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 on the dyno:
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: ignition 12 V wiring, proper spark plug wires and spark plugs, 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.
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, and must be kept away from the ignition 12 V cables.
Ignition 12 V circuit
Every 12 V cable carrying the ignition current is an antenna. The longer the cable and the larger the loop it forms, the more interference it radiates.
Typically, the battery is far from the engine, and the positive wire runs to the control desk, goes through the ignition key and returns to the engine. That's several meters of thick cable emitting interference with every spark and running very close to the PC and the measurement cables.
The right way to reduce interference is:
- Mount the battery close to the engine, on the panel where the coil and ignition are. That way the power cables are short and the loop is smaller.
- Install an automotive relay on that same panel, so the ignition can still be switched off from the control desk. The power circuit stays with short cables next to the engine, and only a thin wire runs from the desk to trigger the relay. The ignition key can be a low-current one.
The relay needs only 4 wires:
| Terminal | Connection |
|---|---|
30 |
Battery positive (thick wire) |
87 |
To the coil / ignition (thick wire) |
85 and 86 |
One to the ignition key on the control desk and the other to negative (thin wires) |
Don't remove the ignition cut-off
Connecting the battery positive directly to the ignition almost completely removes the interference, but the ignition can no longer be switched off from the desk. Use it only as a test, never as a permanent installation.
Spark plug wires
Always use EMI suppression spark plug wires and avoid copper 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 before they turn into waves. Each copper wire is 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 every time an engine is mounted on the dyno; besides improving the measurement, it helps prevent damage to the acquisition system, the PC or the monitor.
The spark plug
Inside, the spark plug has a center electrode, a ceramic insulator, the metal body and, in many models, a resistor that does the same job as the suppression wire. We recommend using resistor spark plugs: many brands identify them with an R in the part number.
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, or surface flashover along the insulator between the plug body and the cap: these are sparks in open air, outside the cylinder.
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 the spark plugs and the plug caps.
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. If you need a longer distance, use an amplified (active) USB cable or a powered USB hub.
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 water pump or the dyno's starter motor 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.
Switching of motors and valves
Three-phase motors, solenoid valves and other inductive loads produce a current spike when switched on or off. That current, flowing through the cables to the load, can generate electromagnetic interference or even cause problems in the room's electrical wiring. Even an air compressor starting elsewhere in the workshop can affect the measurement.
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.
Chassis dynos and motorcycles
All of this also applies to chassis dynos. In that case the battery and the 12 V wiring belong to the vehicle and there is little to change, but the spark plug wires, the spark plugs, the connection and the grounding can be improved.
On motorcycles, the spark plug wire is usually attached to the coil and cannot be replaced. In that case, use a resistor spark plug or a plug cap with a 5 kΩ resistor.
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.
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 computers a few meters from the engine.
Summary
- Ignition positive through a relay, with the battery close to the engine.
- EMI suppression spark plug wires (never copper).
- Resistor spark plugs and plug caps in good condition.
- Measurement and USB cables away from power cables, crossing them at 90°.
- Ground rod with the dyno, the interface and the PC connected to it.
- Short, high-quality USB cable.
If you still have problems after applying all of this, contact us and we'll review it together.