Rotary Injection Timing Calculator

When calculating the injection timing for a rotary engine, additional care needs to be taken as the injectors have a significatly smaller proportion of time with the port closed.

A sequentially injected rotary engine fires an injector for a rotor every 360 crank/ESS degrees, rather than every 720 like a piston engine. This means as the injector duty cycle increases, it is much more sensitive to injection advance. If attempting to perform closed-port injection on a rotary engine, the chance of the injection event overflowing to the previous stroke's intake event is much more likely when using end-of-injection timing advance than a piston engine.

Example piston engine timing:

  • Intake open: 19° BTDC
  • Intake close: 59° ABDC
  • Duration: 258°
  • 258/720 = 36% of cycle open, 64% cycle closed

Example rotary engine timing:

  • Intake open: 45° ATDC
  • Intake close: 50° ABDC
  • Duration: 185°
  • 185/360 = 51% of cycle open, 49% cycle closed

RX-8 13B-MSP rotary engine timing:

  • Intake open: 3° ATDC
  • Intake close: 65° ABDC
  • Duration: 242°
  • 242/360 = 67% of cycle open, 33% cycle closed

Unlike the usual 80% duty cycle maximum recommendation for piston engines, for rotaries it's a good idea to keep the maximum duration much shorter, such as 60%.

Use the calculator below to plot injection advance against DC, to show what stages of the intake cycle the injector is starting and finishing at.

Generally, "top dead center" or "TDC" refers to the piston being at the top of the compression stroke, ready to start the power stroke. Typically the spark plug is fired around or before this point, with ignition advance set usually between 5-40 degrees before TDC.

When discussing camshaft and port timing however, TDC usually refers to the beginning of the intake stroke. That is adjustment is simple to understand and visualise due to the operation of a piston engine, but for a rotary engine it is a bit more difficult.

Due to the rather confusing operation of the rotary, where two stoke events are happening at once at all times, it can be difficult to understand Mazda's provided specifications, as they are 180 eccentric shaft degrees away from where the spark plug fires. This means the rotor is in a different position as compared to the combustion stroke TDC, unlike a piston engine where the piston is in the same place for both.


Conventional TDC

Mazda Port Timing TDC

Ignition BTDC reference
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By @8.touge