Fuel Injector Calculator

[tigerstalon]

RC Engineering has a handy Fuel Injector Worksheet that will help you calculate what size fuel injector you need. However, I'm not sure what to enter for fuel pressure because the DSM fuel pressure regulator raises fuel pressure roughly 1psi for every 1psi of boost.

For instance, if my goal is 300 horsepower at the crank for a 16G/Big 28 setup, then I would enter 300 as the crankshaft horsepower, 4 injectors, 0.60 for brake specific fuel consumption, and 0.80 for max duty cycle. Now, if I enter 43 as the system fuel pressure, then it calculates 595 cc/min as the flow rate for the injectors (which seems higher than what most people run with this setup). However, the engine will be running boost at probably 18-20 psi, which means the fuel pressure will be roughly that much higher than 43psi, also. So, if you enter 61 (43 + 18) as the system fuel pressure, then it calculates a flow rate of 500 cc/min, which sounds more reasonable (perhaps even a bit low!).

So, when entering the system fuel pressure, should you estimate how much boost you will be running at that horsepower and add that to 43psi to get the right number, or should it always be 43?

 

[talonted_one]

As great as that tool is....just get 550cc inectors.

and to answer your question you should input your base fuel pressure. The reason all their figures are so hi, is because it will give you a larger margin of error.

 

[tigerstalon]

As great as that tool is....just get 550cc inectors.

and to answer your question you should input your base fuel pressure. The reason all their figures are so hi, is because it will give you a larger margin of error.

Well, I pretty much knew that the setup I described would be fine with 550's. That's why I asked the question--if you use the base fuel pressure (43) with all the other parameters I mentioned, it tells you that you need roughly 600cc/min, which seems way off considering people can run 320 bhp or more fine with 550cc's. Now, you can play with the margins of error by changing the max duty cycle or the brake specific fuel consumption and get it down to 500cc/min or lower, but that gives you less room for actual margin of error--like if you put them in the car and tune it only to find out that you're running 99% duty cycle and still a tad lean.

The math is all spelled out on the page as to how the calculator works. I'm not looking for a specific injector size for a specific setup. I'm looking for an answer on the fuel pressure parameter that will allow me to run several sets of numbers correctly to determine what injectors I should get for each setup.

 

[kpt4321]

If your base fuel pressure is 43 psi, you are running 43 psi of differential pressure across the injector tip at all times.

The reason you're getting such big numbers is your BSFC is off.

 

[tigerstalon]

Ok, so the fuel pressure regulator keeps fuel psi at 43 above the cylinder atmospheric pressure, which would be normal atmospheric pressure plus any additional boost pressure.

I chose 0.60 for BSFC because that's what the page suggests--use 0.60 to 0.65 for turbocharged engines. I'm guessing this is because the engines run a little hotter than others and require a little more fuel to keep thing cool. I'm also guessing that when we tune our engines, we probably end up lowering this value somewhere closer to 0.50, huh?

 

[DSMeclipse4G63]

I just did it with my fuel pressure set at 50psi(since I had to replace my stocker with the b and m cause it was fvcked) and it said 551cc/52.4lbs injectors would be fine for 300hp. I currently have it set at 57psi to make up for my shitty 450s on a b16g. I am running reletivly low boost at 14-15psi and my o2's read normal stoich at driving and max anywhere from .94-.98v at wot so Im doing pretty good with this setup I would think. After calculating 300hp at 57 fuel pressure it says I need 516cc/49.14lbs injectors. Being as though Im not pushing 300hp, I am still flowing enough fuel(with a good margin of error) for 270hp. Thanks for the link I would say its very help, not accurate to a "T" but still helpful clearing up alot thoughta Ive been having.
Andrew

ps I just did the conversion to find my actual cc with the upped fuel pressure and got 518 cc/49.34lbs, untuned I seem to have no problems. I always heard that much fuel untuned could be a problem?

 

[kpt4321]

I chose 0.60 for BSFC because that's what the page suggests--use 0.60 to 0.65 for turbocharged engines.

BSFC is essentially air to fuel ratio, assuming a fairly constant output per airflow (let's call it 10 horsepower per pound per minute of airflow).

If you have 10 hp per lb/min, then the conversion from BSFC to AFR is:

1/(BSFC*0.166666).

As such, 0.60 is about 10:1, and 0.50 is about 12:1.

A good number to use for a DSM is 0.55. On race gas, you can be as low as 0.50 or even 0.45, if you a little on the lean side (which most DSM'ers tend to be on race gas, when tuning to knock.)

 

[tigerstalon]

Excellent. Thanks for your help, Kyle.