Injector sizing and boost-referencing the fuel injected engine for centrifugal supercharging.

Injector sizing and boost-referencing the fuel injected engine for centrifugal supercharging.

By Sam Logan: If larger injectors are installed—that is, injectors with a greater flow rate—an engine tuning shop is usually required to re-tune the map—the fuel curve—to achieve the correct air-fuel ratio. To acquire a safer tune in a supercharged engine, where cylinder heat increases with boost, tuning shops will often adjust the air-fuel ratio to 12.0:1 at full throttle as opposed to around 12.8:1 on a naturally aspirated engine. In cruising conditions, air-fuel ratios will remain at 14.7:1 The size of the injector limits the amount of horsepower generated, and calculations are used to decide the appropriate injector for desired maximum rpm and horsepower. They are calculated in pounds-per-hour of fuel dispersed. For example, 25lbs-per-hr is sufficient to support 350hp; 45lbs-per-hr is sufficient for 500hp and so on. Here is a table with more particulars: FLOW RATE DEFINITION, CALCULATING HORSEPOWER RANGE. Horsepower is measured at crankshaft. Injector Flow RateNaturally Aspirated EngineEngine w/ Power Adder19 lbs./hr.225-290 hp185-240 hp24 lbs./hr.280-360 hp240-300 hp30 lbs./hr.350-450 hp300-375 hp36 lbs./hr.425-540 hp350-450 hp42 lbs./hr.500-625 hp410-525 hp46 lbs./hr.540-690 hp450-575 hp52 lbs./hr.610-775 hp510-650 hp60 lbs./hr710-900 hp590-750 hp Supporting a supercharged engine with sufficient fuel is a moving target—there are numerous factors to consider, even the model year of the same engine and vehicle brand. The route is determined by the amount of supercharge boost and the capacity of the fuel pump along with the flow capability of the fuel injectors. Avoid confusion: Injector flow capability is measured in lbs/hr—not psi. Supercharging with boost referencing or booster pump or larger fuel injectors: The reason for introducing more fuel pressure is as follows: for every pound per square inch...