720cc primaries 550cc secondaries?
 

Is there any reason everyone runs the larger injectors on the secondary position? If they're in the primaries, it'll give you the extra fuel capability throughout the entire rev range instead of just after 3800, and with the rumour that many engines go due to leaning out before the secondaries kick in , it seems like the way to go.

there is really not that much need for larger primaries on a lightly modded car as the boost doesnt come on until the upper RPM range. Thats why most people start with upgrading the secondaries.

If youll look at the cars that are more modded with larger port jobs and what not, they are running bigger injectors all around.

But even if they're not needed, why not put them in the primaries. You dont get any less fuel in the upper RPM range, theres still 2x550s and 2x720s going. Is it just because its easier to tune this way?

"But even if they're not needed, why not put them in the primaries. You dont get any less fuel in the upper RPM range, theres still 2x550s and 2x720s going. Is it just because its easier to tune this way?"

if u put them in the primary position, u will not give the fuel a chance to attomize as well at high rpm's, thus resulting in less horsepower.

it also has to do with the fact that higher cpacity injectors have a harder time injecting at lower volumes. i dont know all the details but i know that this is why mazda uses 2 injectors per rotor instead of just 1. it makes the feul delivery much more predictable and controlable when using staged injectors instead of just using 2 1200CC injectors for the whole motor.

I have the stock 550 primaries and 1600 cc secondaries! I dont' see any need to upgrade to larger primaries! unless you are running like 25 30psi on a larger turbo!

a little bit of a hi-jack, im sorry, but what's the largest injectors you can run via stock ecu (w/ S-AFC) and stock fuel rail?

--Paul

What turbo are you running, and what did your a/f ratio look like at 3500? I'm taking my car to a dyno soon, and I'll probably be hitting ~12psi on the stock turbo before the secondaries kick in. Want to make sure I have the fuel to tune it right.

And I'm not sure if I agree about the fuel atomization problems, does anyone have proof this is a problem (not saying I dont believe you, I'm just skeptical anyones actually measured it)?

Actually that has nothing to do with it. Once the secondaries are firing, all four injectors are fired at the same time with the same pulsewidth, so it makes little difference which is bigger than the other.

But at low load and low rpm when only the primaries are firing, the injector pulsewidth are quite small, especially at idle obviously. A bigger injector would need smaller pulsewidth to flow the same amount of fuel, and injectors (especially big ones) don't work as well at very low pulswidths.

So smaller primary injectors allow better accuracy at low pulsewidths, improving idle quality, driveability, economy, emissions, etc. So you always use your smaller injectors as primaries, and you always upgrase your secondaries first. :)

I have a hybrid t04 turbo! right now, and I can't remember how I had it tunned I sold my microtech and am now going with a haltech! Sorry i can't help you, but by 3500rpm i was running 10psi! but that was with alot more cfm than the stock turbo!