Leading and trailing plugs
Each rotor housing on a Wankel has two spark plugs. The leading plug lights the charge, and the trailing plug fires a little later to help burn the mixture completely in the long, moving combustion chamber. The gap between them is the split.
On a Nexus ECU, the Base Trailing Split Angle sets that gap. It only appears when Engine Type is set to Rotary. The value is in degrees of retard from the leading timing, so a bigger number means the trailing plug fires later.
A worked example
Many rotaries have factory timing marks for a leading spark 5 degrees after TDC and a trailing spark 20 degrees after TDC. In Haltech terms that's a leading timing of -5 degrees (negative means after TDC), and the trailing plug sits 15 degrees later. So you'd enter a split of 15, meaning 15 degrees of retard from the leading plug. When you check timing with a light, the leading mark should line up with the leading value and the trailing mark should show the split.
Reading and editing the table
The table is laid out like the main ignition table: load across the horizontal (X) axis, from MAP or TPS depending on your ignition load setup, and RPM on the vertical (Y) axis. Values are in degrees, with positive numbers before TDC and negative numbers after TDC.
To change the axis break points, select the table and press F3. You can edit an existing value or add a new one. When you move an existing break point, NSP adjusts the cell values so the overall shape of the table stays the same, so rescaling won't scramble your tune.
Start from the factory split or a proven base map, confirm it with a timing light, and make changes in small steps while watching logs. Split changes affect exhaust temperature and how completely the charge burns, so don't make big jumps on a loaded engine.
Related reading: the base ignition table, cranking ignition, ignition tuning overview and our 10 things to know about rotary tuning.
