The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the Air Fuel Ratio (AFR) is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.
You do not need to bother with increasing fuel pressure when running engine management. Since you are in full control, you simply need to add larger injectors (which you can now control) and utilize the stock fuel pressure regulator. The money saved on the FMU is money spent on injectors that can support your goals.
SO…when you add boost to your Miata you typically need extra fuel flow through your injectors. Why? Because injectors are typically rated in cc/min at 43.5psi of fuel. Which can be converted into how much HP each can handle. By raising the fuel pressure in the rail, you effectively increase the HP capabilities of your injectors. In a nutshell, raising the fuel pressure raises the cc's of an injector! Get it? Section 3
An APFR or FMU are ideal alternatives to expensive ECU modifications. This is accomplished by using the manifold vacuum/pressure to drive the fuel pressure up as the boost rises.
It is strongly recommended that you use an aftermarket engine management system, such as MegaSquirt (most common and inexpensive), AEM EMS, Haltech, MoTeC, etc. Any other "band-aid" mod will get ridicule on the boards. If you can't afford to tune your car properly, you can't afford to turbo your car.
To determine the amount of fuel you can supply with your AFPR or FMU follow this calculation:
Fuel pressure @ rail = Rise x boost + idle pressure
Example: 10 x 5 + 50 = 100psi
So with a ratio of 10:1 (10psi of fuel for every 1psi of boost) you can supply 100psi of fuel to your injectors when boosting 5psi. If you don’t plan on adjusting your boost levels often the simple static disc FMU unit can be had for cheaper than an adjustable unit. However, with the added benefit of being able to easily supply your injectors with the right amount of fuel by the turn of screw adjustments they are worth the extra cost.
Larger injectors, Intercooler, etc., can change the desired fuel levels. Hoever, most do not recommend over 100-110psi of fuel at the rail in boost. *At this point it's time to think about larger injectors that can handle higher HP ratings at 100psi of fuel in the rail. The stock ecu can handle a -+ 20-30% injector. So the fueling with the stock ecu can only go so far. If you feel you're going to breach this point, then EMS should be considered.
*SEE THE SECTION ON FUEL INJECTORS
Hard Line -> Front of Fuel Rail -> OEM FPR -> OFFSET Fitting of FMU -> CENTER fitting of FMU -> Return Line
BEGI Fuel Management Unit
The BEGi standard rising-rate regulators are designed to increase fuel pressure and fuel flow in a fuel injection system by augmenting the factory fuel pressure regulator. These regulators must be used in conjunction with a factory regulator, which provides the base fuel pressure (the fuel pressure at fuel rail during idle conditions). The amount of fuel pressure rise can be adjusted by adjusting a screw on the side of the unit.
The Vortech FMU (standard on the GReddy kit) works in the same principal however the amount of fuel pressure rise is determined by a disc and cannot be changed on the fly. Recalibration kits can be bought for cheap however they require the disassembly of the unit to change the ratio.