I see a lot of people with 550s in the primaries and 1000s in the secondaries. What if you did 720s in both primaries and secondaries? How is that different?

Secondaries come on at 3800, so are the 550s able to supply enough fuel before then?

 

What kind of engine management you running or plan to run? You may flood with 720s in the primary. A piggy back like an SAFC cannot control the injectors during cranking. With 1000cc secondaries you'll have enough fuel unless you are going with a pretty crazy setup. I run 550 primaries and 1000cc secondaries no problems, but I have a standalone.

 

If you're going with a standalone, I'd say go with the smaller primaries & bigger secondaries. It's a bit more work to tune, but you gain control at low power settings (and fuel flows) where it matters - this is why primary & secondary injectors are used, instead of, say, 2x1600s. Getting two big injectors to idle would be difficult at best.

The issue is how short of a time you can cycle the injectors for. Most ECUs have a minimum cycle time (1ms or so), and if you can't get little enough fuel to idle with that cycle time, you have to raise your idle. Also, injectors take a finite amount of time to snap open and snap closed. Fuel flow during these times is not easily predictable. At extremely low duty cycles, these half-open periods dominate the fuel delivery, and it makes it really hard to tune (you're tuning for n+c opening time, where c is unknown but large compared to n).

That's just my opinion.

-=Russ=-

 

I'm planning on 4x720. I'm at 26ms at about 10% on the megasquirt at idle with the 460's, so it will be a snap to change it to 460/720*26 and totally correct the entire map for the new injectors. (Megasquirt y0)