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# EFI Tuning Fundamentals: Injector Sizing

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## Injector Sizing

### 02.27

 00:00 - We've learned a lot about fuel injectors and how they function so far but how do we choose the correct size injector for a specific engine? To work this out we need to know a few parameters first. 00:10 We're going to need to know how much power we're expecting, as well as what the BSFC of the engine is, how many injectors are fitted to the engine and the maximum duty cycle we want to run the injectors at. 00:23 Before we use this equation, I want to talk about injector duty cycle. 00:27 Remember that this is the percentage of time the injector is on versus the cycle time. 00:32 Generally we want to stay away from running the injector at very high duty cycle such as 95-100% in order to ensure that we have some head room to increase the fuel supply if required. 00:44 Some injectors are also known to begin operating erratically if they're constantly operated at very high duty cycles. 00:50 As a good rule of thumb, I don't like to exceed 85% to 90% injector duty cycle so we need to take this into account when sizing our injectors. 01:00 The equation for injector flow is shown here. 01:03 We have our injector size in pounds per hour, our projected flywheel horsepower, the BSFC of the engine, the number of injectors fitted to the engine and finally the injector duty cycle. 01:15 Let's run some numbers through this equation to see how it works. 01:18 We'll assume we have an engine that's expected to make 600 horsepower. 01:23 It's fitted with four injectors and we don't want to exceed 85% injector duty cycle. 01:28 The engine is turbocharged so we're going to assume a BSFC of 0.55. 01:34 From this equation, we can calculate that we need an injector that can support 97 pounds per hour fuel flow. 01:41 If we want to change this to cc's per minute, we can then multiply by 10.2 like we saw in theprevious injector flow module and this gives us 990cc per minute. 01:51 Normally we would round the number up when actually selecting the injector to ensure we have a little extra head room. 01:58 The key takeaways from this module are that in order to select the right size injectors for our project, we first need to have a clear idea of all of our variables. 02:07 The expected power output, the engine's brake specific fuel consumption, how many injectors are fitted and the maximum duty cycle that we're happy to run the injectors at. 02:17 Once we have that, we can use the simple equation we've looked at to work out exactly what sized injectors we need.

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