the aim...

Torque is basically twisting force, measured in pound-feet (except everywhere else in the world, where the metric is newton-meters, but we're ignoring that). Torque is moment (distance from the shaft center to the point of force application) times force. BTW, the term "foot-pounds" is an incorrect expression of torque, as that rendition applies to something entirely different.

Picture a shaft with a one-foot arm attached. A force of one foot applied to the end of that shaft will produce one lb.-ft. of torque. Ten pounds at that point makes 10 lb.-ft. Likewise, one pound of force applied to a ten-foot moment also produces 10 lb.ft. of torque.

So you wanna drag? You have a 400 HP turbocharged gaso engine that produces maximum horsepower at 8,000 RPM. Your opponent has a 250 HP gaso engine that produces maximum HP at 5,000 RPM. Who's going to win the race, all-up weights being equal? Don't bet the farm...

Horsepower is torque x RPM. Acceleration depends on torque...that 250HP engine may suck the doors off your 400HP because of torque produced at lower RPM.

What is the practical application for Barth owners? My Breakaway has a built-to-spec AMG 6.5L TD, named the "Optimizer 6500" (where DO they come up with these names?), producing 420 lb.-ft. of torque at 2,600 RPM and 230HP at 3,600 RPM. Cummins used to produce a 5.9L TD that could be rated at 230HP. Figures are no longer available, but IIRC, peak horsepower was at 2,400 RPM. Torque was around 600 lb.-ft. at 1,400 RPM

Why the difference in RPM for max HP? The AMG Optimizer 6500 is a short-stroke V8, which means it produces less torque at a given RPM than the Cummins. "Stroke" in an engine is the distance a piston will go from the top to the bottom of its travel. Shorter "stroke" means the moment arm is smaller.

So, short-stroke means the crankshaft offset, i.e. the moment of the crankshaft, is shorter than the Cummins, meaning that a piston on the power stroke produces less torque than the Cummins, regardless of RPM. The upside of this is that a short-stroke engine can run at higher RPM than a long-stroke, like the Cummins, since the internals are flailing about a bit less. The downside is that low-RPM torque is less, because the Cummins has a bigger offset on the crankshaft.

Now we come to the practical side: The engine's stroke (as concerns RV acceleration) is important, only in that its torque is multiplied by the transmission and the differential. The transmission is a torque multiplier. For example, if the engine produces 300 lb.-ft. of torque, and the transmission is in a 3:1 gear ratio, 900 lb.-ft. will be delivered to the driveshaft. If the diffy is 4:1, 3,600 lb.-ft. will arrive at the axle.

If the tranny and differential are well-matched to the engine, acceleration performance at the same coach weight, of my 230HP, engine and that of the 230HP Cummins would be about the same.

So they're equal. But not so fast, my friend!

This is what counts for fuel economy: Due to gearing (tranny and diffy) to take advantage of the engine's torque curve, my "Optimizer 6500" runs most efficiently at 2,600 RPM at 63 MPH. The 5.9L Cummins can probably run at the same speed at 1,600-1,800 RPM. Due to lower RPM, the Cummins is more efficient because there are less friction losses (and this includes internal engine friction, the power consumed by power steering pumps,water pumps, and alternators) in an engine at 1,600 RPM than at 2,600. Diesels also can run by design at even lower RPM, so if one swaps a four-speed (like the 4L80E in my Breakaway or the 4-speed Allison in many Cummins-powered coaches) for a 5- or 6-speed, economy is further enhanced, because friction losses are reduced still further because RPM can drop and the engine can produce enough torque to drive the coach at the selected speed.