Most engines are fuel injected these days and modified ones often need larger fuel injectors if the power output has been increased substantially. Going too large on the injector size causes its own problems though because the pulse duration needed at idle and cruise will become very short and sensitive to exact calibration. In an ideal world we want the injectors to run at about 80% to 90% duty cycle so they have a little spare capacity in hand but are no bigger than necessary.
Duty cycle is the percentage of time the injectors spend open. Obviously at 100% duty cycle the injectors never close which is clearly undesirable.
Injectors are rated by their fuel flow in cc/minute at a specific fuel pressure. This is usually 3 bar (43.5 psi) but sometimes you see charts showing flow at different test pressures. You need to be able to standardise flows given at different test pressures to be able to properly compare injectors. Flow is proportional to the square root of the test pressure. In other words as pressure doubles flow only increases by root 2 or 1.41. So to translate flow at one pressure into flow at another we need to do the following.
New flow = tested flow x square root (target pressure / test pressure)
For example. We have a rating of 200 cc/min at 3.5 bar. What would the flow be at 3 bar?
Flow = 200 x square root (3 / 3.5) = 185 cc/min
1 bar is 100 kPa or 14.5 psi which is a little lower than atmospheric pressure (14.7 psi).
The next step is to understand how fuel flow is related to bhp potential but this isn't straightforward and no single formula will apply to every engine despite what you may read on the internet. The amount of fuel an engine needs to develop a given amount of horsepower varies with many things.
1) Mechanical efficiency. Frictional losses in the engine reduce the power developed in the cylinders to that shown at the crankshaft in dyno testing. The lower these losses the more flywheel power you'll get for a given amount of fuel. Dry sumped engines for example will gain flywheel bhp for no extra fuel due to reduced oil drag losses round the crankshaft. Piston ring friction, crank bearing losses etc all contribute to this mechanical efficiency.
2) Thermal efficiency. High compression ratios develop more power from a given amount of fuel but increase the susceptibility to detonation. Thermal barrier coating of piston crowns, valves, combustion chambers etc can reduce internal heat losses. Engines with well designed combustion chamber shapes need less ignition advance and produce more power from a given amount of fuel/air charge.
3) Fuel/air ratio required. Most normally aspirated engines need very similar F/A ratios for best power but heavily turbocharged ones often use richer mixtures to help cooling and suppress detonation.
A good guide to estimate the bhp potential for a normally aspirated engine is to proceed as follows.
a) Calculate the injector flow in cc/min at the fuel pressure actually being used with the equation above.
b) Multiply the flow of each injector by the total number of injectors and divide the result by 6. This will give you a safe working bhp allowance at a duty cycle of about 85%.
Example. Injector flow for a 4 cylinder engine is 185 cc/min. BHP potential at 85% duty cycle is 185 x 4 / 6 = 123 bhp.
Alternatively to work out the required injector size from the target horsepower, multiply bhp by 6 and divide by the number of injectors.
Example. Target bhp is 200 for a 4 cylinder engine. Required minimum injector size is 200 x 6 / 4 = 300 cc/min.
For forced induction engines add 20% to the above figure to give yourself some leeway for rich mixture settings.
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