understanding trigger inputs crank vs cam sensors

From the crank sensor we can get engine RPM and as well as engine TDC but we won't know exactly which piston is firing so for the engine to run we have to use waste spark system . my question is can the engine run and use sequential injection system with the trigger input at the camshaft only ? since we know where exactly the piston is and calculate the engine speed by doubling the cam rotation ? I am pretty new to this trying to work out the best place to put the trigger wheel on my project car 1994 w124 Mercedes m111940 engine. trying to replace stock ecu with aftermarket ecu.

It's best to pick the ECU, then see what it's trigger requirements are. Knowing what the existing triggers for the stock engine can help you pick an ECU that is easy to connect to the factory triggers.

To answer your question -- yes, you could put a trigger on just the camshaft, (often two triggers on one wheel) however that has the issue of being very low resolution typically, unless you are using some type of optical trigger wheel. What was the stock Mercedes trigger setup -- did it run sequential?

Below is the info I got on wikipedia about my engine.

Its a speed density system using 2 ignition coils and no ignition distributor. Cylinders are fired in pairs (dual fire) - 1 and 4 together, and 2 and 3 together. The crankshaft position sensor is sensing the movement of two radially opposed position plates on the flywheel, one of which is magnetized, and the other is not.

So it means that it uses crank sensor and wasted spark system not sequential. as for the ECU I need advise on a cheap entry level ecu that can control both ignition and fuel for a 4 cylinder engine. input required are MAP + TPS + IAT + CLT + lambda sensor . I am not planning to install turbo. I just want to replace stock ecu (PMS) since its a pain getting the original ecu.

I would trust the factory workshop manual over the wikipedia entry for a 23 year old car. I would probably put an oscilloscope on the crank trigger see what the pulses look like.

The description does seem to describe a wasted spark scenario without a CAM sensor -- do you think you could add a CAM trigger easily enough to make is run sequential?