Knock Sensors
 

I have a S4/5 hybrid turbo motor, S4 rotors, S5 irons and housings. It does not have a knock sensor, which goes above the trailing plug on each rotor housing. Are there any differences in knock sensors? Engine will be ran by an Adaptronic standalone ECU with knock sensor support. Can i use a piston engine knock sensor? Or if not are fd knock sensors better, or RX8 ones?

You're gonna want to check the documentation for your engine management. Some knock sensors, known as resonant, are "tuned" for a narrow frequency band for a particular engine. Others are a flat response type. Its likely you'd want to use one of these flat response sensors.

I think it is a flat response one, as I have to set parameters for background noise, etc.., also says this in the manual. What type(s) are used on the RX7's?

Knock input - High impedance, filtered input

all knock sensors from 1987-1995 are the same, they just have different connectors. the rate and voltage i don't know, it should be in the FSM in the ECU pinout input perameters.

I don't know if the stock sensors are resonant or flat response.

The frequency of knock is different from engine to engine. Primarily because of the size of the cylinders, or on a rotary, the rotor housing.
Think of it like a bell. A 3" bell is going to ring a a higher frequency than a 4" bell. And a bell the size of a rotor housing is going to be much lower still.
So I guess if the stock sensor is resonant, because it is tuned for that engine, it MAY be good to use.

I'm sure someone with in depth knowledge will at some point join this discussion.

there is a good knock sensor thread in this section, i'd search for Arghx, as the user and knock sensors as the keyword. there was an Rx8 guy who, basically asked the same question, and got the specs from the manuals, and then went and made his engine knock (its easier than you think), and basically confirmed the book findings. he posted MP3's of the noise too

unfortunately the RX7 knock sensors can vary depending on the condition of the gel encapsulating the knock sensor, so some cars show high knock readings even though there really isn't an issue going on.

the RX8 knock sensor isn't gel filled and is more modern, and likely where i would suggest looking.