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Before you start Before testing you want to know at what rpm the engine produces its maximum power. You'll also want to have a good tachometer so you can observe the rpm during your test run. It's also important that the tach reads accurately at higher rpm, otherwise you may tune several hundred rpm away from the power peak.
Actually watching a tach during the run is not always easy. You may want to remove it from a low console and mount it higher in your field of vision. Reading dial numbers also is a problem when you are running, so on a regular needle tach we mark a strip on the dial where the power peak is, and then you can see if the needle matches with the line or is above or below it. We prefer a needle tach, as observing numbers changing on a digital tach is difficult at speed.
If the numbers are small and hidden below the handlebars, as on some machines, your only recourse is to mount a recall tach in a better position. Auto Meter and VDO were the first to offer recall tachs, about 15 years ago, and they were an instant success. No more watching tachs on the fly, you just pushed the record button, and when you came back from your run you could hit the replay button and watch the rpm of your run displayed in 1/3 slow motion. Not only did the recall tach give great details of clutch performance, it was way safer.
If you have a VDO replay tach it may now be worth some money, because after VDO was bought by the giant Siemens Corp., the tach was discontinued because it didn't meet Siemens' volume requirements. Fortunately EGT Technologies has developed its own playback tach, with even more features than the VDO, so clutch tuners are in luck.
The second bit of info you want is speed, and a quick look on the speedometer as you pass the finish line often is good enough to get an idea of performance changes. Timing the run is a little harder; it requires some timing equipment.
Oval and snocross racers may get perfectly good results with a stopwatch, and we have even used arm drop and stop watches on drag strips with good result, if the run was long enough.
But when you get down to 60- or 100-feet times, you need accurate electronic timing. Electric eyes and cables are time consuming to set up, and need constant adjustment on the ice, but that's what we struggled with for 20 years. A better setup is an automotive lap timer; two beacons, one at the start and one at the finish, start and stop the timer, which is located on the sled. The beacons run on batteries, so no messy wires are needed. Combine the timer with a playback tach and a max recall on your EGT gauge and you can do some serious testing in a hurry.
Factory snocross teams with a good budget, have a complete data recorder system measuring everything from engine speed to secondary speed, belt speed, belt location, G-forces in 3 dimensions, all kinds of temperatures from exhaust to engine, throttle positions and even suspension travel. Add a GPS to the system and you can check intervals on the race track, and compare real speed to track speed for info on slippage and efficiency. This size database is usually too much for the average Joe as it can take several engineers to interpret the data. Mere mortals could quickly suffer from data overload.
The optimum for testing on the ice is a Stalker radar gun and software system like the one AmSnow uses at its annual Real World Shootout (see p. 30). A Stalker system picks up the run from standstill to completion and the data is fed into a computer. That data can then be used to give you elapsed time and speed at any distance you choose. You can overlay runs to see where you gained or lost. A Stalker system isn't cheap, with a full data recording capacity it runs several thousand dollars.
Or, for $25 you can go to an asphalt drag strip in summer and get equally valuable recordings on a time slip, which is why asphalt testing has become so popular and why the AmSnow Shootout at Martin, Mich., each September is so valuable for early sled season evaluations and a look at new aftermarket products. Plus you can win some cash, if you're successful.
In the last three issues we've introduced some of the variables you encounter when tuning your clutches. You always learn something new when you test, and I am surprised that after 40 years, I still enjoy finding new ways to tune and get more performance from the automatic snowmobile belt drive transmission.