Fuel injectors evolved over time, and as you might suspect, their spray patterns and level of atomization have improved with each generation. Here's what I'm talking about: http://www.youtube.com/watch?v=oxImoYMRUCA&feature=related
Guys who have played around with Miata injectors already know this; '99+ Miatas have a different shape injector body, and many have done that upgrade.
Injectors also come in different flow rates.
I recently upgraded mine and my dad's Escort's injectors to a set of EV6 units as used on the Mustang V6 and Windstar from about a decade ago. Same 19-lb/hr flow rate, newer design. We measured with a scan tool and confirmed long-term about a 5% increase in fuel economy. The electrical connectors were more or less compatible -- we had to break some plastic tabs and zip-tie the connectors down so they wouldn't vibrate out, but it worked. Some BMW E30 and Mustang owners seem to do this upgrade as well.
I recently found a crack in one of my injectors, so I figured now's the time to upgrade. I've done a lot of poking around and searching; unfortunately, the cheap and easy to find Ford injectors may not work. Miata injectors use that plastic spacer to seal the injector agains the intake manifold. Most other injectors use an O-ring. Otherwise, I think the length is the same.
So! Mazda injector geniuses! I want EV6 or newer injectors that will fit my 1.6L Miata. But not just fit: I want the same flow rate, too. I'm looking for a drop-in, no-hassle swap. No AFM tweaking, no Megasquirt, no adjustable FPR. This means injectors from a '99+ Miata won't work.
Does anyone have ideas? I think I found Ford SUV injectors with the right flow rate, but again, that plastic-spacer-versus-O-ring thing leaves me uncertain.