Justin@VMP
April 29th, 2009, 01:12 AM
The stock GT500 injectors are not good for nearly as much power as people think.
It's easy to be tricked, until you take a closer look at the data.
First off, I want to say that stock injectors are 47-48lb at the Ford Fuel system pressure, not any larger, this is per FRPP. Really not very big.
The truth is a car with a stock blower and pulley can easily max them out.
Now I am not advocating that everyone go out and upgrade injectors, there are a lot of reasons to stick with stock injectors as long as you can, for reasons of fitment, driveability, and because I don't like to waste money on un-needed parts.
There are also good reasons to go out and change your injectors when needed, primarily for more consistent fuel control and high RPM use.
You can still get enough fuel with a stock injector that is maxed out, but its not pretty. The bad part about maxing out an injector is the lack of control. You may be able to get a rich enough a/f to be safe, but you won't have any control of it once you exceed about 95% injector duty cycle. Anything above that, the injector is basically hanging open and dumping fuel all the time. When an injector maxes out it goes rich and then slopes lean at about a 45* angle, if you have very accurate a/f measuring equipment with very little delay you'll see the rich dip and then lean slope. If you measure a/f in the tailpipe you may never see this happen, as their is a several hundred rpm delay with some types of tailpipe measuring devices (mainly vacuum based ones that don't put a sensor in the tailpipe).
It's important to remember that a fuel injector only has a certain amount of time to inject fuel, as RPM increases, this window decreases. Lack of injector will most often show itself at very high RPM. A good example of this is the new FRPP CJ which has a TVS's GT500 motor, it makes a ton of power (shhhhh), runs 16psi of boost, and uses huge 80lb injectors. In testing FRPP wanted to run the motor to 7000rpm, so they needed a big injector, they also did not want to install a device like a boost a pump due to reliability and engineering concerns. This is why they went with such a large injector, and it payed off, at 16psi the duty cycle is only 60% at 6000rpm.
Raising fuel pressure is a way to get more fuel from an injector, injector flow rate increases by the square root of the change in pressure. A 10psi increase in pressure will get you roughly 5lb/min worth of flow. I have no problem doing this on a small scale, 5-10psi, up until the point where the stock pumps gets maxed out. The factory system has no problem operating at slightly higher pressures, its actually designed to do this in certain situations (like cat protect) and there are parameters to do this. However, there are limitations.
Here is data from a car with a stock blower, upper pulley, and CAI that is maxing the stock injectors out:
http://vmptuning.com/GT500/inj/TP48sM122MAXDC.jpg
You can see that injector duty cycle hits 100%, which leads to a slight loss of a/f control at very high rpm, but you may never see it if you don't have good equipment.
The fix is to go into the calibration and raise fuel rail pressure to deliver more fuel through the injectors, the stock pumps easily handle this, as they are only at 80% duty cycle at 39psi/550rwhp/13psi boost. Increasing the fuel pressure slightly will get the inj duty cycle down to 90-95% and still keep pump duty cycle below 90%. IMO this is the best solution for this car.
Here is data from a TVS blower car with upper pulley and TB running about 16psi and making 625-650rwhp:
http://vmptuning.com/GT500/inj/MM48sDC.jpg
This car has already had the fuel pressure raised to increase flow and lower the inj duty cycle to a reasonable 95%, the fuel pump duty cycle is at 95% as well, still a little bit of safety margin in both. I feel this is a good solution for this car. However, if he installed a motor with better rotating parts and wanted to rev out to 7000rpm it would quickly max out the stock injectors.
This GT500 has a TVS, upper and lower pulleys, TB, makes 18psi of boost, and makes 670-700rwhp depending on conditions and fuel, it is maxing out the stock injectors like crazy:
http://vmptuning.com/GT500/inj/JS48sMAXDC.jpg
107% duty cycle even with fuel pressure raised to 48psi, and the fuel pumps are maxing out too at 98% (BTW, fuel pumps also act funny when the duty cycle is pushed that high, they don't produce steady pressure). This car needs the fuel injectors replaced with larger ones, the stock ones have been exceeded greatly, even though a/f was 11.7:1 (per analog1) the a/f is not smooth and there is no control. Fuel pressure has already been raised and the pumps are maxed out, so this is not an option. When larger injectors are installed the fuel pressure can be brought back down to 39psi and duty cycle for the pumps will drop to 90-95%, which is plenty of margin, this is how the CJ is set up.
It's easy to be tricked, until you take a closer look at the data.
First off, I want to say that stock injectors are 47-48lb at the Ford Fuel system pressure, not any larger, this is per FRPP. Really not very big.
The truth is a car with a stock blower and pulley can easily max them out.
Now I am not advocating that everyone go out and upgrade injectors, there are a lot of reasons to stick with stock injectors as long as you can, for reasons of fitment, driveability, and because I don't like to waste money on un-needed parts.
There are also good reasons to go out and change your injectors when needed, primarily for more consistent fuel control and high RPM use.
You can still get enough fuel with a stock injector that is maxed out, but its not pretty. The bad part about maxing out an injector is the lack of control. You may be able to get a rich enough a/f to be safe, but you won't have any control of it once you exceed about 95% injector duty cycle. Anything above that, the injector is basically hanging open and dumping fuel all the time. When an injector maxes out it goes rich and then slopes lean at about a 45* angle, if you have very accurate a/f measuring equipment with very little delay you'll see the rich dip and then lean slope. If you measure a/f in the tailpipe you may never see this happen, as their is a several hundred rpm delay with some types of tailpipe measuring devices (mainly vacuum based ones that don't put a sensor in the tailpipe).
It's important to remember that a fuel injector only has a certain amount of time to inject fuel, as RPM increases, this window decreases. Lack of injector will most often show itself at very high RPM. A good example of this is the new FRPP CJ which has a TVS's GT500 motor, it makes a ton of power (shhhhh), runs 16psi of boost, and uses huge 80lb injectors. In testing FRPP wanted to run the motor to 7000rpm, so they needed a big injector, they also did not want to install a device like a boost a pump due to reliability and engineering concerns. This is why they went with such a large injector, and it payed off, at 16psi the duty cycle is only 60% at 6000rpm.
Raising fuel pressure is a way to get more fuel from an injector, injector flow rate increases by the square root of the change in pressure. A 10psi increase in pressure will get you roughly 5lb/min worth of flow. I have no problem doing this on a small scale, 5-10psi, up until the point where the stock pumps gets maxed out. The factory system has no problem operating at slightly higher pressures, its actually designed to do this in certain situations (like cat protect) and there are parameters to do this. However, there are limitations.
Here is data from a car with a stock blower, upper pulley, and CAI that is maxing the stock injectors out:
http://vmptuning.com/GT500/inj/TP48sM122MAXDC.jpg
You can see that injector duty cycle hits 100%, which leads to a slight loss of a/f control at very high rpm, but you may never see it if you don't have good equipment.
The fix is to go into the calibration and raise fuel rail pressure to deliver more fuel through the injectors, the stock pumps easily handle this, as they are only at 80% duty cycle at 39psi/550rwhp/13psi boost. Increasing the fuel pressure slightly will get the inj duty cycle down to 90-95% and still keep pump duty cycle below 90%. IMO this is the best solution for this car.
Here is data from a TVS blower car with upper pulley and TB running about 16psi and making 625-650rwhp:
http://vmptuning.com/GT500/inj/MM48sDC.jpg
This car has already had the fuel pressure raised to increase flow and lower the inj duty cycle to a reasonable 95%, the fuel pump duty cycle is at 95% as well, still a little bit of safety margin in both. I feel this is a good solution for this car. However, if he installed a motor with better rotating parts and wanted to rev out to 7000rpm it would quickly max out the stock injectors.
This GT500 has a TVS, upper and lower pulleys, TB, makes 18psi of boost, and makes 670-700rwhp depending on conditions and fuel, it is maxing out the stock injectors like crazy:
http://vmptuning.com/GT500/inj/JS48sMAXDC.jpg
107% duty cycle even with fuel pressure raised to 48psi, and the fuel pumps are maxing out too at 98% (BTW, fuel pumps also act funny when the duty cycle is pushed that high, they don't produce steady pressure). This car needs the fuel injectors replaced with larger ones, the stock ones have been exceeded greatly, even though a/f was 11.7:1 (per analog1) the a/f is not smooth and there is no control. Fuel pressure has already been raised and the pumps are maxed out, so this is not an option. When larger injectors are installed the fuel pressure can be brought back down to 39psi and duty cycle for the pumps will drop to 90-95%, which is plenty of margin, this is how the CJ is set up.