I have determined that the best size injectors for my 88 TII are 550 primary and 720 secondaries....if anyone has any other suggestions, please let me know before i buy them...thanks

 

Depends on the mods, boost, etc.....

 

Thats good for a mild, bolt on mods only engine.Downpipe,intake,mufflers,hybrid,12psi or less........
Im running 800 secondaries on a streetported S5 13BT,3 years now.Runs great,I hit 14psi all the time and it still stays rich.TO4B hybrid,Supra pump,Big FMIC,3" exhaust,stock S5 controls.
Get a decent pump too,like an FD or Supra drop in.

 

im going to be using 550 primaries and 1600 secondaries on my setup.

 

This is entirely based on your setup. 550 primaries should be fine in most cases but if you're using an aftermarket ECU you can just use 1600's and control them well. If you're suing the stock ecu, you should use a piggy back. People don't seem too worried about running a SAFC or anything with bigger secondary injectors but when you think about it, the car doesn't know it's adding that much extra fuel so it's just more or less winging it.

If you're running a S-AFC, I'd say 720's or GSL-SE injectors (if it's a s4) would be the way to go and if you're running a stand alone, just go buy some low impedence bosch 1600's.

 

Running marginally bigger injectors in the secondary(680-800) is fine without a piggyback,provided you use a stable boost controller to match the airflow to the increased fuel output.True,the ECU doesnt know theres more fuel being deliverey(mechanically),but if the airflow into the combustion chamber is increased an equal amount(either by porting or increasing boost) then the mixture will remain correct.The trick is picking the correct size secondaries and the correct boost levels that generate a safe AF mixture,and not overrunning the air flow meter's capacity to measure.Since rotaries prefer a rich mixture,you can go a bit large/rich and itll still pull well and protect the engine from lean out.

No, its not the best way to do it.We all know a standalone is the true,safe approach to setting mixtures.But smog states wont have it,and piggybacks are a pain in the ***,especially with a metered air type EFI which is already calculating airflow into the engine mechanically.Ive never run a piggyback.My setup has run flawlessly for years on 550 and 800 injectors with an Apexi AVCr keeping boost at 14psi.The A/F deliverys are matched mechanically,so the ECU doesnt know its making 300HP instead of 200.The only time it riches out is if I tap into the higher revs(above 5000) and the throttle position is light(no boost),which isnt that often,and has to be deliberate.Normally Id just spool it up or shift, way beforehand.
Before that I ran a stock S4 engine/ECU,stock S5 turbo with 550 and GSL-SE 680 injectors and FD fuel pump.Max boost was about 12psi with the same boost controller.Never missed a beat until the water jacket seals rotted out from a previous overheating.(80K junkyard engine)