This is what I found on VFAQ! And this is also the reason I am doing this upgrade.
Background As we look to get more power from our engines, we naturally want to get a greater air/fuel charge through the system. Lets consider how to increase fuel.
One common method to supply more fuel to the engine is to increase the pressure at the fuel pump by increasing the voltage. This definitely increases the pressure at the pump, but that doesn't necessarily translate to more volume at the fuel rail. This is because the whole system was designed to deliver the volume of fuel that the stock injectors need, and not much more. The same analysis holds for installing a high flow pump. The fact is that you can only push so much fuel through the 'coffee straw' stock system. Its a capacity problem.
With the help of a local performance shop with a liquid flow bench, I was able to run some flow tests on the stock fuel delivery system. It was found that from the fuel pump outlet to the fuel rail inlet fitting, @ 60psi, a flow of 1.9 liters per minute could be measured. When the pressure was increased to 75psi, the flow only went up by 0.1 lpm. That's only ~5% more flow for a 25% increase in pressure... not good. Couple that with the fact that stock injectors are 450 cc/m so that four of them can approach a flow of 1.9 lpm (4 x 450) , it can be seen that we are approaching the point where the injectors can outflow the fuel system.
What are the reasons for this low flow? There are several restrictive points in the system. Starting from the front and working back, they are:
* The fitting at the fuel rail only has a diameter of 5.5mm
* The banjo fitting at the outlet of the fuel filter.
* Fuel filter is a low-flow design (2 lpm).
* The steel fuel line is only 3/16" ID. [BTW: the -6AN fitting we'll be using has an actual measured ID of 5/16"]
After market pumps can develop a lot of pressure though, so what's the problem? One significant issue is based on how we tune our cars air/fuel ratio. Notice in figure 1 that in this case the fuel pressure sender is in the small diameter restrictive section of the delivery system. Now the diameter of the fuel rail is much larger than the fuel line leading up to it. So when the injectors are at a high enough duty cycle as to approach the maximum capacity of the preceding delivery system, the pressure in the fuel rail will see a drop off at each successive injector as indicated by the wavy blue lines. There will be enough pressure to deliver sufficient fuel for the first two cylinders and probably the third. But the last cylinder (#1) will be starved and thus risks a lean condition.
Since the EGT is usually tapped to the #1 runner, a lean condition is indicated by a high temperature in that cylinder. But typically, an 02 reading is also used to tune and the O2 value we see is based on all four cylinders. Hence we have been accustom to thinking that our lean threshold is much higher ( > .85V) than it needs to be. And our fuel pressure gauge has not given us a proper indication of the problem as it is far upstream and is not subject to the pressure drop off in the rail. It would be better to mount the fuel pressure sender at the end of the rail but that would be a difficult installation.
It is asserted that this leads to a loss of power because we effectively have two very rich cylinders, one about right and one lean. Because we have to protect the lean cylinder, we sacrifice power in the first two. The fuel pressure regulator also does not help alleviate this problem. Since it is at the end of the rail, it can only regulate the maximum pressure provided there is enough flow. It does nothing to correct the low pressure, low flow situation.
The solution is to upgrade the flow capacity of whole fuel delivery system to minimize the pressure differential seen by all four injectors. Or, put another way, a high flow pump and big injectors are great. But you've got to have big enough line to actually get the extra gas to the intake.
After the new fuel delivery system was designed and built, we took it back to the flow bench and found that it flowed ~10 lpm @ 60psi. Very nice :-)
A word of caution: The following procedure is pretty involved and is not recommended for those without a reasonable amount of mechanical knowledge and automotive repair experience.
We have been very pleased with the results obtained with this modification to our cars. In general, we have found that our EGTs are measurably lower and O2s a little higher which allows a greater tuning range. For those interested, our entire fuel and control system consists of:
* Stock DSM fuel pump
* SteveTek High Flow Delivery System
* 660 ND injectors
* APEXi SAFC
We find this combination to be very well integrated. Enjoy!
