How To: Properly Size Fuel Injectors For Your Engine (2024)

You may have seen horsepower estimates for EFI systems or injectors where the peak horsepower potential changes between normally aspirated and supercharged or turbocharged applications. The fuel flow numbers will change mainly due to the BSFC number applied to that particular supercharger or turbocharger. Conventional wisdom states that it is wise to choose a larger injector for boosted applications in order to have sufficient fuel flow beyond that required for the engine’s expected power level. This is important because the consequences of running a too lean air-fuel ratio on a boosted engine are nearly always disastrous. However, there are multiple reasons for choosing a larger injector.

Boosted engines, especially belt-driven supercharged engines consume a significant amount of crankshaft power merely to drive the supercharger and the larger the supercharger, the more horsepower it takes to spin the blower under boost. It is not unusual for a large centrifugal supercharger to consume more than 100 shaft horsepower to move the air to create boosted power. Even small centrifugal superchargers generally demand upwards of 50 to 60 horsepower to spin.

This additional power required to drive the blower must be accounted for when calculating the size of the injector necessary to make the boosted power. This can be accommodated by changing the BSFC curve. In our earlier N.A. examples, we made a point of noting that those BSFC numbers were for normally aspirated engines. Supercharged engines on gasoline will be represented by far less efficient numbers, meaning the BSFC number will be numerically larger. Typically, a supercharged engine will demand a 0.65 BSFC for gasoline. That’s roughly a 33 percent increase in the amount of fuel required. This begins to account for the fuel required to make the power that will be used to drive the blower but won’t show up as actual horsepower at the crankshaft.

As an example, if we are planning to build a 900 horsepower LS engine on gasoline using a centrifugal supercharger, we might use a BSFC number of 0.65 to help us estimate the size of the injector. With a BSFC of 0.65 and a projected 900 horsepower – this would mean 900 x 0.65 = 585 / 8 = 73 lb/hr. Now we need to account for the 70 percent boosted engine duty cycle safety margin so we take 73 lb//hr x 1.30 = 94.9 or a 95 lb-hr injector. So for a 900 boosted engine, a 95 to 100 lb/hr injector would be a good starting point. Of course, an even larger injector would be a good idea for several reasons.

Sizing the injector larger than you might think offers other advantages. For boosted engines with the injector located in the intake manifold, the injectors are forced to open against much higher pressures than with a normally aspirated engine. As an example, let’s assume our 900 horsepower engine with a supercharger making 15 psi of boost. The injectors must now work against that 15 psi pressure in the intake manifold which effectively reduces the fuel pressure in the fuel rail from 43 psi down to 28 psi, which is extremely low.

To compensate, all boosted applications use a fuel pressure regulator that has the capacity to sense pressure in the intake manifold and boost match the fuel pressure. So when running a 43 psi line pressure with 15 psi of boost, the rail fuel pressure will actually be 58 psi but pressure at the injector outlet (differential pressure) will be equal to its rated delivery at 43 psi.

Sizing a fuel injector on the larger side offers a benefit should you decide to boost the power at some point because the larger injector offers enough additional fuel flow to offset the minor power increase. Tuners are often tempted to boost fuel pressure when faced with adding power to an existing package. If the injectors are on the small side, adding fuel pressure will help. Keep in mind every injector has a maximum operating pressure, above which it physically won’t open. This differs greatly across all injectors.

There are limits to this move, however. The problem quickly becomes a matter of fuel pump capacity. As an example, let’s use our 900 horsepower supercharged engine that we now want to raise to 1,000 horsepower. This would normally demand a 105 lb/hr injector using our previous equation. But a tuner might be tempted to bump the fuel pressure from 43 psi to 58 psi. As we’ve seen from our previous fuel pressure math, this is worth 15 percent additional flow.

However, this also raises the base fuel pressure to 58 psi but then with 15 psi of boost-referenced pressure, the fuel pressure increases to 73 psi. This may work if you are using a high quality, high flow fuel pump that is capable of producing the necessary large amount of fuel at this equally high pressure. This also dangerously assumes that the entire fuel delivery system is capable of minimizing restrictions to be able to deliver this large amount of fuel at this higher pressure. This is a lot to demand from the fuel delivery system. If the pump is not capable, the tuner may unfortunately discover this after the engine runs lean and burns a piston.

The smart move in this case is to size the injectors much larger by perhaps 20 to 30 percent in anticipation of adding more power while not requiring a large increase in fuel pressure. From the Holley fuel pump lineup, this will demand one of Holley’s twin fuel Dominator EFI fuel pumps that would have the capacity to meet this kind of demand. At 60 psi, the Holley Dominator 12-1200 is rated to feed a supercharged 1,000 hp. The point to remember is that when sizing injectors, it’s crucial to also include the fuel pump needed to feed the injectors at the appropriate fuel pressure. Even the best injector will never perform properly if fuel is not delivered in both the required volume and pressure.

If you know a little bit about alternative fuels then you may already be aware that while E85 has much going for it with regard to higher octane number performance, it also demands a much larger injector because this fuel commands a roughly 30 percent higher BSFC number. This is because when we burn a pound of pure ethanol, it will produce only about 70 percent of the heat generated by a pound of gasoline.

This means ethanol requires approximately 30 percent more fuel in order to produce the same amount of heat – which equates directly to cylinder pressure and horsepower. So sizing an injector for use with E85 automatically will demand an injector that is at least 30 percent larger. Calculating injector size for a 500 horsepower normally aspirated engine on E85, we would use the 0.70 BSFC number. Including a maximum 70 percent duty cycle, this would put the injector size at 57 to 60 lbs/hr compared to gasoline’s 42 lb/hr size. Comparing these two injector sizes, you can see that the E85 injector comes out to 42 percent larger injector.

Beyond the size of the injector, there are also the physical dimensions of the injector to consider as well as its electrical connections. There are three popular electrical connectors that are not interchangeable so it’s important that if you already have a wiring harness that the injector you choose should sport the correct connectors. The most popular are EV1, EV6 (USCAR), and Multec 2 connectors but there are also several import styles as well.

If you’ve stuck with us through this entire process, you can see there’s quite a bit of tech involved with choosing injector sizes for a performance engine. If there’s a lesson here, it’s that you can be safe with a slightly larger injector with very minor consequences but you certainly don’t want to shoot low and have to band-aid your EFI system in order to compensate. The smart move is to carefully choose your injector size based on realistic estimates of power and how the system will be used.