Justin@VMP
February 9th, 2008, 12:13 PM
I worked on a 19psi/3inch pulley whipple 03/04 cobra a few weeks ago.
Before I tuned it, this car had some WOT A/F inconsistencies that were showing on the A/F graph. After some further datalogging I realized that fuel pressure was bouncing all over the place. The oscillation was so bad that the computer could not control pressure, and thus A/F was all over (this erratic A/F was seen very clearly with a WBO2 installed directly in an exhaust bung before the cats). The yellow line is fuel pressure, black line is rpm, and blue line is A/F from my AFM1000.
http://www.vmptuning.com/dynographs/johnmysticfp.jpg
This car was pegging the factory MAP sensor. A few changes later, this was fixed, and the fuel pressure and thus A/F were under control again. When the factory MAP (boost) sensor pegs on a high boost car the computer thinks the sensor has failed, so it ignores it and thinks there is no boost. This results in the computer jumping up and down on the Y axis of the fuel pump voltage table as seen in the yellow line below. The computer uses a PID controller to run the fuel pump on a returnless car, but it still needs a good reference point to start with, and thats where the fuel pump voltage table comes in.
The Cobras and FGTs are unique because they use a MAP sensor to measure boost, this is important to reference the correct row from the fuel pump voltage table. All other Fords use ineferred/calculated MAP, including the GT500. On the naturally aspirated 05+ PPC Fords SCT has been able to modify the code so the computer will see boost and reference the correct line of the fuel pump voltage table, just like a GT500. I helped figure this out about 1.5yrs ago.
When the computer can properly use the Y axis of the fuel pump voltage table you will end up with much better fuel pressure control on a boost car, especially during transient situations. This is why tuning the voltage table properly helps the hesitation issue.
http://www.vmptuning.com/dynographs/johnmysticfp2.jpg
Before I tuned it, this car had some WOT A/F inconsistencies that were showing on the A/F graph. After some further datalogging I realized that fuel pressure was bouncing all over the place. The oscillation was so bad that the computer could not control pressure, and thus A/F was all over (this erratic A/F was seen very clearly with a WBO2 installed directly in an exhaust bung before the cats). The yellow line is fuel pressure, black line is rpm, and blue line is A/F from my AFM1000.
http://www.vmptuning.com/dynographs/johnmysticfp.jpg
This car was pegging the factory MAP sensor. A few changes later, this was fixed, and the fuel pressure and thus A/F were under control again. When the factory MAP (boost) sensor pegs on a high boost car the computer thinks the sensor has failed, so it ignores it and thinks there is no boost. This results in the computer jumping up and down on the Y axis of the fuel pump voltage table as seen in the yellow line below. The computer uses a PID controller to run the fuel pump on a returnless car, but it still needs a good reference point to start with, and thats where the fuel pump voltage table comes in.
The Cobras and FGTs are unique because they use a MAP sensor to measure boost, this is important to reference the correct row from the fuel pump voltage table. All other Fords use ineferred/calculated MAP, including the GT500. On the naturally aspirated 05+ PPC Fords SCT has been able to modify the code so the computer will see boost and reference the correct line of the fuel pump voltage table, just like a GT500. I helped figure this out about 1.5yrs ago.
When the computer can properly use the Y axis of the fuel pump voltage table you will end up with much better fuel pressure control on a boost car, especially during transient situations. This is why tuning the voltage table properly helps the hesitation issue.
http://www.vmptuning.com/dynographs/johnmysticfp2.jpg