Interference

The engine revs normally, but the tachometer shows impossible RPM jumps, or the connection to the software suddenly drops. In most cases this is interference generated by the engine's ignition.

The following video (in Spanish) explains where interference comes from, how it travels and how to eliminate it:

Threshold, noise and spikes

The signal entering the acquisition system has an interference tolerance threshold. There is always a background electrical noise level, and every time a spark fires, a noise spike appears.

As long as the spikes stay below the threshold, nothing happens. If a spike crosses it, the system counts pulses that don't exist: the RPM reading jumps, or the connection drops and the software freezes.

There are two ways to solve it, and it's best to apply both:

• Lower the noise: proper spark plugs, plug caps and spark plug wires, and measurement cables kept well apart.
• Raise the threshold: a good connection to the PC and a good ground.

How interference travels

Through the air

A cable doesn't have to touch another one for interference to occur. Each spark is a very fast current pulse that emits waves, like a radio. The receiver, in this case, is the measurement system a few meters away from the engine.

WIRELESS PROPAGATION

Electromagnetic interference propagates through the air. Its intensity decreases with distance: keep the PC, the interface and the USB cable as far from the engine as possible.

Through cables: coupling

When a cable carries a current pulse, it creates a magnetic field around it. If another cable runs in parallel, that field induces a voltage in it.

• The closer the cables and the longer the parallel run, the stronger the coupling.
• Separating them quickly reduces the effect.
• If two cables must cross, cross them at 90°.

Rule of thumb

Sensor cables and the USB cable must never share a cable tray or be tied together with power cables, ignition cables or spark plug wires.

The spark plug

The spark plug is suspect number one. Inside it has a center electrode, a ceramic insulator, the metal body and, in many models, a resistor that dampens fast pulses before they turn into waves. We recommend always using resistor spark plugs: many brands identify them with an R in the part number.

In normal operation, the spark fires at the tip, inside the cylinder, enclosed in metal. But some faults go unnoticed in the engine yet show up in the measurement:

• Internal crack in the insulator: the spark fires in the crack and at the tip. The engine keeps running normally, but that extra spark is in an unshielded area and interference skyrockets.
• Loose or damaged plug cap: the spark fires in open air, outside the cylinder.
• Surface flashover along the insulator, between the plug body and the cap.

These faults produce spikes so strong that they exceed the threshold even when everything else is properly installed.

Strange readings out of nowhere

If a dyno that has always worked well suddenly starts showing strange readings or connection drops, the first step is to check or replace the spark plug and the plug cap.

Resistor spark plug cap

On motorcycles and karts, the spark plug wire is usually attached to the coil and cannot be replaced. In those cases, use a resistor spark plug, a plug cap with a 5 kΩ resistor, or both.

Two-stroke engines

On some engines, for example two-stroke engines, the resistor can weaken the spark and cause misfires. Test the engine before taking measurements.

Spark plug wires

When the spark plug wire can be replaced, always use EMI suppression wires. A common wire is just a copper conductor with insulation; a suppression wire has a resistive core (carbon, or a spiral wound over a ferrite core) that dampens fast pulses. A copper wire works as an interference antenna.

Racing wires

Many "racing" wires have little suppression or are plain copper. On the dyno you don't need racing wires: you need real EMI suppression wires. It's good practice to replace them when the vehicle goes on the dyno; besides improving the measurement, it helps prevent damage to the acquisition system, the PC or the monitor.

PC connection

How the PC is connected directly affects the tolerance threshold:

• A short, high-quality USB cable provides a good reference. With a longer cable the threshold drops and the highest spikes start getting through. The USB cable should not be longer than 3 m.
• With the data connection over WiFi, the acquisition system is electrically isolated from the PC: the threshold rises and you gain a lot of margin. In that case the USB cable is used only for power. This is the recommended option when there are interference problems or a longer distance is needed.

Ground leakage

There is a problem that looks like interference but is something else: ground leakage.

• A desktop PC may have a small insulation fault in its power supply, and part of the mains voltage reaches the case. That current looks for ground and often finds it through the USB cable, via the acquisition system. The result is communication drops.
• It can also happen the other way around: a leak in a lamp, a pump or an electric motor of the dyno looks for ground through the sensor cables, enters the acquisition system and exits through the USB towards the PC.

The solution is a dedicated ground rod for the dyno, connected to the dyno frame, the acquisition system ground and the PC ground. That way the leakage has a direct path and doesn't go through the measurements.

Other sources of interference in the workshop

Noise doesn't always come from the engine. Three-phase motors, solenoid valves and other inductive loads produce a current spike when switched on or off. Even an air compressor starting elsewhere in the workshop can produce spikes that travel through the air and through the electrical wiring.

SPREAD THROUGH THE ELECTRICAL NETWORK

Conducted interference propagates through the cables. It can affect long distances, depending on the electrical wiring.

• Keep the measurement cables and the USB cable away from the cables that power pumps, motors or solenoid valves.
• If the electrical wiring is poor, install a dedicated power line for the PC and the acquisition system.

Dyno ≠ track

All these measures are for the dyno, not for racing. On the track, components are designed for impacts, vibration and much more demanding conditions. The dyno doesn't have those demands, but it has something the track doesn't: sensors, data acquisition and a computer a few meters from the engine.

Summary

• Resistor spark plug, in good condition and properly tightened.
• Plug cap in good condition, preferably with a 5 kΩ resistor.
• EMI suppression spark plug wires (never copper).
• Measurement and USB cables away from power and ignition cables, crossing them at 90°.
• Ground rod with the dyno, the interface and the PC connected to it.
• Short, high-quality USB cable or, even better, WiFi connection.

If you still have problems after applying all of this, contact us and we'll review it together.