Posted for "Ed Anderson" <eanderson@carolina.rr.com>:

MessageTommy, I fly with the stock 440 cc/min NA Mazda Injectors.  I have had fuel flow of over 20 GPH on take off on a cold <32F morning indicating that power was around 200HP.  So the stock 440 cc/min will support any NA engine just fine.  I've tried the 550 cc/min and just found that they provided more fuel capacity than I needed and at least in my installation, did not idle as smoothly.    Now if you are going to turbocharge, then the 550 cc/min may be needed depending on how much boost you are going to run.  Also, the physical size of the 440 and 550 are different.  The 550s are a bit shorter and fatter although the nozzel sizes are the same dimension (can plug into the same size hole).  I had to use a smaller spacer (that came off my 91 fuel system) to space the 550 cc/min injector properly for the block injector holes on my 91 block.  If I am not mistaken, the 550 cc/min (at least on the 91 turbo II block) are high impediance injectors (approx 12-16 ohms) and the electrical connectors are different than the 440 cc/min injectors.  The high impedance injector connector have a engagment key off set from center on the connector socket.  The 440 injectors are low impedance ( approx 2 -3 ohms) and their injector connector has the key centered on the connector socket.  Obvioulsy to keep your from accidently plugging the incompatible impedance injector for the Mazda EFI the engine might have.  You can use either one with Tracy's EC2, however, if you use the low impediance 440 cc/min injectors  then you must follow his instructions on adding some 25 watt resistors in series with the injectors.  If you are using the 550 cc/min High Impedance injectors then you do not need the resistors.

FWIW

Ed Anderson