[Shane@DBPerformance]

Take the injector size the factory ECU is set for, divide that by the new injector size and then subtract 1 from it. That will give a general starting point for you.

(360cc/550cc) - 1 = -34%

You have to be extremely careful on the 3s cars though with bigger than stock fuel injector and a piggyback systems that try to fool the stock ECU like an SAFC. The 3s cars run way too much timing in the upper RPMs, so you can run in trouble if you try to crank the boost up a lot or if you put in bigger fuel injectors, since going negative with a system like an SAFC has the unwanted side effect of making cars run even higher ignition timing. I wouldn't run 550s on a 3s without a system that can control/retard ignition timing like an AEM EMS or MAFT Pro. Since your car is 1991, you can retard the timing using the CAS, but that will make the timing too low in the lower boost RPMs and at cruising RPMs hurting power and gas mileage.