From
Holley Tech Articles:
TECH ARTICLE # 7 - Proper Injector Selection
Selecting proper injector size if often misunderstood. There is no magic to proper selection. If an injector is too small, not enough fuel will be available when tuning an engine and damage can result. If an injector is selected that is much larger than is needed, the injector pulse width (time the injector is open) at idle may be too low and tuning problems at idle may occur.
The following formula is used to properly determine injector size:
Injector Size = (Engine HP (Flywheel)) x (BSFC) / (# of injectors) x (Duty Cycle)
Injector Size = Flow rate in lbs/hr
Engine HP = Maximum engine horsepower at the flywheel
BSFC = Brake Specific Fuel Consumption (BSFC) is the lbs. of fuel an engine consumes per HP per hour. It is simply a measure of how efficiently an engine is at converting fuel to horsepower. It is very important to use a BSFC number that is close to your actual number. If it is not, the injector will be too small or larger than is necessary.
The following are some general guidelines when choosing a BSFC number:
- Low to medium performance street engines: 0.50
- Performance engines with good cylinder heads: 0.45, 0.40
- Race engines with very efficient cylinder heads: 0.45, 0.55
- Supercharged and Turbo-charged engines: 0.55, 0.60
Supercharged and Turbo-charged engines run at richer air/fuel ratios that raise the BSFC number. They require larger injectors for the same horsepower as a naturally aspirated engine.
Add 0.05 for marine applications, as they need to run richer than a comparable automotive application due to continuous wide open throttle use.
Duty Cycle
The duty cycle is the maximum amount of time you want the injectors to be open at a certain horsepower and injector size. Under most circumstances you don't want an injector to be open more than 90% of the time at the most. Marine applications shouldn't exceed 80%. Injectors are rated at 100% duty cycle (static flow).
When calculating injector size, round up to the next nearest size needed. For example if you calculate 26 lb/hr and have a 24 lb/hr and a 30 lb/hr to choose from, select the 30 lb/hr injector.
Examples
400 HP street engine
Number of injectors = 8
Injector size = (400 HP) x (0.5 BSFC) = 27.7 lb/hr / (8 injectors) x (0.9)
600 HP Supercharged engine
Number of injectors = 8
Injector size = (600 HP) x (0.57 BSFC) = 47.5 lb/hr / (8 injectors) x (0.9)
Injector HP Chart for Typical Performance Engines
The following chart indicates recommended injector size for typical naturally aspirated V8 performance applications. A BSFC of 0.45 and duty cycle of 90% is used for the following recommendations. Use the formula above to calculate your injector size if a different BSFC and duty cycle is required.
Injector Size Horsepower Range
... 14 lb/hr ... up to 225
... 19 lb/hr ... 225-300
... 24 lb/hr ... 300-385
... 30 lb/hr ... 385-480
... 36 lb/hr ... 480-575
... 42 lb/hr ... 575-670
... 50 lb/hr ... 670-800
steve
