This is a draft. Please review and offer constructive comments/corrections.

SIZING FUEL INJECTORS (Normally Aspirated Applications)

If you want the Reader’s Digest version of this writeup, read the next sentence, and exit:

Multiply crankshaft HP by 0.070 to get the required injector size.

For a more rigorous analysis:

There are several sites on the I’net that help you size injectors. All of these sites use the following equation:

(HP x BSFC) / (# CYL x DUTY CYCLE) = LB/HR injector required

In this equation:
• HP = Flywheel Horsepower
• BSFC = Brake Specific Fuel Consumption, expressed in LB/HR/HP
• # CYL = Number of cylinders (injectors) in the engine
• DUTY CYCLE = Duty Cycle. This is a measure of % of the time in a 4-stroke cycle that the injector is open. An injector that is open ½ of the time has a 50% duty cycle. An injector that stays open continuously is at 100% duty cycle.

Here is a typical website, and a very good one:

http://www.rceng.com/technical.aspx


An example of the calculation:
Given:
• Flywheel HP is 500 HP, normally aspirated
• It’s an average engine, with an average tune, nothing special, so to play it safe, we assume a BSFC of 0.50 LB/HR/HP
• It’s an 8 cylinder engine, with one injector per cylinder
• We want to limit the duty cycle to 80% (0.80). All injectors can handle this level of operation. Some will start to have problems achieving rated flow, or will experience degradation of the spray pattern if run at greater than 80% DC. Others will operate satisfactorily at 90% or higher.

(500 x 0.50) / (8 x 0.80) = (250) / (6.4) = 39.06 LB/HR

To be completely safe, in virtually any normally aspirated application, this is the injector size to use. But this is a VERY conservative basis, which usually results in an injector size that is larger than you really need.

A well-designed combustion chamber, with a good tune, in a purposely built high performance engine will usually have a BSFC lower than 0.50 LB/HR/HP. In the world we live in, the value will typically run in the range of 0.45 – 0.50. The lower this number is, the more efficient your engine is operating, and the less fuel you will need to achieve any given HP level. If you are designing a NASCAR engine, operating in an extremely narrow power band, you can do even better (lower) than this. But we aren’t designing a NASCAR engine. We’re part-time racers, trying to have a little fun.

As you can see from the formula, the required injector size is highly dependent on the limitation we put on duty cycle. 80% works…. But in most cases, so does 85%.
So let’s look at a less conservative basis for sizing injectors. Let’s assume we have a BFSC right in the middle of the range 0.45 – 0.50. Let’s assume we can go to 85% DC. When you do this, you can arrive at a “rule of thumb” for selecting the minimum required injector size. And that is (for normally aspirated engines ONLY):

Multiply crankshaft HP by 0.070 to get the required injector size.

Example:

500 crank HP x 0.070 = 35.0 LB/HR

That’s about as simple as it can get, and it works. If you want to use the original equation, use a BSFC of 0.476 #/HR/HP and a DC of 0.85 (85%). I've downloaded the injector sizing program from MSD. It gives results very similar to the above:

http://www.msdfuelinjection.com/MSDInjectorV1/main.html

Many people try and size the injectors based on rear wheel HP. This does not work as well. Yes, you can assume a drivetrain loss, and back-calculate flywheel HP from rear wheel HP, but that is just introducing another “unknown”.

Other usefull links:

Injector flow rates by part number:

http://users.erols.com/srweiss/tableifc.htm

http://www.juscuzmotorsports.com/injector.html

Injector Tech:

http://www.sdsefi.com/injectors.htm

http://www.witchhunter.com/variousfuelinjectors.php4