PART 6 - Interpreting the Data


Story and Photos by John Copeland

By now you're probably wondering when the heck we're going to stop fooling around on this dyno business and tell you what all those numbers really mean. Well, your patience is rewarded this month. This time we're going to take a look at some real data, corrected for environmental conditions like we learned last time, and figure out what it's telling us about the engine and how to set it up before we go to the track. We'll use a 5HP stock class Briggs, but the principles are pretty much the same, 4-cycle or 2-cycle.

One of you correctly pointed out that the dyno data we used last month to illustrate proper data correction did not reflect the entire usable range of RPMs available to the average Briggs racer. Right you are! It turns out that the data I used was taken for a specific dirt-track setup where, once the green flag dropped, the clutch was never a factor. Consequently, the engine builder in this example was concentrating on getting the most out of the engine beginning in a higher RPM range. To help you get a clearer picture of how to transfer the numbers on the dyno to your track setup, we'll use data that starts at a lot lower RPM, 4000 RPM to be specific. The Corrected Data is shown in Figure 1.

It will probably be helpful to look at this data in graphic form as well. We'll graph only the Torque and Horsepower and exclude the CHT for now. See Figure 2.

SERIAL # #12345 TEMP 80
HEADER 990 x 15 BAR. PRESS. 29.5
4000 9.14 6.96 363
4100 9.18 7.17 363
4200 9.19 7.35 365
4300 9.21 7.54 367
4400 9.26 7.76 366
4500 9.13 7.82 364
4600 9.00 7.88 362
4700 8.98 8.04 361
4800 8.88 8.12 359
4900 8.79 8.20 357
5000 8.67 8.25 356
5100 8.52 8.27 359
5200 8.71 8.62 359
5300 8.68 8.76 354
5400 8.61 8.85 352
5500 8.34 8.73 352
5600 7.94 8.47 351
5700 7.76 8.42 350
5800 7.53 8.32 350
5900 7.34 8.25 349
Figure 1

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.

SERIAL # #12345 TEMP 65
4000 9.25 7.04 324
4100 9.29 7.25 325
4200 9.30 7.44 325
4300 9.32 7.63 327
4400 9.34 7.82 327
4500 9.37 8.03 328
4600 9.40 8.23 328
4700 9.38 8.39 328
4800 9.29 8.49 327
4900 9.20 8.58 327
5000 8.98 8.55 326
5100 8.87 8.61 326
5200 8.78 8.69 324

Figure 4

It may be tough to get much detail from the graphs in the size that they can appear here. But if you look at the tables, you'll see that changing headers did a couple of things. First of all, we've shifted the Torque peak from 4400 RPM to 4600. And at 4600 RPM, the peak is a bit higher too. However, at the upper RPM range, the Torque falls off pretty hard, resulting in lower Horsepower from 6000 RPM on up. And take a look at the CHT readings. For some reason its header is generating cooler cylinder head temps by about 40 some degrees. We'd sure want to try a jet change next to see what happens if we push the CHT back up where it was with the other header.

But, on the basis of just these two runs, if we were running a track that put a premium on lower RPM performance, a track where you rarely needed to run over 6000 RPM, we'd sure want to give this header a try. Pretty neat, huh?  

Well, that's about it for this month. Next time we'll start to wrap all this dyno business up and talk about how to successfully transfer what you learn from the dyno to the race track.

Remember, just because you don't have a dyno of your own doesn't mean you have to be out of the loop. Lots of good engine builders have serviceable dynos of various sorts.

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!