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**How To: Properly Size Fuel Injectors For Your Engine**

# How To: Properly Size Fuel Injectors For Your Engine

As with just about anything else regarding the art of hot rodding, the more you know about a subject, the better informed and more accurate your decisions will become. Nobody likes to invest hard cash in a product only to discover it won’t work due to an oversight or a lack of knowledge.

So when it comes to fuel injectors, there is an amazing array of opportunities and with that comes the realization that you should know about as much about fuel injector sizing and selection as possible beyond the question – “Is it big enough?” This story will attempt to uncover some of the less well known aspects of sizing injectors. Our focus in these paragraphs will be on normally aspirated street engines with some additional information regarding boosted engines and alternative fuels such as E85.

Let’s start by categorizing electronic fuel injectors into two main areas: high and low impedance. For nearly all street applications the high impedance injectors are the far more common selection. These injectors are driven with what is known as a saturated injector driver. High impedance injectors have a resistance value of around 12 ohms. This results in a continuous current draw of around 1 amp. This design reduces heat buildup allowing these injectors to run for extended periods with excellent reliability.

Low impedance injectors require a peak-and-hold injector driver. These drivers cost more and are typically only used in aftermarket ECUs. Peak and hold drivers allow for a higher opening current (peak) and then drop and maintain a lower current (hold) after the injector is open. This makes it easier for the injector to operate, especially with high fuel pressures. All low impedance, higher flow rate injectors require a peak and hold injector driver. Nearly all OE and many aftermarket ECUs are designed to drive only high impedance injectors. As a point of reference, both the Holley Dominator and HP ECUs are designed with the capacity to drive either the high or low impedance versions. Both of these ECUs contain 8:2 Amp injector drivers. This allows the HP ECU to drive a total of 16 low impedance injectors while the more robust Dominator is equipped to control as many as 24 peak-and-hold injectors in a 4:1 Amp mode. Some very high flowing applications require each injector to be driven in a 8:2 Amp mode.

With these categories established, we can look at some basic specs that are common to all fuel injectors. All injectors are rated to deliver a specific amount of fuel at a rated pressure. As an example, let’s take a typical high impedance Holley injector rated at 42 pounds of fuel per hour (lbs/hr). You may also see injectors rated in cubic centimeters per minute (cc/min). In an accompanying chart, we offer simple conversion formulas that allow you to convert these different specs.

This 42 lb/hr rating may not necessarily mean very much unless you work with injectors every day. A more appropriate question might be – “How much power can I make with a 42 lb/hr injector?” - or the inverse of this question as “How can I determine the injector size I need in order to make 500 flywheel horsepower?” Let’s take this second question first.

Using 500 normally aspirated horsepower as our goal, the first thing we need to address is something called Brake Specific Fuel Consumption (BSFC). This number represents the amount of fuel an engine will require to produce one horsepower in one hour expressed in pounds of fuel per horsepower per hour (lbs/hp/hr which is commonly abbreviated to lbs/hr).

The conventional BSFC number for a gasoline engine has been 0.50 which means the engine would burn a half a pound of gasoline per hour to make one horsepower. With advances in engine combustion efficiency and reduced friction this number often drops to 0.45 or lower. For this discussion we’ll use 0.50 as the BSFC number.

The simple formula is horsepower times BSFC – or 500 horsepower x 0.5 = 250 lbs/hr of fuel. This is the amount of fuel we need to supply to the engine to achieve 500 hp. If we divide that volume by eight injectors (for a V8 engine), this comes to 31.25 lbs/hr necessary to feed the engine. In theory a 32 lb/hr injector should feed a 500 horsepower engine.

Unfortunately, the realities of internal combustion engines are not quite that simple. First of all, this allows absolutely no room for variation or if we overachieve with our engine and actually make more power. This simplified calculation does not allow room for additional fuel flow or other potential errors. We will not delve deeply down the rabbit hole on this but one important aspect of injector performance has to do with how well the injector delivers fuel at both the top and bottom ends of its flow scale.

Injectors deliver fuel in a linear fashion through a majority of their operating range. However in the lower and upper 5 to10 percent of their flow ranges, the output becomes non-linear. Although most EFI systems have tables to correct this non-linearity, it is best to avoid operating the injector in these ranges. We’ll expand on this in a moment.

For this and other reasons it is common practice to reduce the injector’s duty cycle. This is expressed as a percentage of operation. An injector that is held open continuously would be rated at 100 percent duty cycle. At a 50 percent duty cycle, the injector is flowing fuel only half the time. To allow for inherent injector variables, Holley recommends sizing fuel injectors for a normally aspirated mild street engine where the maximum duty cycle is limited to a maximum of 70 percent. This allows room for growth in anticipation of future upgrades.

Taking this into account, this effectively reduces the amount of fuel the injector can supply by 30 percent from its 100 percent duty cycle. This means if we were sizing an injector for 500 horsepower, we’d add 30 percent to the 31.25 lbs/hr (31.25 x 1.3 = 40.6) and then rounding that result up to a 42 lb/hr injector. This offers multiple advantages that will become clearer as we progress in this discussion.

Now let’s address the first question which was “How much power can I make with a 42 lb/hr injector?” This is a little easier since all we have to do is multiply the flow rate times the number of injectors and then divide that by the engine’s BSFC number. With a V8 engine, we multiply 42 lb/hr x 8 = 336 lbs/hr and divide by 0.5 BSFC = 672 horsepower. But now we must reduce that power to our 70 percent duty cycle safety margin and now those 42 lb/hr injectors can be counted on to safely make 470 horsepower on a normally aspirated engine (672 x 0.7 = 470.4). Those 42 lb/hr injectors would now be considered slightly on the small side for a projected 500 horsepower engine so you can see that injectors perhaps slightly larger than 42 lbs/hr would be a safe bet.

When we double-checked this information with Holley EFI Engineer, Doug Flynn, he added that the oft-mentioned 85 percent duty cycle number is the absolute maximum that should be seen on an ECU duty cycle display at max power. What this means is that 70 percent would present a much wider safety margin for computing injector use.

To emphasize this point, Flynn said that he would recommend a maximum effort, normally aspirated race engine to reference a 30 percent duty cycle for tuners looking for that last horsepower. That may sound like overkill. However, this allows for the opening and closing of the injector to be adjusted by the tuner such that it occurs ideally in conjunction with valve opening events. If an injector is operating at an 85 percent duty cycle you will likely end up injecting fuel during valve overlap, leaving no ability to optimize injector timing.

For most race engines, Flynn says that the ideal maximum you should see on a duty cycle display for the fuel injectors is 70 percent. Duty cycles higher than 85 to 90 percent risk running into inconsistent fuel delivery issues. You also may not have enough flow for individual cylinder fueling, which might require up to 10 to 20 percent of injector flow.

The reason for this conservative duty cycle is, again that injectors do not typically operate the same at the extreme ends of the output cycle as well as they do in the middle of the flow curve. At low output (typically below 1.0 to 1.4 milliseconds of pulse width depending upon the injector) and the top end (above 85 to 90 percent), injectors operate in the non-linear fashion. That’s engineering speak that means the injector no longer can be trusted to deliver exactly the fuel that is commanded to flow. If less fuel is delivered, this can be catastrophic for engine survival at WOT. This is a major reason for using a conservative duty cycle when determining injector size.

Larger injectors should typically be run in a sequential mode as opposed to batch fire mode. Batch fire mode commands each injector twice in the cycle. Sequential operation fires the injector once per engine cycle, timed during intake valve opening. This single fire mode creates a longer pulse width, which positions the injector in a linear range (especially at idle) and allows for the opening and closing to be individually adjusted which is not possible in a batch fire mode.

There’s another reason to size injectors on the larger side and this works in concert with fuel pressure. Most Holley injectors are rated at 43 psi unless otherwise specified. This is an important specification because fuel pressure has a direct impact on fuel flow. Lower pressure will reduce the flow rate while higher pressure will increase the flow rate. We can determine this change in flow rate with a simple formula.

As an example, let’s say we need a slight increase in flow from our 42 lb/hr injectors rated at 43 psi. Let’s calculate how an increase in fuel pressure to 58 psi will increase flow. We show the math in Chart 04 where a 15 psi increase in flow pushes the 42 lb/hr injector to a touch over 48 lbs/hr. This same equation can be used to estimate the flow of an injector rated at a higher pressure (58psi) operating at a lower pressure.

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.

## Injector Conversions

**LBS/HR to CC/MIN:**

- 1 lb/hr of gasoline =10.5 cc/min
- Convert a 42 lb/hr injector to cc/min: 42 x 10.23 = 429.7 cc/min

**CC/MIN to LBS/HR:**

- 0.098 lb/hr = 1 cc/min
- Convert a 429.7cc/min injector to lb/hr: 429.7 x 0.098 = 42.1 lb/hr

## BSFC Chart

**NATURALLY ASPIRATED**

- Gasoline: 0.45 - 0.50
- E85: 0.65 - 0.70
- Methanol: 0.90-1.0

**Turbocharged/Supercharged**

- Gasoline: 0.60 - 0.65
- E85: 0.85 - 0.90
- Methanol: 1.80 - 2.0

## Power/Potential Conversions

**Calculating Individual Injector Size from a HP Goal – 8 Cyl. Engine**

Goal: 500 HP Normally Aspirated with a V8 Engine

(HP x BSFC) / 8 = Individual Injector Size in lbs/hr

(500 x 0.50) / 8 = 31.25 lb/hr at rated pressure

We then increase the size by 30 percent to compensate for the 70 percent duty cycle;

31.25 x 1.30 = 40.6 or rounding up to a 42 lb/hr injector

**Estimating HP Potential from Injector Size (lb/hr) –8 Cyl. Engine**

Example: 42 lb/hr injector

(Injector Size x 8) / 0.5 BSFC

(42 x 8) / 0.5 = 672 HP

Reduce this by 30 percent to compensate for 70 percent duty cycle

672 x 0.85 = 470 HP

## Estimating Injector Size with Changes in Fuel Pressure

(New PSI / Base PSI) = Y

√Y = Z

Z x 42 = new size

(58 / 43) = 1.34

√1.34 = 1.16

42 x 1.16 = 48.7 lbs/hr

## Estimating Engine Performance

If you are not sure how much horsepower your engine can potentially make, there is a simple formula that is easy to use. This estimator is based on pump gas, street engines with decent heads and roughly a 10:1 compression ratio. The base estimate number of 1.25 lb/ft per cubic inch is a generic estimate. Weaker engines may drop to perhaps 1.2 or lower while others are capable of 1.35 and better.

Displacement x 1.25 lb/ft /ci = Peak Torque

Peak Torque x 0.9 = Torque at Horsepower Peak RPM / 5,252 = Peak HP

Example:

350 SBC with a peak HP RPM of 6,000

350 x 1.25 = 437.5 lbs-ft x 0.9 = 393.7

393.7 x 6,000 rpm / 5,252 = 449.8 rounds up to 450 horsepower