Most guys who buy a tach buy it to see one number: how many RPM the motor turned down the straight. That's fine — but you paid for a lot more than that, and if you're only glancing at peak RPM you're leaving the best part of the tool sitting in the toolbox. A data system with a cylinder head temp sensor on it will tell you whether your gear is right, whether your jetting is right, and whether the problem you think you have is actually the problem you have.
Here's how we read the numbers at the shop, and how to turn them into a change on the kart.
What a kart data logger actually gives you
A basic tach — a Mychron or an Alfano — logs RPM off a sensor wrapped around the plug wire, and reads head temp off a sensor under the spark plug or screwed into the head. That's two channels, and those two channels cover most of what you can control at the track on a Saturday.
RPM tells you what the gear is doing. Head temp tells you what the fuel and air are doing. Read them together and you stop guessing.
The hardware side is simple. You need the gauge, an RPM sensor and a clip to hold it on the plug wire, a CHT sensor, and a lead to run it back to the gauge. Then you need somewhere to bolt the gauge where you can actually see it — either an angled steering shaft mount or a clamp-on mount for 1" tube. We stock all of it in tachometers and data.
Reading peak RPM: is your gear right?
Run the kart, come in, and look at the highest RPM the motor saw at the end of the longest straight. That single number is your gearing report card.
RPM too low
If the motor never gets up into the range it's supposed to pull, you're geared too tall. The motor is dragging the kart around instead of spinning. It'll feel lazy off the corner and it'll usually run warm, because a loaded-up motor makes heat. Add teeth to the axle sprocket.
RPM too high
If you're screaming past the top of the window and the kart quits accelerating halfway down the straight, you're geared too short. You ran out of gear. Take teeth off the axle sprocket.
The rule that matters
One tooth is a real change. Two teeth is a big change. Don't stack three adjustments into one session and then try to work out which one helped — you can't, and you'll waste the night. Change one thing, run it, write the number down.
If you're not sure what your combination should turn, start with our outlaw kart gearing guide and work from there. Sprockets are cheap compared to the time you'll burn guessing.
Reading cylinder head temp: is your jetting right?
Head temp is the other half of the picture. On a clone-type motor, most people want to see a steady reading somewhere around 400°F once the kart is up to temperature and working. Above roughly 450°F you're into territory where you should be paying attention rather than shrugging.
Those are ballpark numbers, not gospel. Different heads, different sensor locations, different classes and different engine builders all move the target. Ask your builder what he wants to see on his motor and use that number instead of ours.
Running hot
Richen it up a step and open up the cooling. More fuel cools the chamber. Check that your flywheel fan is actually moving air, that the shroud is in place, and that nothing has been taped over from a cold night three weeks ago and forgotten about. Our clone carburetor jetting guide walks through which way to go on the jet.
Running cold
Cold isn't safe, it's slow. A motor that never comes up to temperature is running fatter than it needs to and isn't making the power it should. Lean it out a step and tape off some air until the number comes up into range.
Temp moving around
If the number wanders 60 or 80 degrees run to run with no change to the kart, look at the sensor before you look at the jet. A CHT washer that isn't torqued flat under the plug, or a chafed lead, will give you garbage data — and garbage data is worse than no data, because you'll act on it.
Reading both numbers together
This is where the tach earns its money. Neither channel means much alone. The combination tells you what to touch.
RPM low and temp high
The motor is loaded up and fighting the gear. It's working hard and not spinning. Gear down first — usually the temp follows the gear back into range on its own. Don't start chasing it with jetting until the gear is right, or you'll end up rich and slow with a motor that still can't pull.
RPM high and temp low
The motor is spinning free but it isn't pulling the kart. That's a driveline or a traction problem more often than a gearing problem. Check clutch engagement and slip, and check whether the tires are hooking. Our clutch tuning and rebuild guide covers what slip looks like in the data and what to do about it. Have a look at clutches and clutch parts if yours is due.
RPM climbs, then falls off at the end of the straight
If the motor pulls clean and then goes flat at the top of the straight every single lap, that is almost never gear. That's fuel delivery, a dirty filter, or heat. Pull the filter, check the fuel pickup and the line, and look at where your temp was when it went flat.
Actually using it at the track
The system only works if you write things down. Keep a notebook or a note on your phone with four columns: track condition, gear, jet, and the RPM and temp you saw. After a half-dozen race nights you'll have a baseline for your regular track, and you'll show up already close instead of burning practice figuring out where you are.
The discipline is the whole thing. Log it. Change one thing. Log it again. Guys who do that get faster. Guys who change three things and remember it wrong stay where they are.
What to run
If you're starting from nothing, a Mychron with a one-piece cylinder head temp sensor gets you both channels out of the box and is what most of our customers end up on. The Alfano is a solid alternative if that's what your crowd runs — the 14mm CHT sensor and patch cable get it wired up.
Already have a gauge and just need to get temp on it? Grab the CHT sensor and the extension lead and you're reading head temp this weekend.
Not sure what fits your setup, or which sensor goes with the gauge you already own? Shop tachometers, sensors and mounts or give the shop a call and we'll get you the right pieces the first time.