Story and Photos by John Copeland

A close examination of the data table confirms what we see on the graph; namely that Torque peaks at 4400 RPM and Horsepower peaks at 5400 RPM. Also please note that the CHT also peaks at about the same RPM as the Torque peak. This is the point at which the engine is performing most efficiently. Remember, the engine is converting the majority of its fuel energy into heat energy; lower heat probably indicates less complete combustion. Of course, if the heat is way out of whack with the Torque curve, jetting is probably not correct.

You've probably also noticed the little "hiccup" in both the Torque and the horsepower curves at about 5200 RPM. You'll see this sort of data, both in 4-cycles and in 2-cycles, somewhere in the transition phase between middle and high RPM. It reflects changes in airflow characteristics through the carb and the engine, fuel pickup changes, and, in some 4-cycles, certain  cam/lifter behaviors. In any case, what is this data telling us?

For openers, experience has taught us that clutch engagement is generally best at 200 to 300 RPM below Torque peak; 4100 to 4200 RPM in this case. That allows for solid clutch hookup before the torque begins to fall off. Setting the clutch at or above the Torque peak may, in some cases, cause the clutch to "chatter" as the hookup drops the RPM slightly and the clutch can no longer develop the needed pressure to engage solidly. This is not to say that there are not occasions where a higher clutch engagement won't work, but in general 200-300 RPM below peak works well. What can we tell about gearing from this data? Well, depending on your race track, you're going to have some options about the usable RPM range. You'll note that, overall, Torque drops steadily as RPM goes up. At some point you want to limit the top end RPMs with gearing just because you're tapping into a diminishing resource. You've heard people say how their engine just doesn't "pull" on top end? Remember "pull" is basically a torque effect. If your engine doesn't want to "pull" on top, you need to lower those top end RPM with gearing until it does. At a local short track where it's a struggle to gain more than 1500 RPM on the longest straight, you're going to need plenty of gear to try to get to that 6000 RPM figure. But never forget that, in this example, the Torque is falling off pretty hard from 5400 on, so if you have any elevation changes to negotiate, headwinds on the straight, etc., you're better off to take teeth off to get down more into the "meat" of the Torque curve. A special example of this is 4-cycles running at enduro events on big tracks. Where aerodynamics becomes the limiting factor on top end, you'll generally go faster if you gear to keep the top RPMs lower, like 5500 RPM, where you still have enough Torque to drive you though the wind resistance.

So this dyno run has given you several valuable bits of data: Where is the Torque peak so I can better judge where to set the clutch engagement? What happens to Torque and Horsepower at higher RPMs so I can adjust my gearing accordingly? And how close is this fuel/air mixture, based on the relationship of CHT to Torque? Finally, we now also have a baseline to compare this engine setup to other engines or other setups on this engine.

A serious dyno program is a tremendous tool. It can give you a completely unbiased look at the slightest change in your engine setup. We ran this same engine a few minutes later, changing just the header. Look at figure 4 for this data. Look to figure 3 for the graph.

You can either contract for dyno time with them, or at the very least try to get all the information you can about what they've learned in their dyno studies. The kind of information that can be gained from a good dyno program shouldn't separate the "chosen few" from the rest of us karters. Each shop and engine builder should share it with their customers so they can run better and have more fun. So let's get going!