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EFI Tuning Fundamentals: Injector Flow

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Injector Flow

06.23

00:00 - Injectors are available in a range of different sizes, and this information is vital for choosing the correct injector for a specific engine.
00:07 If you're tuning a factory ECU or an aftermarket ECU that uses a VE fuel model, then you're also going to need to provide information to the ECU so that it knows how much fuel your injectors can flow.
00:20 The problem is that some injectors are rated in terms of volume flow, like cc's per minute, while others are rated in terms of mass flow, such as pounds per hour.
00:29 This can cause some confusion if we want to convert between these units of measurement.
00:34 Let me explain a little more thoroughly.
00:37 If we test an injector and power it on for one minute and find that it flows 1000 cc's, then we could say that the volume flow for the injector is 1000 cc's per minute.
00:47 To find out what the mass flow is, though, we need to weigh the fuel that is being supplied during that minute, and the result is going to depend on the density of the particular fuel being tested.
00:58 We can't, therefore, directly convert from units of volume to units of mass without knowing what the fuel density is.
01:06 Fuel density is normally referred to as specific gravity and its value will also depend on the fuel and the temperature that the fuel is being tested at.
01:15 For example, the specific gravity of pump gasoline is normally considered to be approximately 0.74 grams per cubic centimetre.
01:23 In this case, units of grams per cubic centimetre is the normal way of representing the specific gravity of fuel.
01:31 What this means is that when tested with pump fuel, a 1000 cc per minute injector will flow 740 grams per minute.
01:39 Injector mass flow, though, is normally referred to using units of pounds per hour.
01:45 So to convert between these units we need to divide 740 by 7.56 and we get the result, 98 pounds per hour.
01:54 While the specific gravity of the fuel and its temperature will affect the conversion from volume flow to mass flow, a quick and easy factor to approximate mass flow from volume flow, or vice versa, for pump gas is 10.2.
02:08 If you have an injector rated in cc's per minute, you can divide by 10.2 to get an approximate mass flow result in pounds per hour.
02:16 If, on the other hand, your injector is rated in pounds per hour, you can multiply by 10.2 to convert to cc's per minute.
02:23 At this point, you might be wondering why we would want to rate injector flow in units of mass flow in the first place.
02:29 This is because when we're talking about air fuel ratio, we're talking about a mass ratio.
02:35 So an air fuel ratio of 14.7 to one, means that we're mixing one pound of fuel with 14.7 pounds of air.
02:44 In the majority of OE ECUs, air flow is either measured directly or calculated in units of mass air flow, and then the required injector pulsewidth to achieve a target air fuel ratio is calculated from the mass air flow and the mass flow rate of the injector.
02:59 Now let's move on and consider the effect fuel pressure has on injector flow.
03:04 The volume of fuel an injector can flow depends on the size of the injector, as well as the fuel pressure.
03:11 An injector will have a rate of flow or reference flow which is what we're talking about when we say we have 1000 cc per minute injector.
03:20 However, this flow value will be measured at a specific pressure and we need to know this to get a complete idea of what the injector is capable of.
03:29 It's reasonably common for injectors to be flow-rated at a fuel pressure of three bar, or 43.5 psi, but this isn't always the case so it does pay to check.
03:39 If you're deciding on the fuel pressure to run on an engine, there isn't a single value that you must apply, but you can take some guidance from what OE manufacturers do.
03:48 A good starting point for a fuel system that uses a manifold pressure referenced fuel pressure regulator is three bar or 43.5 psi differential fuel pressure.
03:58 For a returnless system, a slightly higher base pressure of four bar or 58 psi is common.
04:04 As the fuel pressure is increased, the injector flow will also increase, and as we reduce fuel pressure, likewise the flow decreases.
04:12 We can calculate how injector flow will change with increased pressure using this equation here.
04:19 To calculate how it will vary, we need to know the original injector flow and the pressure at which this was measured.
04:25 We know this, we can then calculate what the new flow rate will be as we change the fuel pressure.
04:31 Note that this relationship isn't one to one, so doubling the fuel pressure won't result in double the injector flow.
04:38 You do need to be a little careful when increasing fuel pressure though as the extra fuel pressure makes it harder for the injector to open, and at some point, the fuel pressure can actually make it impossible for the injector to open and it simply will cease operating.
04:51 Most aftermarket injectors are supplied with suggested maximum pressures, so it's important to make sure that you stay within the safe operating region.
05:00 As we've already seen, increasing the fuel pressure will also affect the injector deadtime, so you need to consider this too.
05:07 The other issue is that as the fuel pressure increases it puts more stress on the fuel pump and the flow capability of the pump will decrease.
05:15 If you're chasing a little additional flow from your injectors to support the fuel demands of your engine, this can become a problem as the fuel pump may not be able to keep up with the increased fuel pressure.
05:26 A good rule of thumb is that raising the fuel pressure is useful if you need to gain perhaps an additional 10 to 15% more flow from your injectors.
05:34 If you need much more than this, you should probably consider fitting larger injectors.
05:40 In summary, understanding the flow capabilities of your injectors is crucial.
05:44 This gets a little tricky with some injectors being rated in cc per minute while others are listed in pounds per hour but we've given you a simple formula to convert between units of mass flow and volume flow.
05:55 Remember also that the volume flow of an injector will depend on its size as well as the fuel pressure but particularly with the fuel pressure, this isn't a linear relationship and doubling the fuel pressure won't double your injector flow.