I've been getting a lot of questions lately about this, so I'm going to lay out the cold hard numbers.
Ford has laid out the fuel flow characteristics of the stock injectors, in fact they use airflow via Maf readings to calculate fuel flow, so we can accurately calculate the maximum lb/min of airflow an injector can support given a known stoich.
The high slope of the injectors is programmed as 0.0121999997645617 in pounds per second. To convert to lb/min simply multiply by 60, in this case .7312. Multiply this # by 8 (for the number of injectors) and you have the total volume of fuel the injectors can flow at *full* programmed fuel pressure at 100% duty cycle.
.7312 * 8 = 5.856 lb/min
Now we have the total fuel volume at full fuel pressure, which is regulated at the fuel hat in the tank. On the 15+ cars Ford has set this pressure at 59.595 psi at zero fuel flow. As fuel flow increases there is a pressure drop at the injector (basic physics) due to the location of the fuel pressure regulator. Ford has calculated this loss and programmed it into their fueling model, but it is only scaled to 4.0 lb/min fuel flow, however this model is linear so we can calculate the fuel pressure at the injectors at 100% duty cycle (5.856 lb/min), which would be roughly 55psi.
At 100% injector flow capacity pressure at the injector has been reduced 55psi. So what does a drop in fuel pressure at the rail do to injector flow? We need to recalculate the fuel volume thru the injectors at 55psi vs 59.595 at zero fuel flow. Ford has already done this calculation. The high slope multiplier is .9269 at 54.97psi.
So, at 100% pulse width on the injectors pressure drops to 55psi and fuel volume is calculated to 5.428 lb/min at 100% injector duty cycle.
Now that we have the maximum fuel flow at 100% injector duty cycle we can calculate maximum airflow the injectors can support, the variables being stoich of the fuel and the programmed lambda at WOT.
Stoich of the average ethanol blended gasoline in the US is calculated to be 14.08 by Ford. If you know the lambda commanded at WOT the math gets very simple.
Lambda at WOT * Stoich * fuel flow = lb/min Airflow
Below are examples:
.820 lambda * 14.08 * 5.428 = 62.6
So, on average gasoline with a commanded lambda of .820 at WOT the injectors can support 62.6 lb/min airflow.
Now lets look at E85:
.850 lambda * 9.7 * 5.428 = 44.75
On true E85 at 100% duty cycle on the stock injectors they can support 44.75lb/min airflow at .850 Lambda.
Looks pretty good right? Well, not really once we start looking at a few other variables.
1. Factory injector tolerance is +/- 6%
2. Factory fuel pressure regulators vary as well
3. Running at 100% duty cycle on the injectors leaves no room for error
4. Ethanol blends vary, we do see over 90% in the summer time in some places around the country
The tolerances can stack up against you just as easily as for you. 90% ethanol and 6% lower flowing factory injectors and you're out of fuel on a factory stock setup.
Food for thought, but it's why AED requires larger injectors on ALL coyote E85/Flex Fuel applications. Better safe than sorry is how we roll.
Ford has laid out the fuel flow characteristics of the stock injectors, in fact they use airflow via Maf readings to calculate fuel flow, so we can accurately calculate the maximum lb/min of airflow an injector can support given a known stoich.
The high slope of the injectors is programmed as 0.0121999997645617 in pounds per second. To convert to lb/min simply multiply by 60, in this case .7312. Multiply this # by 8 (for the number of injectors) and you have the total volume of fuel the injectors can flow at *full* programmed fuel pressure at 100% duty cycle.
.7312 * 8 = 5.856 lb/min
Now we have the total fuel volume at full fuel pressure, which is regulated at the fuel hat in the tank. On the 15+ cars Ford has set this pressure at 59.595 psi at zero fuel flow. As fuel flow increases there is a pressure drop at the injector (basic physics) due to the location of the fuel pressure regulator. Ford has calculated this loss and programmed it into their fueling model, but it is only scaled to 4.0 lb/min fuel flow, however this model is linear so we can calculate the fuel pressure at the injectors at 100% duty cycle (5.856 lb/min), which would be roughly 55psi.
At 100% injector flow capacity pressure at the injector has been reduced 55psi. So what does a drop in fuel pressure at the rail do to injector flow? We need to recalculate the fuel volume thru the injectors at 55psi vs 59.595 at zero fuel flow. Ford has already done this calculation. The high slope multiplier is .9269 at 54.97psi.
So, at 100% pulse width on the injectors pressure drops to 55psi and fuel volume is calculated to 5.428 lb/min at 100% injector duty cycle.
Now that we have the maximum fuel flow at 100% injector duty cycle we can calculate maximum airflow the injectors can support, the variables being stoich of the fuel and the programmed lambda at WOT.
Stoich of the average ethanol blended gasoline in the US is calculated to be 14.08 by Ford. If you know the lambda commanded at WOT the math gets very simple.
Lambda at WOT * Stoich * fuel flow = lb/min Airflow
Below are examples:
.820 lambda * 14.08 * 5.428 = 62.6
So, on average gasoline with a commanded lambda of .820 at WOT the injectors can support 62.6 lb/min airflow.
Now lets look at E85:
.850 lambda * 9.7 * 5.428 = 44.75
On true E85 at 100% duty cycle on the stock injectors they can support 44.75lb/min airflow at .850 Lambda.
Looks pretty good right? Well, not really once we start looking at a few other variables.
1. Factory injector tolerance is +/- 6%
2. Factory fuel pressure regulators vary as well
3. Running at 100% duty cycle on the injectors leaves no room for error
4. Ethanol blends vary, we do see over 90% in the summer time in some places around the country
The tolerances can stack up against you just as easily as for you. 90% ethanol and 6% lower flowing factory injectors and you're out of fuel on a factory stock setup.
Food for thought, but it's why AED requires larger injectors on ALL coyote E85/Flex Fuel applications. Better safe than sorry is how we roll.
