Injector size vs ENGLISH bhp

[Adam W]

I know there are a few sites on the web which tell you how to this, but they're all written in american

 

Is there a well respected formula for going from cc/min to max bhp, without going to nasty duty cycles or silly base fuel pressure?

[THOR Racing]

Hope this helps Adam, doesn't appear to be too difficult?]

....... Pete

 

Select

duty cycle = 80%

BSFC = 0.55

# of injectors = 6 (obviously)

 

 

QUOTE: "

The following formula is used to properly determine injector size:

 

Injector Size = (Engine HP (Flywheel)) x (BSFC) / (# of injectors) x (Duty Cycle)

 

[Where / means divided by and x means multiplied by.]

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

Race engines with very efficient cylinder heads: 0.40 - 0.45

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) / (8 injectors) x (0.9)= 27.7 lb./hr

 

600 HP Supercharged engine - Number of injectors = 8

Injector size = (600 HP) x (0.57 BSFC) / (8 injectors) x (0.9) = 47.5 lb./hr

 

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

14 lb./hr

19 lb./hr

24 lb./hr

30 lb./hr

36 lb./hr

42 lb./hr

50 lb./hr

Horsepower Range

up to 225

225-300

300-385

385-480

480-575

575-670

670-800

 

The following chart provides maximum horsepower levels based on injector size and various BSFC values. Note that this is at 100% duty cycle and 43.5 psi; raising the fuel pressure will increase the maximum horsepower. See the text below for fuel pressure and injector flow calculations.

 

 

Fuel Pressure and Injector Flow

The pressure at which an EFI system runs affects the flow of an injector. Most fuel injectors are rated at 43.5 PSI. Increasing the pressure above this increases the flow and lowering it decreases the flow. If an injector does not flow enough at 43.5 PSI, the fuel pressure can be raised to increase the flow rather than having to change to a larger injector. It is best not to raise the fuel pressure much above 60 PSI. Certain injectors will not open properly at higher pressures. Fuel pump flow also decreases at higher pressures, so make sure your fuel pump flows enough if the pressure is raised. Remember that on a supercharged or turbo-charged engine the fuel pressure will be raised above the base fuel pressure by the extra boost that is produced. In other words if the base fuel pressure is 43.5 PSI and you have 10 PSI of boost, the total fuel pressure will be 53.5 PSI."

[Adam W]

Thanks Pete, much appreciated!