SanJoRX7

So, I'm looking around at some stuff and I keep seeing that people have upgraded their secondary injectors to 1200, 1300, and even 1600cc units. Now, I understand that when the injector duty cycle gets to be ~85+%, you're looking at the possibility of fuel starvation, and thus detonation. That concept does not escape me.

What I want to know is why only the secondaries? Also, people tend to refer to injector duty cycle in a % that seems to indicate that it's an aggregate of primary and secondary duty cycle numbers. Is this true? Why not get 1200cc primaries too?

Please to explain

P'cola FD

With bigger primaries you would have to seriously lean out the idle mixture and, from what i've heard, it makes the car run very poorly. If the secondaries do the trick then why bother with the primaries.

jspecracer7

From what I understand of injector duties(which is probably not too much, The primaries fire at idle, and up to 40% injector duty when the secondaries start firing. The problem with upgrading the primaries is that there is such a thing as too big a primary injector for idling purposes.

I've seen people use 850 primary and 1600 secondary and complain about how much richer their idle is.

TYSON

SanJORX7, I put 850 cc injectors in place of my primaries last night. I see in your mods list that you have a PFC. Mine fired right up and idled solid after I adjusted the PFC using the info in the manual. Another concern people seem to have is that because the injectors are actually shorter, they may not seal properly and allow fuel to leak into the engine. I will start my car when I get home and see how she runs. Before I did this, my car idled at 4.4% injector duty when it was at operating temperature. Now it idles at 3.8% injector duty.

maxcooper

It is cheaper to just upgrade one set of injectors if you can get enough flow from such a setup. A rich idle seems to be the other problem with big primaries, but I have heard a lot of reports that it works fine (and also many that it doesn't). Perhaps the better atomization of smaller primary injectors is also a factor in the idle situation.

Another option to increase the effective size of your injectors is to run higher fuel pressure. I understand that this may also improve the atomization when using large injectors.

After the secondaries come on, the primaries and secondaries are running at the same duty cycle (at least with the stock ECU and the PowerFC), so a single number is sufficient to describe what duty cycle you are running.

-Max

TYSON

BTW it is a lot easier to get at the secondaries. To remove the primaries you have to remove two big machine screws, (stripped them both, of course, why aren't they bolts?) and get past the solenoid rack.

Max, is there an easy way to increase the fuel pressure? This looks like more work.