The system MUST have a brake/roller RPM sensor, but it is completely ok to not have an engine RPM measurement. In this case you use the brake RPM sensor + gear ratio to know the Engine RPM. Gear ratio can either be entered directly if you know it, or you can use the engine’s tacho and hold the RPM at a set point (for example 4000RPM) then press a button. YourDyno will calculate the gear ratio.
Alternatively you can add a direct Engine RPM reading in one of the following ways:
Spark plug wire/coil wire signal pickup
If the engine you test do not have OBD2 support (see below), then you can use a signal pickup.
The MSD 8918 Tach Signal Pickup is suitable for picking up the RPM signal from the coil primary wire. It senses current passing through a cable, so it works on the primary where the current is high and the voltage is low (this is opposite for the secondary that goes to the spark plug).
It needs 12V supply. Its output is also 0-12V so you need an interface between the MSD and YourDyno. If you try to connect the output directly to YourDyno then something will smoke, so don’t do that. Here is the connection diagram (two alternatives):
The 12V power supply (or battery) shall not be connected to YourDyno at all.
DTec also sells a suitable RPM adaptor. It can pick up the signal from the spark plug wires, from the coil or a VR sensor. There have been some reports of questionable quality and the MSD is recommended instead. But if you choose to go with the DTec, here is how to connect it:
DO NOT ADD A PULLUP ON THE “SIG” SIGNAL like the documentation says. YourDyno has an internal pullup. If you attempt to add a pullup to V+ you may damage the YourDyno box. Also, use a separate 12V power supply to this box, not connected to YourDyno at all. The only connection between YourDyno and the RPM box shall be the two signals as in the figure above.
You need to program “Pulses per revolution” to either 1 or 0.5, depending on your engine (1 for 2 stroke or 4 stroke with wasted spark, 0.5 otherwise).
RPM signal from ECU or coil
If you go this route, make sure to let the signal from the ECU or the coil trigger an optocoupler. No pullup is necessary, this is included inside the YourDyno box.
RPM reading from OBD2 is supported via Bluetooth or USB using an ELM327 unit. These units are very low cost and works well with YourDyno. They can also be used if both RPM inputs are used on the YourDyno box. WiFi version is not supported.
OBD2 is supported also via a plugin called ScanTool. This plugin supports more OBD2 units than the built-in OBD2 support does. If you use the ScanTool plugin, make sure to disable the OBD2 in Options->OBD2.