There appears to be some confusion amongst consumers as to the benefits and time to fit larger injectors. Many times, I hear of customers dictating to the tuner that larger injectors are needed or supplies larger injectors to be fitted for tuning seemingly under the impression a larger injector will make more power.  It poses the question. How big an injector should you fit? 

What are the ultimate plans for your engine? No one wants to buy things twice, although those of us in the industry see that often.  This is the time to be realistic, buy once and save. If you have a HP goal in mind, first make sure it is achievable in your budget, a finished 600hp engine in your car is a lot more fun than an unfinished 1000hp engine on a stand.  Be aware of the fuel you are going to run, E85 requires roughly 30% more fuel flow than Petrol under the same engine conditions.

What determines how much power an engine makes?  The main factor is airflow, the more airflow the greater the Horsepower. Fuel should be delivered into the engine in a controlled fashion relative to the air flow, the fuel flow required is a ratio of the air flow. The maximum fuel required is a ratio of the maximum airflow achieved.  The metered air fuel ratio can vary depending on the application typically from 11.7 to 14.7 to 1 for Petrol, this is the equivalent of Lambda .79 to 1.  If we take the example of 14.7:1 or Lambda 1, this means for every 1 gram of fuel there is 14.7 grams of air, at the richer end of the scale at 11.7:1 or Lambda 0.79 there is only 11.7 grams of air for every gram of fuel.

There are multiple calculators online that help with converting fuel flow to Horsepower, these should be used as they provide a good guide as to the correct injector choice based on the Horsepower your engine can make.  These calculators use proven theory and formula to estimate an engines airflow primarily based on its size, the maximum RPM and induction pressure, is it boosted and by how much. The larger the engine, the harder it revs and the more boost it has the more fuel it will require.  From the airflow a fuel flow can be calculated, from the maximum fuel flow the correct injector can be sized based on the number of cylinders (injectors). 

If you have a stock engine that is not going to rev substantially higher there will be no need for larger injectors, in most cases where a high flow exhaust and intake is fitted with high flow catalytic converters the tuner will tune the car leaner than stock as this will make more power, the manufacturer tunes richer so as to not damage the catalytic converter and other components such as 02 sensors.  The aftermarket performance tune rarely takes this into consideration which may or may not cause an issue depending on the use if the vehicle the exhaust heat generated adn the length of time the heat is generated. It is not uncommon to have less injector duty (peak open time), on a tuned car than a factory tune as the mixtures under full load will be considerably leaner.  Larger injectors at this point will be of no advantage but may in fact cause issues with idle and economy. 

An injector has a range that it can operate in, there is a minimum pulse width (open time) the injector can achieve below which the fuel delivery becomes to unstable and inconsistent.  If too large an injector is fitted the car may run overly rich on overrun and idle where the fuel required is below the minimum Pulse width of the injector, this may mean the idle speeds need to be raised unnecessarily just to use the fuel delivered and fuel economy will suffer, in addition at low Pulse widths the injector may not spray effectively, poor atomization can result in rougher idle due to poor combustion.

An injector is too small when the peak duty cycle (on time) exceeds approximately 85%, fuel injectors are not made to be permanently on. 

Choose the injector that suits your build, don’t waste money on parts you don’t need and don’t buy 1000 hp worth of injector for your 600 hp build as ultimately the best injector is the smallest that will do the job while remaining under under 85% peak duty cycle.