Choosing a Fuel Pump and Regulator

Fuel pump first

You have already decided on the size of your fuel injectors and you know what fuel pressure you will run them at [at maximum load]. So that tells you the maximum fuel demands that you will place on the fuel pump. You need to find a pump that will flow at least enough fuel to supply all of the injectors at 100% DC [duty cycle] at the required fuel pressure. Since your fuel injectors likely won't be fully open all at the same time, you will end up getting a pump that is slightly oversized. But that is fine, you want to be absolutely sure that the pump can supply all the necessary fuel. I also suggest oversizing the pump even more, just to have an extra margin of safety. A fuel line might develop a small kink or you might see a voltage drop at the pump for some reason. Buy the right pump the first time. In order to do this right, you will want to get a pump characteristic curve from the vendor. It will show how much fuel the pump can pump at different pressures. Here is an example of a pump curve:

Pumps are usually rated in GPH [gallons per hour] and injectors are usually rated in cc/min [cubic centimeters per minute]. Here's the equation that sorts out the units and other details:

Now you have got the maximum fuel flow rate that the pump should have to supply and the pressure it must supply it at. Just draw a line up to the pump curve from the Pressure axis. Then draw a line across to the Flow Rate axis, starting at the point on the pump curve where you just drew the line up from the Pressure axis. The point where the horizontal line you drew hits the Flow Rate axis is the flow rate of the pump at that pressure. If the pump flow rate is higher than the maximum fuel flow rate that you calculated above, then you are all set.

Now the fuel pressure regulator

If you have a 1995 car you will probably want to disable your stock FPR [fuel pressure regulator]. If you have a 1996-1999 car, you will probably want to remove your stock FPR. With the addition of an adjustable FPR, you will have full control over your fuel pressures. You need to purchase a stand-alone FPR that can adjust static fuel pressure and increase fuel pressure with boost. Getting a FPR with a choice of calibration disks is also important. This will give you maximum flexibility when trying to build a suitable fuel curve with your new injectors and FPR. Static fuel pressure is the fuel pressure at 0 psi boost [atmospheric conditions]. At idle, the engine creates vacuum, so the idle fuel pressure will be lower than the static pressure. While boosting, the pressure in the intake manifold will be positive, so the boosted fuel pressure will be higher than the static fuel pressure. See the Fooling the S-AFC page for more information about measuring pressure. The calibration disks tell the FPR how much to increase the fuel pressure with every psi of boost. For example, a 6:1 disk will increase fuel pressure by 6 psi for every 1 psi of boost pressure. Personally, I really like the Vortech S-FMU. It is a stand-alone unit with a static adjustment, various disks, and the option of installing a bleed valve. The bleed valve can be used to "bleed off" some boost pressure at the FPR to fine tune the fuel curve. It's not cheap, but it is a nice unit. We'll assume that you bought a FPR that works similar to the Vortech S-FMU.

Here's that fuel pressure equation again. You're going to be sick of it soon. So you know what size injectors you bought and what fuel pressure you need at maximum load [full boost]. Now you need to figure out what fuel pressures will be right for the rest of the operating points [idle, cruise, partial boost]. For this next step to work, you will need some data from your previous fuel set-up. You will want the fuel pressure at several boost pressures [idle vacuum, 0 boost, low boost, high boost]. If you were using stock injectors on a bolt-on turbo kit, that stuff is in the instructions. You want this data, because you know that the car ran OK with these fuel pressures. The point in altering fuel pressures when you change injectors, is to provide about the same amount of fuel that you had before the change. This way, you keep the fuel flow within a range that the ECU can still control. If you know what fuel pressures [at each manifold pressure] made your car run before you put in the new injectors, you can calculate approximate flow rates. This will let you build an approximate fuel curve versus intake manifold pressure. Since the duty cycle is low when you're not at full throttle, the flow rate calculations don't really represent how much fuel is going in, but how much fuel the ECU has to work with. Its just a good starting point for figuring out how to make your new injectors work right. Try to build the new fuel curve so that it looks like the original fuel curve. You may have to be very creative to accomplish this. An S-AFC can be used to alter the fuel curve even more. Leaning X% with a S-AFC actually changes the fuel flow rate by almost the same X%. You can use the S-AFC to lower an entire fuel curve or just modify part of it. Earlier, you chose an injector based on a duty cycle less than 100% [more like 80-85%]. The ECU will try to drive this injector at nearly 100% if you don't tell it not to. You need a S-AFC to fool the ECU into running a lower duty cycle. Fooling the S-AFC by making it adjust fuel relative to intake manifold pressure instead of throttle angle is a modification that I think is very worthwhile. Read that page for more details about that.

So now you have some baseline data from your previous fuel setup. You've built a graph that looks like this. Like I said earlier, the object is to try to build the new fuel curve so that it looks like the old one. It is a good idea to start by matching up the static pressure point [0 boost]. Figure out what fuel pressure will make the new injector supply the same amount of fuel at that point. Then subract the new static pressure from your fuel pressure at maximum load [full boost]. Divide that number by the boost pressure at full boost. That tells you what calibration disk will give you the proper fuel at 0 boost and full boost. If that number falls between 2 calibration disks, choose the larger one. So if you calculate that you need 6.58 psi of fuel for every 1 psi of boost and you have a choice of 6:1 and 7:1 disks, then choose the 7:1 disk. You can use a bleed valve or S-AFC to lean things out some. There isn't much you can do to change the fuel pressure at idle without changing the static fuel pressure. You can just use the S-AFC to adjust the fuel at that point. The ECU can adjust for any small errors in fuel flow rate at the idle point and other low loads. See the MFI system page for more details on that. Also check out the Example problem page that shows a fuel system built from start to finish.

Contributed by Corbin
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Cars Modifications Power Turbo Fuel_System Choosing a Fuel Pump and Fuel Pressure Regulator

Document statistics: Last modified on 2010-10-26 15:07:46 by DarkOne