I'm not even quite sure where individuals got the 21# rating, unless it was from a site that sells Bosch injectors, and they regularly have the information incorrect on many of them.
Actually, the injector calibration values are created by Ford at approx. 39.15 psi (varies in the 39.x range). They are rated by the manufacturer at 3-bar, which is 43.5 psi. GM rates their injectors at 4-bar (58 psi).
Also, Ford has kept 39.x psi as their used fuel pressure since they've been using fuel injection (all the way back to the SVO and '86+ Mustangs). On the Mustang, Ford switched from return-style to returnless beginning with the '99 model year.
Actually, Ford used the Jetronic/Minitimer connector on ALL EFI Mustang 5.0 and 4.6 2V engines from '86 to '04, and '96-'98 4V. The '99 & '01 Cobra, '03-'04 Mach 1, and '03-'04 Cobra, and ALL '05+ Mustangs, use USCAR connectors. All Lightnings use Jetronic/Minitimer connectors.
To note, many people confuse the "injector body" call tag (EV1, EV6, EV14) with the injector connector. Some new style bodied injectors have old style connectors, and vice versa. See here:
Furthermore, The "pink" 19# injector still uses the Jetronic/Minitimer connector, just as the orange 19# injector did. The injectors are identical, save for part #. See here:
Ford Racing Performance Parts [M-9593-C302*]
That is 100% spot on. The Bernoulli equation is used when resizing injectors for varying flow rates (the flow rate increases with the square root of the pressure ratio). HOWEVER, this also changes the injector dynamics of the injector operation. Instead of attemping to explain it out, I suggest one take the time to read the below "Fuel Injector Article" by my friend Greg Banish (Calibrated Success):
Are all Fuel Injectors Created Equal?
Read the latest article from Greg on the topic. <---click! (PDF Format)
(go to page 3)
You're basically right! See here for the calibration data on both of those injectors by FRPP (Ford Racing, a division of Ford):
Terminator injectors:
http://www.fordracingparts.com/parts/ics/m-9593-m39.pdf
Lightning injectors:
http://www.fordracingparts.com/parts/ics/m-9593-f302.pdf
Take the injector "AHISL" ("high" injector slope) and multiply it by 3600 to get #/hr. So in the case of the Lightning, 0.011947 * 3600 = 43.0092 #/hr, and for the Cobra, 0.011133 * 3600 = 40.0788 #/hr. Now here is where it gets odd - Ford production uses different values than that on the Lightning and Cobra. On the Lightning they use 0.011300 * 3600 = 40.6803 #/hr, and on the Cobra they use 0.011133 * 3600 = 40.0794 #/hr. Personally, I'd trust Ford production data more closely. With all that being said, the similarities between those two injectors stop cold there, as ALL the other calibration data is drastically different between them. Therefore, you could NOT use Cobra injector values on a Lightning, and vice versa, or the vehicle would run like complete crap.
Regards,