Firing order: Should the front rotor fire first every time?
 

I think I read somewhere that FDs (or was it rotaries in general?) don't fire the spark plugs (or injectors?) until the eccentric shaft has made a full rotation.

I take this to be because the G signal determines rotor position and it only gets one signal per rotation (from the little metal tab).

I also take this to mean that one should always fire first. Presumably the front rotor, since it's called Rotor #1.

Is this true?

Injection timing and Spark timing interaction with crank angle sensor
 

Short answer (TL;DR): front rotor fires first because it starts injecting fuel first according to the TDC signal. Starting with the first TDC signal (G sensor), every ~180 degrees one rotor injects and the other fires.

Detailed answer:

There are sensor limitations but it's going to depend on the ECU code as well. Let's assume we have the stock ECU. I don't have any kind of reverse engineered code, but I do have the 3rd gen service highlights document (page F-34 through F-43). It specifically says that:

cranking mode = engine speed under 500rpm with start signal ON

ignition timing = fixed 5 leading and 5 trailing.

injection pulsewidth = fixed according to lookup tables of water temp and barometric pressure

Now look at the crank angle sensor, ignition timing, and injection timing diagram:

https://www.rx7club.com/attachment.p...1&d=1461166646

Every time the ECU receives a G signal, it indicates TDC compression on one of the 3 chambers on the rear rotor, and the end of the exhaust stroke on one of the chambers in the front rotor. Halfway between G pulses (180 crank angle degrees), you have TDC compression on the front rotor and the end of the exhaust stroke on the rear rotor.

So the spark timing for the rear rotor is scheduled according to the G signal. The spark timing for the front rotor is scheduled to be halfway between G signals.

The injection timing (start of injection event) for the front rotor is scheduled according to the G signal. The injection timing for the rear rotor is scheduled to be halfway between G signals. The end of injection timing is determined by converting the injection pulsewidth (a time measurement) into crank angle degrees. Longer pulsewidth = later injection end timing in crank angle degrees. Faster engine speed with same pulsewidth = later injection end timing in terms of crank angle degrees

When you start the engine, the ECU doesn't know where the two rotors are positioned. It can't know until it receives a TDC signal for the rear rotor (based on the G signal). At that point it knows the front rotor has finished the exhaust stroke and it can inject some fuel into the front rotor chamber. The fuel injects, which takes a certain number of crank angle degrees, then it has to wait until another reference point (halfway between G signals) to fire the spark, or 360 crank angle degrees later. That's the basic idea.

Put another way:

For injection timing, it starts spraying at the end of the exhaust stroke. It knows the end of the exhaust stroke based on the G signal for the front rotor, or 180 degrees after the G signal for the rear rotor. There may be some offset to the start of injection timing in the stock ECU maps, most likely advancing based on engine speed. The G signal isn't exactly at TDC; it's some number of degrees before, and the software accounts for that.

For the spark timing, it starts firing the plug at TDC compression. It knows TDC compression based on 180 degrees after the G signal for the front rotor, and the G signal itself for the rear rotor. The spark timing is offset from TDC according to the spark timing advance. So 20 degrees spark timing advance means 20 degrees before TDC. The actual physical TDC is calculated to be some fixed number of crank angle degrees after the G signal.

G signal: some degrees before TDC
TDC: some degrees after G signal
Exhaust stroke end: some degrees after G signal, but on the opposite rotor.

So this there something wrong with my wiring if T2 always sparks before T1 when cranking?

Or does it spark as soon as there's a G signal?

What do you mean by T2 fires before T1?
They should be 180 degrees apart. It isn't like watching a Harley motor or something where they fire really close to each other where one is definitely before the other.

Now leading/trailing will fire close enough to each other that one is clearly before the other. Do you have your wiring all crossed up between rotors/leading/trailing?

Previous owner had the coils in the wrong order (Rev. 1-3 order instead of Rev. 4 order) and I want to make sure the harness wasn't modified.

Based on slow motion video I can see that T2 spark plug fires before T1 after I start cranking every time. I'm talking about which plug fires first, not talking about actual combustion.

I suspect this may be normal if the G signal shows the rear rotor's position (unless the intentionally doesn't fire the rear spark plug until fuel is in the system).

I have a PFC by the way.

Oh, you're talking about when first cranking the car to start it the very first trailing spark comes from t2?
I'd never thought about that before, but seems that arghx answered that question then. Interesting info.

I took a slow-mo video showing my CAS and both trailing plugs and I would bet money that my T1 plug is firing every time metal tab passes by the G signal sensor. T2 seems to be firing roughly 180 degrees opposite of the tab.

I'm praying to his noodly appendage that this is the reason my engine isn't running right.

Maybe I should just go and buy a used stock engine harness...

There doesn't seem to be anything wrong with the coil > ignitor harness or the ignitor > harness, so what the hell is going on?

T1 is almost certainly firing when the trigger wheel tab passes the sensor, which seems to be when T2 should be firing.

The trigger wheel can't be put on backwards, right?