Fuel Injector Size Calculator
 

After having a few brief discussions with some friends of mine lately about injector size, and them hearing different things from different people, I think this should get some airing out.

Question: When you have one size of injector, intend on making a certain HP number, you talk to a tuner, they suggest a larger injector, are they trying to screw you?

Answer: Most likely not. A tuner will be trying to cover their ass by recommending an injector they know will be SAFE for the application at hand. An example of this would be cars making 500RWHP with 42lb/hr injectors. While it may WORK, its not SAFE because you are maxing out the injector's duty cycle doing it.

There are a pile of different injector calculators available on the Internet, and, if you use them, put in the recommended 80% duty cycle, you will find that, normally, the stock injectors on your vehicle pretty much perfectly match the power numbers the engine makes, its almost like the engineers knew what they were doing :)

Example, using the calculator I provide below, lets use the stock fuel pressure of 39psi, and a BSFC (Brake Specific Fuel Consumption) of .50, which is erring on the high-side for an N/A engine. Lets put in 225HP and 80% max duty cycle. What injector size do we end up with? 18.56lb/hr, go figure.

Now, lets put in 570HP, which is about average flywheel for a 500RWHP supercharged car. Use a BSFC of .6 because its boosted and 80% duty cycle and whoa whoa whoa, what do we have here! 56.44lb/hr injectors at stock fuel pressure! Now, increase duty cycle to 90% and we end up with 50's............No where near 42lbs/hr........

Is this information gospel? No. But I think its a good guideline for determining what injectors to run with your intended power numbers...........

Here's the link to one of the calculators if anybody is interested, calc is located about 1/2 way down the page.

http://www.rceng.com/technical.htm

RC Engineering is a manufacturer of high performance automotive fuel injectors, so I'd assume the info they provide is somewhat accurate.

YES I AGREE WITH MOST OF THE .BUT WHAT WOULD HAPPEN IF YOU WENT THE OTHER WAY.. SAY YOU ONLY NEEDED 42S AND YOU WENT WITH 50s. Would the computer know enough to compinsate

If you get a chip burnt or are using a tuning device, you tell the ECM what size injectors you are running so that it DOES know. Its relatively easy for a tuner to tune-out too big of an injector..........

Also for supercharged applications, you have to think of the amount of HP being sucked up by the SC drive. That can sometimes be as much as 60 - 80 hp. So when the engine dynos at 400 hp, it actually needs fuel for 460+ hp to keep from leaning out.


cheers
Ed N.

Very good point

Run in to this with the newbie Super Coupe guys all the time. My 95 had 36# stock, with bolt-on stuff I now have 42#. I have an upgraded blower to go on, that can push as much as 20 psi ... so I'll likely be upgrading again to 50# sprayers to make sure I don't kill the injectors when open-track lapping.


cheers
Ed N.