I mentioned this in another thread but it was requested I post the math. I felt it deserved its own discussion. I am not an engineer. All of this is based on research for building my own engine.
So with the disclaimer out of the way, here goes.
Fuel injectors are selected based upon Brake Specific Fuel Consumption (BSFC), Horsepower (BHP) and Duty Cycle (DC).
The exact math is:
BHP x BSFC
------------------- = injector flow rate pounds per hour (pph)
injectors x DC
BSFC is a bit of a variable. The standard I will use is .60. That means it takes six tenths of a pound of fuel to make one horsepower. Depending on who you ask it can be .6 to .7. The duty cycle really comes down to the injector mfg. You can get injectors rated to 100%, but most are rated at 80%. I am using 75% for our formula because it makes the math easier.
A stock 5sp is rated at 170HP. Our formula will be (170*.6) / (4 * .75) = 34pph.
The XR4ti comes with 35pph injectors. It needs to be noted that the rating is at 39psi. Most injectors are now rated at 43.5, 45 and 55 psi. That is very important when shopping for injectors.
With an intercooler, big VAM, Ranger CAM, 65mm TB lets estimate 210HP.
(210 * .6) / (4 * .75) = 42pph
Now lets do the same thing for 300HP. This number was chosen because it is doable with the 2.3 motor and so many people have this as a goal.
(300 * .6) / (4 * .75) = 60pph
The reason for keeping the duty cycle down is because we are running turbos. Under boost the fuel pressure has to increase pound for pound. The stock fuel regulator with vacuum line disconnected is supposed to be 35-45psi, we will just assume 39lbs since that is what the injectors are rated at. With 18lbs of boost the fuel pressure has to be 57psi to overcome the boost. This is one of the main reasons our injectors have leakage problems. It is also why fuel pumps go out.
That brings us to this part.
For most aftermarket injectors you will have to go to an ajustable regulator and a fuel pump that can keep up. If you go with a 42pph injector that is rated at 55psi you have a base psi of 55lbs. Under boost that increases to 73lbs! That is a bit hard on fuel pumps. The solution is to use a larger injector and lower the fuel pressure. We will use an example of a 52pph injector rated at 55psi for our motor. We will use the 210HP requirement of 42pph.
The formula is as follows: New pressure equals rated pressure times the desired flow rate squared, divided by the base flow rate squared.
required_pressure = rated_pressure * (rate_desired^2 / rated_flow^2)
p = 55 * (42^2 / 52^2)
p = 36psi
By using the higher rated injector we can lower the fuel pressure to stay within the specs of our fuel system. Most injectors will function properly down to about 25psi. Anything lower and you start altering the spray pattern. This comes into play at idle. You already know the pressure regulator increasses with boost, but it also decreases pressure with vacuum. Each 1" of vacuum drops the pressure .49psi. So at idle our 36psi system with 18" of vacuum will be at 27psi. It is important to remember this.
----> Please Note <---- In order to do this properly you need a fuel pressure guage and an Air/Fuel mixture guage. The process to do this is for another write-up.
Now, when is a bigger injector not a bigger injector?
Mopar sells a performance injector that is a drop in replacement for our stock bosch injectors. They are rated at 42pph @ 55 psi. The price is good at $150 per set of 4. Now I will explain how they are not bigger.
Using our formula above we plug in the following numbers:
rated_pressure = 55psi
rate_desired = 35pph
rated_flow = 42pph
p = 55 * (35^2 / 42^2)
p = 38psi.
Just so happens that is within the tolerance of our stock regulator.
There are some injectors out there that are rated in cc's. These are usually found on rice burners. The formula for converting to pph is: cc / 10.5092
So a 550cc injector is 52pph. Most of these I have seen is based on 43.5 psi.
Fuel injector sizing math / Upgrading Fuel Injector Size
Moderator: John Brennan
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I forgot about this thread - thanks for the reminder Vince.
I realize it wasn't really the intent for this to be a tuning post per se, but I think it's important to make two points in that regard:
1) Installing larger injectors alone will not magically create more power. Using Vince's 300HP example above, you cannot simply install 60PPH injectors and make 300HP. Quite the contrary in fact, because...
2) The stock computer (PCM, ECM, EEC-IV, whatever you want to call it) is designed to work with 35PPH injectors. If you install larger injectors (or smaller, for that matter), it doesn't somehow automagically know this at all. As a result, if for example, you install 70PPH injectors, twice as much fuel will be injected and your motor will run horribly rich (probably won't even start). Bottom line, you cannot simply install injectors significantly different from stock without changing the computer's calibration.
That said, there are ways to "trick" the stock computer into delivering closer to the proper amount of fuel when making small changes from the stock 35PPH injectors, but these methods are mostly flawed (some more than others). Generally speaking, "Tuning" on a fuel injected motor is done via software changes, not hardware.
I realize it wasn't really the intent for this to be a tuning post per se, but I think it's important to make two points in that regard:
1) Installing larger injectors alone will not magically create more power. Using Vince's 300HP example above, you cannot simply install 60PPH injectors and make 300HP. Quite the contrary in fact, because...
2) The stock computer (PCM, ECM, EEC-IV, whatever you want to call it) is designed to work with 35PPH injectors. If you install larger injectors (or smaller, for that matter), it doesn't somehow automagically know this at all. As a result, if for example, you install 70PPH injectors, twice as much fuel will be injected and your motor will run horribly rich (probably won't even start). Bottom line, you cannot simply install injectors significantly different from stock without changing the computer's calibration.
That said, there are ways to "trick" the stock computer into delivering closer to the proper amount of fuel when making small changes from the stock 35PPH injectors, but these methods are mostly flawed (some more than others). Generally speaking, "Tuning" on a fuel injected motor is done via software changes, not hardware.
Brad
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Re: Fuel injector sizing math. <FAQ Peer Review>
I think these formulas tend to be a little conservative. Maybe our motors our more efficient than 0.60 BSFC. I'm pretty sure there are guys putting down 300HP with stock injectors. 0.60 BSFC at 100% DC requires 45pph. Who knows. Theory and reality don't always agree.vcallaway wrote: Now lets do the same thing for 300HP. This number was chosen because it is doable with the 2.3 motor and so many people have this as a goal.
(300 * .6) / (4 * .75) = 60pph
The reason for keeping the duty cycle down is because we are running turbos. Under boost the fuel pressure has to increase pound for pound. The stock fuel regulator with vacuum line disconnected is supposed to be 35-45psi, we will just assume 39lbs since that is what the injectors are rated at. With 18lbs of boost the fuel pressure has to be 57psi to overcome the boost. This is one of the main reasons our injectors have leakage problems. It is also why fuel pumps go out.
That brings us to this part.
For most aftermarket injectors you will have to go to an ajustable regulator and a fuel pump that can keep up. If you go with a 42pph injector that is rated at 55psi you have a base psi of 55lbs. Under boost that increases to 73lbs! That is a bit hard on fuel pumps. The solution is to use a larger injector and lower the fuel pressure. We will use an example of a 52pph injector rated at 55psi for our motor. We will use the 210HP requirement of 42pph.
I'm not sure if the injectors are rated for 80% duty cycle, but I think anything you read about injectors will recommend that you size the injectors for 80%. You want to leave youself some room. And usually price isn't too much different between injector sizes, so you might as well give yourself some room.
If you're buying new injectors, you might as well just set the fuel pressure for what they're rated for. I guess if you're trying some backwoods tuning on the stock computer, do whatever you want. But, if you're at the point where you need new injectors, either modify the comp for the size you're getting or go aftermarket comp, and leave the fuel pressure alone.
-Nate
'85 Merk. Back together. Only thing stock is the head and block.
'85 Merk. Back together. Only thing stock is the head and block.
One thing I did not state is minimum pressures.
When you start tweeking fuel presures to use bigger injectors you can cause an idle problem. Most injectors start failing at about 20 to 25 psi. If you run a higher rate injector and dial down your base fuel pressure to under 30 psi you will be hosed at idle. A stock engine should pull about 15 psi of vacuum. That will put the fuel pressure at idle at about 15 lbs. Way too low to fire injectors properly.
The purpose for this writeup was not really for modifying the fuel system. It started when I was looking to replace my stock injectors. There are lots of performance injectors on the market but trying to figure out what will work is hard. The companies rate them based on a target market. If the target is a japanese car they will rate them in cc's and at a fuel pressure a specific car uses.
When you start tweeking fuel presures to use bigger injectors you can cause an idle problem. Most injectors start failing at about 20 to 25 psi. If you run a higher rate injector and dial down your base fuel pressure to under 30 psi you will be hosed at idle. A stock engine should pull about 15 psi of vacuum. That will put the fuel pressure at idle at about 15 lbs. Way too low to fire injectors properly.
The purpose for this writeup was not really for modifying the fuel system. It started when I was looking to replace my stock injectors. There are lots of performance injectors on the market but trying to figure out what will work is hard. The companies rate them based on a target market. If the target is a japanese car they will rate them in cc's and at a fuel pressure a specific car uses.
Vince Callaway
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yeah those numbers are pretty conservative, but good for a baseline. I know I was making pretty close to 300 engine hp on stock injectors (put 258 to the wheels).
in general, low impedance injectors can be safely run at 100%, high impedance injectors shouldnt be taken over 80%.
in general, low impedance injectors can be safely run at 100%, high impedance injectors shouldnt be taken over 80%.
86 Silver XR4Ti. 'squirted. 12.7@111
03 VW Golf tdi
05 Yamaha FZ6
03 VW Golf tdi
05 Yamaha FZ6