Updated: Oct 18, 2019
Understanding how much fuel your engine needs.
In order to determine what size fuel injectors you’ll need, you first must understand how they are sized. The flow rate of a fuel injector is typically measured in CC/Min or LB/Hour.
This is the maximum amount of fuel that can flow through them in a given time frame. Some common ones are 17LB/Hour, 19LB/Hour, or 197CC/Min. The more horsepower your engine makes, the larger the fuel injectors will have to be.
With that being said, you can run into problems if the flow rate of your injectors is too high, or too low.
Now, let’s get down to figuring out the flow rate you need for the horsepower you’re making.
There are four pieces of information you will need: injector duty cycle, BSFC (Brake Specific Fuel Consumption), number of cylinders your engine has, and horsepower.
The injector duty cycle is the amount of time the injector spends open. This is usually around 0.85 for high performance injectors. This means that the injector is open for 85% of the time, and closed for 15%. The manufacturer will normally have this information readily available.
The BSFC is usually understood to be 0.50 for naturally aspirated engines. Horsepower can be tricky as well if you’re engine isn’t running yet. You’ll have to just take a ballpark of what you should be making with all the modifications you’ve done.
It’s better to err on the higher side, as you can use injectors that are slightly too large, but if they’re too small you won’t be able to get enough fuel.
Now that we have all the numbers we need, there’s a simple equation we’ll plug the numbers into, to get our optimum fuel injector size.
(HP x BSFC) / (# of inj. X Duty Cycle) = Flow rate in LBS/HR
Let’s do an example. I recently built an engine that I had a goal of 300HP at the crank for. It’s a high compression, N/A inline 6. My BSFC will be 0.50, and my duty cycle will be 0.85.
(300x0.50) / (6x0.85) = 29 LBS/HR
As you can see, with my engine specs, I’ll be needing injectors with approx. 29 LBS/HR flow rate. In this case, I would probably go with the common 30 LBS/HR flow rate.
The important thing to remember is that fuel injectors don’t make power, they simply support it. Buying new bigger fuel injectors won’t make your car any more powerful.
If you’ve modified your engine, and need to support those modifications, then yes, you need bigger fuel injectors.
If you have a stock engine that just seems to be running a little sluggish, bigger fuel injectors will actually cause a huge headache, because they will supply way too much fuel to your engine without the proper tuning.
A better solution is to get your stock fuel injectors restored. Most manufacturers recommend cleaning your fuel injectors after about 100,000 miles.
This is typically the way to go if you haven't completely rebuilt your engine to make loads more HP than it was before.
In fact, even if you haven't modified your engine at all, there's plenty of benefits to restoring your stock fuel injectors.
After a while, your injectors will get gummed up with old fuel residue, and they won't be able to do their job correctly.
The spray pattern will be affected, meaning the air and fuel won't mix as well, leading to wasted fuel buildup in your intake manifold and ports.
The injectors won't have the same flow rate, which will cause your engine to run less efficiently, and can even cause serious damage.
Sometimes they can even get stuck open or closed, always leaking fuel into the cylinder causing further wear and tear on your engine.
All of these problems can be solved by simply having your injectors restored. It's a fraction of the price of new injectors, and your engine will thank you.