What Your Gauges Are Trying To Tell You
By John Copeland
Time was when karters had nothing to tune their engines with but their ears and the seat of their pants. Oh sure, in the pits they looked at the spark plug, and some even looked at the top of the piston, to get an idea of how things were going in there, but once they hit the track is was all up to "feel" and, if you were good, an educated ear to keep your engine at its peak. The legendary Gary Hartman used to feel the head of his McCulloch with one ungloved hand while he was racing and dial the carb according to what he felt. Was it a psych job, or could he really tell? Who knows, but he sure was fast.
Today's karters have the benefit of a variety of gauges to tell them all sorts of things. Some will even remember what's happened a various points around the track, so the driver doesn't have to. Some have internal lights to help night racers and flashing displays to catch your attention if things are getting out of hand. Let's look at the various functions most karters are likely to consider and see what each one can do for you and your on-track performance.
While this was not the first type of gauge karters used, it is probably the most universally accepted now. As you most likely know, it simply reads and displays the RPM (revolutions per minute) the engine is turning at any given moment. In most cases this data is collected by a sensor attached to the spark plug wire, although some gauges use a pickup off the crankshaft. The spark plug sensor picks up a signal everytime the coil fires and the circuitry in the gauge translates that into the readout that you see. Both 2 cycle and 4 cycle racers need this information to determine proper clutch settings and to help select the right gears. Your engine builder will tell you where he wants the clutch to engage and how many RPM you should see at the end of the straight, and the Tach gives you the info you need to get it there. Here's a couple of hints that may save you some grief later on: Digatron makes different tach sensors for 2 cycles and for 4 cycles. In most cases I've seen the 2 cycle sensor used on both 2 and 4 cycles without any problems, but some 4 cycle ignitions just don't get along with the 2 cycle sensor and you'll get bad readings, or no readings at all. So, if you're a 4 stroker, ask your local dealer for a 4 cycle sensor, just to be safe. Also, some 2 cycle ignitions may give your gauge problems with RF (radio frequency) interference. Swapping the stock spark plug cap for a resistor type, like the NGK Plug Boot, will cure that and give you better, more consistent, readings.
CYLINDER HEAD TEMPERATURE (CHT)
The oldest type of instrumentation in karting, and still probably the best known, the CHT measures just that, the temperature of the cylinder head. This is almost universally measured by a sensor that goes on the spark plug before it is threaded into the cylinder head. While some CHT units use a true thermocouple (two wires of different metals twisted together that, when heated, generate an electric current), the vast majority use a device called a "thermistor" which is a wire whose electrical resistance changes with temperature. The CHT is the most widely accepted method of determining the proper carb settings for your engine. Since the introduction of sturdy, reliable CHT gauges, karters have relied on them to report whether their engines were running too rich or too lean. When properly used, the CHT gauge can give you some important clues to what's going on inside your engine. But don't forget, the CHT sensor is mounted to the OUTSIDE of your engine. This means you have to take a couple of very important factors into consideration. First of all, the outside air temperature is going to have an effect on what the gauge reads. An engine that runs at 400 degrees on a 90 degree afternoon, may read 20 or 30 degrees cooler when the air temperature drops to 50. Leaning the carb out to get back to that 400 degree CHT will most likely lead to a premature exit. Along the same lines, remember that the fire inside the engine has to heat up the head casting BEFORE it gets to the sensor. That takes time, so remember to allow for things to heat up (or cool down) before you put too much stock in CHT readings. Another note; the CHT sensor is even more succeptable to RF interference than the TACH sensor. Again, the NGK Plug Boot is a sure fix. CHT can be a valuable tuning tool for both 2 cycle and 4 cycle racers. All kart engines need to run at the optimal fuel/air ratio within the limits of allowable INTERNAL temperatures. We'll discuss the factors that effect those temperatures, and how to determine the best settings, in another article. For right now, let your engine builder guide you. He'll tell you how hot he wants things to get, and what to do to adjust for the proper temperature.
EXHAUST GAS TEMPERATURE (EGT)
The advent of EGT instrumentation has given the karter an entirely new dimension to looking at fuel mixture analysis. While EGT has been primarily used by 2 cycle racers, some 4 strokers are seeing the advantages and adapting their setups to utilize this technique. As the name implies, EGT measures the temperature of the exhaust gas stream as it exits the engine. EGT has several advantages over CHT in terms of response and reliability. First of all, because the EGT probe in inserted directly into the hot exhaust gas stream, response time is almost instantaneous. The slightest carb adjustment or pipe change shows up right now! It can even display how hard your engine is pulling at different parts of the track. Also, because it uses a true thermocouple as a sensor, EGT is not succeptable to the RF problems that can plague CHT. But EGT also requires special care to properly install. It is extremely important that the reading be taken a specific distance from the exhaust port. The exhaust gases cool rapidly in the header and if you compare the readings taken from a probe inserted in the header 2 inches from the port with those taken from a probe 2 1/2 inches from the port, the readings will be meaningless. If you are going to try to use EGT effectively, set the sensor position in every header EXACTLY the same: Same distance from the port, same depth of insertion into the header. Using EGT or CHT is not necessarily an either/or situation. Both have valuable information to deliver and, while they can be used effectively alone, together they provide a powerful look inside your engine. While conditions that lead to increased CHT usually generate increased EGT readings as well, such is not always the case, and how these two reading change, relative to each other, can tell you alot about your setup and, sometimes, help avert disaster. More on that another time.
Like many of you, I'm sure, when Speedometers first appeared for karting use I really couldn't think of what possible use they could be. No, that's not true. There was one use I could think of. Now, when some non-karter asked "How fast will it go?", I could give them something better than "Oh, about a hundred." I was only guessing, and they probably didn't believe it anyway.
But a few years ago I spotted a Speedometer on the kart of a veteran racer who, I was sure, didn't need to worry about impressing anyone. His explanation led me to try putting a Speedo on my own kart, and now I wouldn't be without it. The Speedometer calculates (that's important) vehicle speed by measuring axle rotations. It is, in effect, a tachometer for the axle. You tell it what the circumference of your rear tires are, and it interprets the axle RPM into Miles per Hour. It's very important that you keep track of tire circumference and adjust the gauge when you change tires, or you won't be able to make best use of your speedo readings. But back to the fundamental question, what good are speedometer readings anyway? In an ideal setting, your Tach would tell you exactly what the kart is doing. With no clutch slip, the engine RPM, divided by the gear ratio, should be the axle RPM. But the fact is there IS clutch slip. Even on the longest straight, with the engine approaching maximum revs, the clutch is slipping, even if just a little. At lower axle speeds, it may slip a significant amount. And, of course, at axle speeds below the clutch's stall speed, we want it to be slipping. But what if we want to know what the chassis is doing at a point on the track where we have clutch slip? How can we get that info? Obviously the speedometer is the answer, but the more important question is "Why would we want to know how fast the kart is going at a point where we are slipping the clutch?" The answer is in the driving. Ask any professional race driver and he'll tell you that "Exit speed is everything." Getting off the corners with the maximum vehicle speed will almost always yield faster lap times. Sprint Car and Midget drivers frequently use the expression "Slow in, Fast out" to describe the best way to the fastest laps. The same applies to karting, except that our slipping clutches rob us of the ability to determine just which line through a corner, which apex point, turn-in point, braking point, etc. will give us the best exit speed. With a speedometer to guide you, and willingness to experiment, I'm willing to bet you'll find faster ways to get through almost every turn you encounter. I became a believer at an enduro track that I had driven literally thousands of laps on. The track has one particularly critical turn, long, sweeping, and decreasing in radius. I picked a spot right at the exit of the turn, where the kart was pretty well tracking straight onto the next chute, and checked my speed there for several laps, driving the line that had always seemed the best for me. Then I started to experiment. I tried all sorts of things; early apex, late braking, "hustling" the kart in hot and deep, easing it in from farther out on the track. Most were slower at the exit, some were about the same. But one line, a line I had never considered, was 4 MPH FASTER! I tried it again and got the same result. Then I headed for the pits and, when I stopped, my buddy with the stopwatch said "Boy, you really ripped off a couple of hot ones those last 2 laps! Let's go racing!" The point is, I would never have tried all those different lines without some way to get IMMEDIATE feedback about whether they were better or worse. There are so many variables during the course of a whole lap, that trying different line in one turn and trying to interpret the result on the watch is a hit-or-miss thing. The speedo gives you that information when you need it, right now. Learning to use a speedometer effectively can take BIG chunks off your lap times, if you'll use it to analyze your driving and experiment.
So there you have it, the 4 major gauge functions you're likely to need or want on you kart. Not all of them may be of interest to you, but all 4 have their uses. Digatron, the industry leader in kart instrumentation, offers these functions in any combination of 1,2,3 or 4 displays. You need to determine which is most important to you, and how much instrumentation is right and affordable for you. Then, once you've made your selection, make it work for you. In karting, like in most things, KNOWLEDGE IS POWER.