KARTING DYNOMOMETERS                
PART I - What are they?

 

By John Copeland

Dyno-tuned, Dyno-tested, Dyno-flow, Dyno-carbs, Dyno-cams, Dyno, Dyno, Dyno. Who the hell is this Dyno guy? It seems like he's stuck his nose into just about every aspect of karting. A quick spin through the pages of National Kart News will reveal no less than 100 references to the word "Dyno". But what does it all mean to the average karter? In the next few months we're going to take a close look at just what a "dyno" is, what it does, how it works, and how all that affects you. Hopefully in the process we'll get a feel for what separates a good dyno from a worthless one, and how you can tell the difference. We'll also look at what sort of information you can reasonably expect to get from a good dyno, and how it may, or may not, apply on the racetrack. It's a lot to cover, so let's get started.

Webster's Dictionary defines a dyno (more properly a dynomometer) as "a device for measuring power (in most cases mechanical power) output from a source." Now that's pretty clear, huh? A more scientific definition comes from the American Society of Mechanical Engineering (ASME) which defines a dynomometer as " Any device for measuring the tangential forces incident to a rotating "mass." In terms of karting interest, a dyno is typically a means of accurately measuring the specific output of an engine, either at the crankshaft or somewhere further down the driveline, for the purposes of comparison. That comparison might be between engines, or between different carbs on the same engine, or between pipes, and on and on. The key element is the term "accurately measuring." Dynos typically apply some sort of measurable resistance on the engine. This resistance is called the "load" and it serves as a benchmark, something to measure the engine's power against. As we will see, there are a variety of ways to measure this output, and these are dictated, in most cases, by the basic mechanism of the dyno. As I mentioned before, the engine output in question can be measured either at the crankshaft itself, or further down the driveline, so let's look at that element first.

Chassis dynos have been around for a long time, in automobile manufacturing as well as in all forms of motorsports. The idea here is just to move the vehicle, whether its a go-kart or a bigger car, into position on the dyno, then to drive the measurement mechanism, either off the drive shaft, or the axle shaft, or the driving wheels. A crankshaft dyno, on the other hand, picks up its input directly off the engine, sometimes directly coupled, sometimes via some speed-reduction mechanism. From the perspective of ease of use on an existing vehicle, the advantages of the chassis dyno are obvious. No need to remove the engine from the vehicle, plus it gives you a more realistic look at the output that is really going to the ground. A chassis dyno can help unravel what is going on throughout the drive line; in the engine, the clutch, the transmission, the drive shaft, the differential, the axles, everything. It gives the skilled operator a lot of latitude in investigating all the driveline components from stem to stern. But this very flexibility is a drawback as well. If the operator wants only to evaluate specific engines or changes to those engines, all the other components just muddy up the water, so to speak. And, as you can imagine, it can take a lot larger facility to accommodate a chassis dyno than something suited to just engine examination. Nonetheless, chassis dynos have found an important place in motorsports. Perhaps the most significant jump in chassis dyno technology occurred in 1965 when Ford Motor Company, having been thoroughly embarrassed by Ferrari at the 24 Hours of LeMans, built the first "programmable" automated chassis dyno, capable of subjecting their GT40 prototypes to all the rigors of a 24 hour race. Mechanical actuators actually operated the gearshift, the clutch, brake, and throttle pedals, everything. The racecar sat on the dyno platform and howled up and down through the gears for 24 hours at a time as the rear wheels drove rollers in the floor to absorb the output. Although no official figures were ever released, the project was rumored to have cost Ford more that $5 Million, but it had the desired results. Ford finished first and second at LeMans in 1966, and returned to win again in 1967.

Fortunately for us, karting doesn't require such extreme measures, nor are chassis dynos all that desirable in our case. Because most karts in this country use centrifugal clutches, except gearbox karts of course, and because these clutches all slip to some degree, using a chassis dyno makes it somewhat harder to sort out what is really happening in the engine. Small variations in engine output can get "lost" on their way to the axle shaft. While it is certainly a valid argument that any changes so small as to not show up at the axle can't very well affect kart performance, changes are still changes. And the sum total of lots of little changes may be significant when added up. Those changes can be good, as in horsepower gains, or bad, but if you can't accurately measure them, you'll never know. And because we're dealing with such relatively small power outputs, in comparison to bigger racecars, it's very important not to lose track of any changes.

That brings us to the area of shaft dynos. As the name implies, these measure the output of the engine either directly at the crankshaft, or through some speed reduction device. Like their chassis dyno cousins, shaft dynos apply some "load" to the engine output, then use a variety of means to compare that output to the known load. There are a variety of ways to apply this load, and each required its own measurement technique. Probably the simplest and most widely used shaft dyno system in use in karting is the "Water Brake" dyno. The basic concept is pretty straightforward. The engine is coupled to a water pump, generally a 'positive displacement" type where the pump impeller mechanically forces the water through the pump. A valve is fitted in the output line of the pump to generate backpressure against the pump. The pump is mounted so as to allow it to move slightly, as if it were trying to rotate around it's own input shaft. Once the engine is running and up in the powerband, the valve is closed slowly to increase the backpressure on the pump. This causes the engine to work harder to overcome the backpressure. As the backpressure in the pump increases it resists the efforts of the engine to drive it. This has two effects; it makes the engine work harder, slowing it down (or at least preventing it from gaining RPM, and it tries to rotate the pump housing around it's own input shaft. This "torquing" of the pump housing can be measured by something as simple as a spring scale or as sophisticated at an electronic load cell. A load cell is similar to the sensing device used in electronic bathroom scales, only much more accurate. The harder the pump housing wants to twist at a given RPM, the higher the torque output is. And the higher the torque at a give RPM, the higher the horsepower. With the throttle wide open, the operator can adjust the engine RPM by opening and closing the valve on the output side of the pump. Once the engine has stabilized at the desired RPM, the operator can then read the spring scale, or the load cell, to determine the twisting force of the engine at that RPM. Pretty neat, huh? And if all you're interested in is relative numbers for comparison purposes (is this carb better, or that one?), that's really all the farther you need to go. But if you want to convert those output numbers into "real" torque (and horsepower) figures, you'll have to go a little farther. Torque is measured in foot/pounds (or inch/pounds, or some similar unit), and a foot/pound is defined as the amount of energy required to raise one pound to a height of one foot.

For our purposes, you'll need to convert the output of the measuring device, be it a spring scale or a load cell, to foot/pounds. The simplest way of doing that is to locate the measuring device exactly one foot from the center pivot point of the pump. Then, if the measuring device indicates a load of 10 pounds, and its located one foot from the fulcrum point, you have an output of 10 foot/pounds of torque. Can't get much more direct than that. Of course, if the measuring device is only 6 inches from the fulcrum point, a 10 pound reading will be 10/.5 feet, or 5 foot/pounds. See how it works? But what about horsepower? I thought you'd be asking. Well, horsepower is a function of Torque and RPM. The actual formula is (Torque x RPM)/5252.1 = Horsepower. In other words, horsepower is a derived number, dependent on RPM as well as Torque (power). I heard it said once that, if you could spin it fast enough, you could get 100 horsepower out of electric pencil sharpener. But remember, without a significant amount of torque, you can't drive anything of any mass. In other words, YOU DRIVE YOUR KART WITH TORQUE, NOT HORSEPOWER. Later in this series we'll talk about how to interpret torque output readings from a dyno, and how to take maximum advantage of what those readings are telling you. I know that everyone wants to compare horsepower output figures, but the really knowledgeable folks are looking at the torque numbers. That's where the critical information is.

As I said before, there are lots of different ways to configure a dyno, to provide the load, even to measure the output. For example, rather than measuring the torque on the pump housing in our example, you could just as easily measure it as torque on the engine mount plate. That can be a little trickier, but it works. In the months to come, we'll look at different kinds of dynos, how they work and the kind of information they can give you. We'll look at commercially available units and ones you can build yourself. We'll be contacting manufacturers of dynos suitable for the karting world and passing on information about their products. Finally, we'll analyze the kind of information you can get from a good dyno and how you can apply it to get more out of your karting effort.