Fuel Pressure Up/Free Boost Mod

By Robert Pina

I will add more detail pictures soon


This is a modification I did on my ST165 All Trac to achieve a higher fuel pressure. It was basically for experimental purposes. The results of this mod have not been verified at all. The mod did give me a much better performance when switching from non boost to a boosting situation. It cured my slight "delay" for the fuel pressure to kick up. Your results may vary.


Your fuel pressure regulator works by sensing vacuum or boost and adjusting fuel pressure as necessary. There is a base fuel pressure, which is the fuel pressure at zero vac/boost (outside air pressure). If vacuum is applied the fuel pressure is further lowered. If boost is applied it raises the fuel pressure with respect to the amount of boost presented.

If you could make the fuel pressure regulator "see" more boost than you actually had, you could theoretically raise your fuel pressure slightly without expensive modifications. Why more fuel pressure? This is a trick used by many import tuners to compensate for some of the modifications to your engine such as air intake, etc. Also with stock injectors there is a point where a little more fuel pressure can grant you extra safety when boosting by causing the fuel injectors to spit out more fuel.

If you have a boost gauge on your car, you see a boost figure which might seem like the turbo is producing. Actually in our cars the turbo is producing 2 more PSI than what you ever see on your boost gauge. When the turbo boost goes through the intercooler some of the pressure is lost due to the restrictions of the intercooler. This is called pressure drop. You can verify this by plumbing your boost gauge into the turbo/wastegate hose and you will see how much more boost you are actually making.

So the idea is simple, since the turbo is "showing" 2 more psi than the intake manifold, plumbing the turbo pressure directly to the FPR will fool it into thinking more boost than is actually going into your engine. Since a turbo only produces positive pressure, your idling fuel pressure will be raised to the base figure. Let's say you are driving at 3 psi. Well your turbo is producing 5 psi and the FPR is pumping out more fuel for 5 psi. So all along the range you end up with a slightly higher fuel pressure. How much it is raised is still in question. I will perform this mod again on my new All Trac with a fuel pressure gauge to verify.

Finally since we know that the boost pressure at the manifold is 2 psi less than the turbo, we can trick the turbo wastegate into opening later than it normally would without the use of a boost controller. Since the wastegate no longer has a signal from the turbo, we take a reading from the manifold and gain 1.5-2 psi for free. This should raise your stock boost (assuming intake and exhaust) to about 10-11 psi.


The problem with doing this is that the turbo outlet "nipple" is much larger than the nipple that goes to the FPR. So you must find something small enough to fit on the FPR. At the same time it must be able to stretch over the turbo air nipple. I would get about 6 feet of suitable hose. Plus a T nipple to tap into the intake manifold. Finally if you don't have a boost gauge this would be a good time to get one. It would be a good idea to buy some very small clamps to hold the vacuum hose on. Sometimes under boost it is possible for the hose to be forced off.

1. There is a nipple here that is the original source for the FPR. To find out which one, unplug the vacuum hose from the FPR. Then run your hand along the path of the hose to behind the intake manifold where you cannot see. You will feel it end at some sort of plastic connector. This is the fuel pressure up VSV. Feel around it and coming out of the same device will be another vacuum hose that leads to the manifold. This is the source of vac/boost for the FPR. Unplug each hose from the manifold and the FPR. Now you have an air nipple on the manifold free which you can use for the boost gauge. As shown in line #1 the vacuum line would run from there to the inside of your car.

2. Now you want to run a vacuum line from the FPR all the way to the turbo. There is no easy way to do this as it has to travel over the entire engine. Disconnect the turbo to wastegate hose and save for a rainy day. Plug one end to the FPR and one end to the turbo outlet nipple that is coming right out from the turbo.

3. There is a large vacuum hose that is coming out of right side of the intake manifold. Cut this hose about halfway between the manifold and the other end. Insert the T nipple into each end, leaving you with free end. Run another vacuum line from the wastegate to this T pipe.

Clamp as necessary. Give the car a start and check for air leaks or hissing sound.

Upon driving you should notice much more of a kick when boost begins to start. The boost pressure should have raised by 1.5-2 psi. Note that your idle and starting characteristics may have changed. The duty of the Fuel Pressure VSV is to lower the fuel pressure at idle in normal conditions. When the car has been sitting for a few minutes and is still hot, the VSV cuts vacuum to FPR temporarily (raising fuel pressure) to make the car start easier.


I believe that this mod also causes the fuel pump to go into constant high mode. That or at least makes the high mode kick in very fast. The results of this mod are similar to the 14 volt fuel pump mod. After having the mod for a few weeks, I decided that it's not suited for daily driving as it simply made me get lower gas mileage. It is a good mod however for a day when you plan on racing or driving more aggressive. Any situation where gas mileage is not an issue.