FAQ : How to Determine Injector Size

FAQ : How to Determine Injector Size


Determining what size fuel injectors you need for your project car is a common question we are asked here at Pro Street. No matter what car or engine you are working on, you will need properly sized injectors to maximize your fuel efficiency and horsepower. The problem is not everyone knows How to Determine Injector Size properly, so that’s what this writeup is for.

Choosing what size injectors is easy to do once you have determined what kind of power adder you’ll be factoring into your vehicle or engine. Finding out what size injectors you should be running is easy once you have determined the forced induction factor for calculating your horsepower goals.

Power Adder Factors:

Normally aspirated: 0.5
Supercharged: 0.6
Turbocharged: 0.625

Now take your target horsepower and multiply it by the power adder factor of your choice, and then divide by the number of cylinders. If you followed this page from our LS swap guides, you will be dividing by 8. If you have found our page from the 2JZ or RB25 guides, you will be dividing by six.

Learning How to Determine Injector Size can assist everyone by just using the rest of the mathematical equation.

Now with that value you will be dividing by the maximum percentage of injector duty cycle ( 90% ) or 0.90. Let’s try a few examples so that you can see how easy this whole process is.

Companies like RC Engineering provide support and services for your injector needs.

Example 1 : Target 600 hp – Naturally Aspirated LS1

  • Injector size = [(600 x 0.5) ÷ 8] ÷ 0.9
  • Injector size = (300 ÷ 8) ÷ 0.9
  • Injector size = 410cc or 41 lb/hr injectors

Example 2 : Target 900 hp – Turbocharged 2JZGTE

  • Injector size = [(900 x 0.625) ÷ 8] ÷ 0.9
  • Injector size = (562.50 ÷ 8) ÷ 0.9
  • Injector size = 780cc or 78 lb/hr injectors – rounded up to 790cc or 810cc

Example 3 : Target 350 hp – Supercharged B18C1

  • Injector size = [(350 x 0.6) ÷ 8] ÷ 0.9
  • Injector size = (210 ÷ 8) ÷ 0.9
  • Injector size = 290cc or 29 lb/hr injectors – rounded up to 330 or 310cc

If your calculation does not return an injector size that is commonly available, it’s a general rule of thumb to simply round up to the nearest available size as shown with examples 2 and 3.

If you are intending to run high boost on your application, being generous while rounding up is never a bad thing unless you do not have the supporting mods with which to both provide fuel and control the fuel pressure in your rail.

Q: Do I need to calculate based on maximum duty cycle?

A: No but this will allow you to calculate the minimum required injector size. An experienced tuner will be able to tune safely even with larger injectors.

Q: Can’t I just run larger injectors and not care?

A: Not always unless you are willing to purchase a lot more than injectors. Your factory fuel pressure regulator may not be able to keep the fuel pressure down depending on the pump flow at 13.5 volts.

DeatschWerks Fuel Pump flow chart

Often times customers choose fuel injectors and fuel pumps that either do not match, or provide too much flow for part throttle or idle situations.

Calculating the fuel pressure, load and injector duty cycles aren’t always the easiest thing to figure out, so if you are confused or need help, ask your favorite tuner of choice for some recommendations.

Take the proper steps and time to select your fuel options wisely as to not repeat your work or have to scrap the modifications that you mistakenly or incorrectly purchased previously.


Looking for a fuel pump install how-to? Check out our how-to guide here.

You now know How to Determine Injector Size, so what are you waiting for? Get out there and upgrade those fuel injectors!