SD questions

[Unit 5302]

The reason the FPR is absolutely necessary with SD is because the computer cannot measure fuel or air entering the engine by volume. It has a finite table of adjustments that it can make, and once your car is running outside that 20% or so, it can no longer compensate for the modifications, and you develop problems.

All the mods you're making increase the amount of air flowing through the engine, but the computer doesn't know anything about that. All the computer knows is air density, rpm, throttle position, and engine temp. Based on that, it says "okay, I think I should be injecting fuel for about this much time" When you add all the mods, it injects what it thinks is the appropriate amount of fuel, but it's reading knock sensors and oxygen sensors that are screaming lean condition at it. By increasing the fuel pressure, you trick the computer into injecting more fuel than what it thinks it's injecting, and things work out.