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EFI Tuning Fundamentals: Calculating Cycle Time

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Calculating Cycle Time


00:00 - Now that we know how much fuel the injector is capable of flowing, and how much fuel we need to supply, we need to find out how much time is available for the injector to actually get the fuel into the cylinder.
00:11 To do this, we've got to figure out how long each engine cycle takes to complete.
00:15 First of all, we need to calculate how many cycles per second are occurring, and this depends on the engine RPM.
00:22 Let's start by converting RPM into cycles per minute.
00:25 We can do that with this equation, where cycles per minute, is simply equal to the engine RPM divided by two.
00:32 Remember that the reason we're dividing RPM by two, is because it takes two full revolutions, to complete a single engine cycle.
00:41 Solving this equation, we get an answer of 3,000 cycles per minute.
00:46 Now that we have the number of cycles per minute, we can simply divide this by 60, to calculate the number of cycles per second, since there are 60 seconds in a minute.
00:56 Doing this, gives us an answer of 50 cycles per second, so this means that at 6,000 RPM, the engine is completing 50 engine cycles every second.
01:06 So now we know how many cycles occur every second, the next step is to calculate how long each of these cycles takes.
01:14 We can do that with this equation, where we're calculating the inverse of the number of cycles per second, Putting the numbers into this equation, we get an answer of 0.02 seconds.
01:26 When we're dealing with engine cycle times, the numbers are typically very small, and we usually talk in milliseconds, instead of seconds.
01:34 0.02 seconds, is the same as 20 milliseconds, since there are 1,000 milliseconds in one second, so our cycle time is 20 milliseconds.
01:44 Now that we've gone through each step to calculate cycle time, I'll let you in on a little secret that's going to save you a lot of time.
01:51 All we actually need to do is use this equation in order to convert directly from engine RPM to cycle time, simply divide 120 by the engine RPM.
02:01 This is one of the key points from this course that I'd urge you to remember.
02:06 It's surprising how often I use this quick calculation in my day-to-day tuning, to work out the cycle time, at a certain RPM.
02:13 Now we know the cycle time, we can move on to the last step, and calculate the required injector pulse width.

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