Should I use 850s or 550s?
 

I'm debating on if I should use my stock 550s in my primary block or if I should use a second pair of 850s I have, and go with 4-850s. Here are my current, and up coming upgrades to my car:

Engine has rebuild with approx. 2k miles
Downpipe
Catback exhaust (Stock Cat)
Stock mount NPR Intercooler (custom piping)
Non-Seq Conversion
Boost Controller (will be running 12psi)
Custom Cold air intake
Complete block off plate kit
Apexi PFC

Please help me decide. 2-850s/2-550s or 4-850s. Thanks all.

if you have a PFC i would go with the bigger injectors.

They wouldnt' be that hard to control and keep you from running lean before the secondaries come online.

It would also provide a little more room.

if you can get the 850s to seal right go with them no question.

(sometimes they have problems sealing)

Unless you are planning to run over 13 psi on a regular basis, there is no point to running larger injectors. The 850cc primaries are harder to tune for than larger secondary injectors and are harder to pass emissions with, if you have to worry about it.

That said, if you get it working correctly, it will give you room to run 14-15 psi with lower duty cycles, for a lot less than upgrading the secondaries.

i hated my 850cc primaries...

so bigger secondary is better than small primaries ??

IMO the way the ECU uses the "staged" injector setup, yeah...

93BlackFD,
Why did you hate your 850's? Im about to install mine and have mr. Kan tune it up on the PFC.

in general, having a bigger secondary is better due to the fact that a larger injector is harder to control (pulse width due to flow).

Not 100percent sure on how the injector is setup in FD's but in FC they came online about 3500rpm and would assumes it's rpm based in the FD as everthing is "premapped on taht damn ecu :).

If your mak'n alot of power before that u can run lean.

Too big and it will bog when they first come online.

12A's had 680cc stock ;) -now that was overkill

It is nearly impossible to max out the primary injectors running the stock twins, unless you are tuned way too rich. That is why the larger secondary scheme works better.