How to Choose Injector size

Choosing the correct injector size for your (RHP Low Buck Truck or Truck Plus cammed) NA or RHP turbo motor is just simple math. The injectors provide the required fuel to achieve the proper air/fuel mixture at idle, under cruise and (most importantly) at WOT. The greater the power output of the motor, the greater the fuel flow necessary to support that power level. On a fuel injected motor, the fuel pump and injectors must be sized to provide the necessary fuel supply to support the desired power output. The proper injector size is a function of many variables, including the power output, the choice of fuel and whether your motor is naturally aspirated or run with an RHP single or mirror image twin turbo or other form of forced induction. We will cover these individually, but performance motors are often a combination of these.

Naturally Aspirated Injector Math:

Using the following formula, we can calculate the injector size for any hp level of a stock or modified eight-cylinder application (4 & 6 &12 cylinders are a multiple of the V8 math).

V8 Injector Sizing Formula: Injector size (flow in lbs/hr) x 16 = HP. This simplistic sizing/hp formula assumes a BSFC (Brake Specific Fuel Consumption, how efficiently the motor converts fuel to hp) of .5. The reality is that many NA motors (LS, Honda, Coyote etc…) are more efficient than this number suggests, but this is a good starting point for NA motors that leaves an acceptable safety margin.

Here are a couple of examples to help understand the formula.

50 pound/hr injectors will support 800 (NA) hp, since 50 x 16 = 800 (NA) hp. The RHP 50-pound injectors will support just about any possible NA combination up to 800 hp.

25 pound/hr injectors will support 400 (NA) hp, since 25 x 16 = 400 (NA) hp.

100 pound injectors will support 1600 (NA) hp, since 100 x 16 = 1600 (NA) hp. The RHP 1000 cc injectors (slightly less than 100 pounds/hr) will support just under 1600 hp and are most often used with forced induction. It is a good idea to step up to these RHP 1000 cc injectors on an NA motor if you plan on adding one of the RHP turbos at a later date. They can be tuned to run perfectly on a mild (cammed) NA 4.8L, 5.3L or 6.0L, or any other engine family or displacement and will be ready with the required fuel supply once you add boost.

This math works for any injector size and works in reverse. If you have hp output in mind and want to know what size injector to use, take the NA hp (let's use 500 hp) and divide by 16 (500/16=31.25). You would need a minimum of 32 pound injectors to feed a 500-hp motor, but never run the minimum size, since injectors should not be run at 100% duty cycle. For the 500-hp example, pick 40 pound injectors (or RHP 50 pounders), so you have some safety margin. If you plan on running boost at a later date, always get the 1000 cc RHP injectors.

Boosted Injector Math:

Compared to an NA motor, a turbo or blower motor will require even more fuel flow for two reasons. The first is you will likely be making more power than an NA application, but also because the boosted application runs richer (meaning a higher BSFC number). The higher BSFC number means a boosted motor requires more fuel at any given power level. A typical NA motor will want 12.0-13.0:1 air fuel ratio, while a boosted motor will want to run richer (11.0-11.5:1 AF).

The simple injector formula for the amount of power a given set of 8 injectors can supply on a boosted motors is as follows:

HP = Injector Size x 16 x .85

50 pound injectors will support (50x16x.85) or 800 x .85=680 hp (15% lower than NA)

100 pound injectors will support (100x16x.85) or 1600 x .85=1360 hp (15% lower then NA)

Expressed another way for the same HP Level,

Boosted Injector Size = NA Injector Size x 1.15

Example: For an 800-hp boosted V8, we would go with the RHP 1000 cc injectors. It is ALWAYS better to have too much injector fuel flow and not use all of it than not have enough. The tuning will decide how much injector you use (meaning bigger injectors won't run rich if tuned properly).

 

Fuel Math:

Gas, E85 or Methanol all require different amounts of fuel based on their stoichiometric values. The stoichiometric values for the three most common fuels are as follows:

Gas 14.7:1

E85 9.7:1

Methanol 6.4:1

What this means is that E85 requires more fuel to reach the same AF value as gas. Methanol requires even more than E85. The greater fuel flow required to maintain a given AF ratio means the injectors must flow more fuel at any given power level. This requires bigger injectors for any given power level.

 

The injectors for E85 must be 30% larger than those used for gas (for the same power level). The injectors used for Methanol must be (twice as big) 100% larger than those used for gas for the same power level. Expressed another way, the same injectors will support 30% less hp on E85 than they will on gas and only ½ (50%) of the power on Methanol. These numbers can vary slightly with E85, since there are many concentrations of pump E85 (ranging from E50 to E85). The RHP 1000 cc injectors have been run at 800 whp on E85 using the RHP 535 fuel pump. For power levels above 800 whp on E85, it would be necessary to run dual RHP 535 pumps and larger (1500 cc) injectors.

 

Choosing the right Turbo:

When choosing the right RHiP Turbo, there are several variables to consider. The variables (in order of importance) include desired power level, response rate and engine size (also single or twin).

Power Level:

The first step in choosing a turbo is to decide on your desired power level. This is important as the RHP turbo (or RHiP Mirror image twin turbos) must be chosen to support the desired power level. We offer different RHP turbos to support power levels ranging from 800 hp (RHiP800) to over 2000 hp (twin RHiP1200s). When choosing a turbo or mirror image turbos for a given power output, make sure you do not run them at (or beyond) the maximum listed power level. For example, if you are going to push your 4.8L or 5.3L LS combo to 800 hp, you SHOULD NOT get the RHiP800 turbo. You should step up to a larger (RHiP1200) turbo. Running the turbo at its maximum flow/boost/power rating is hard on the turbo and will result in reduced efficiency and turbo life. While the RHiP turbo can (and has) support the listed max power rating, that doesn't mean you should run it there. Be realistic about the desired power level and choose accordingly. Also take into account that most turbo owners might initially be satisfied with the extra power at 10 psi, but almost every turbo owner will turn up the boost and power (it's so easy). Choose your turbo not just for the current desired hp level, but for future power levels, knowing you will likely crank up the boost.

Response Rate:

If one end of the performance turbo spectrum is max power potential, the other end is boost response. These two features of the turbo are at odds, as a smaller, less powerful turbo is going to offer better response rate, while a larger, more powerful turbo, will offer reduced response rate. Extensive testing and development has produced turbos that offer the best possible combination of both, there is always a trade off in one for the other. On any combination, the RHiP800 (GTX3584RS) will always be more responsive than the RHiP1200 (G42). The same goes for the difference between the mirror image, twin RHiP1300 and the RHiP1600 turbos. One of the most common questions is will twin (smaller) turbos spool up quicker than one single (bigger) turbo? The reality is that, when sized to provide the same power level, the response rates will often be comparable. Twin turbos can offer the advantage of packaging, as it is often possible to package two smaller turbos in a tight engine bay rather than 1 larger single turbo. Twin turbos might also improve response rate by positioning the turbo closer to the cylinder head. Plus, you get to say I have a twin turbo! Mirror Image turbos at that!

 

Engine Displacement:

The displacement of the motor has a dramatic effect on turbo selection. Not surprisingly, bigger, more powerful motors require bigger turbos, but there is more to the equation than that. Where turbos are concerned, engine displacement is a double-edged sword. On the one hand, anything that improves low speed torque production (like increased displacement) also improves boost response. We all want improved boost response. Unfortunately, the improved boost response comes with the penalty of higher back pressure (not unlike a smaller turbo).

If we look at the RHiP800 turbo as an example, it is actually rated to support over 900 hp. Why do we call it the RHiP800? That is because the RHiP800 will only support near 800 hp on a larger LS (or other) V8 application. In fact, it will make more max power on a smaller 4.8L than on the larger 6.0L. In fact, it will only support 900+ hp on an even smaller-displacement motor (are you listening JDM Honda, Toyota and Nissan guys?) Though the compressor map plays a part (it is more efficient at higher pressure ratios), the real limitation on bigger motors is back pressure. For any given turbo, the bigger the motor, the higher the back pressure for any given boost pressure. Once again, strike a balance between turbo sizing, displacement, power output and boost response.