How To Set Dynamic Fuel Pressure For Nitrous

Checking dynamic fuel pressure is the only way to know for sure that the correct pressure is pushing on the fuel. Nitrous can be unforgiving for those who discover only too late that the system was a bit too lean because the fuel pressure was incorrectly set.

By Jeff Smith

The military has a saying that you train like you fight and fight like you train. The same can be said for tuning high performance engines. An essential tuning requirement to ensure your engine is safe is you must ensure that the fuel pressure and volume is sufficient when the engine is under load at rpm – not idling in your shop. Rather than make estimated guesses or make multiple passes to get the fuel pressure dialed in properly – there’s a much easier way. ZEX offers a dynamic fuel pressure test gauge that will allow a nitrous racer to accurately dial in his fuel pressure. The tool is affordable and, with the engine not running, can accurately simulate the amount of fuel used by a nitrous system and the pressure delivered by the system under load.

First, let’s cover some nitrous basics. All plate nitrous systems are designed to deliver a given amount of fuel along with the nitrous. The tune is determined not just by the size of the supplied nitrous and fuel jets but also relies heavily on proper pressures for both nitrous and fuel. As an example, the standard ZEX plate nitrous system (PN 82040) uses a combination of a 62 nitrous and 53 fuel jets for a 150 hp tune with 6 psi of fuel pressure and 950 psi nitrous pressure. Common sense dictates that if the nitrous or fuel pressures change, the flow rates will also change. On the nitrous side, ZEX assumes the bottle pressure will be 950 psi. On the fuel side, the tuning recommendation for the 150 hp kit demands a fuel pressure of 6 psi. You might also note that other nitrous companies will use a different combination of jetting for the same horsepower setting. This is because their tune may only demand 5 or 5.5 psi as a base fuel pressure. Generally speaking, a higher base fuel pressure will require a smaller jet to flow the same volume of fuel compared to a lower fuel pressure. As for nitrous pressure, most systems operate with a base pressure of 950 psi. There’s a reason that tuners should avoid higher pressures, but we’ll save that discussion for a separate story.

Where many enthusiasts can get into trouble is that they assume that fuel pressure is the same whether the engine is idling or making full power. With nitrous, this can be a very costly oversight. When an engine is idling, it is using very little fuel so almost any fuel pump can supply both pressure and fuel flow. But under load, there are dozens of variables that can affect fuel pressure ranging from a slightly undersized pump, a hydraulic restriction in the line such as too many 90-degree fittings, or perhaps the fuel filter is a restriction. The result of any combination of these situations is that as demand for fuel increases, the pressure will drop. This reduces the overall volume which means the engine runs lean. Under nitrous, this can result – at best – in a loss of power. At worst, this can cause melted spark plugs, melted pistons, and other equally nasty repercussions. To avoid these expensive lessons, it should be obvious that dynamically testing the fuel delivery system is a smart move before you squeeze the nitrous button.

Fuel Delivery Systems

Carbureted plate type, wet nitrous systems are the most popular systems on the market so we will deal with that kind of a system in this story. But the basics of checking dynamic fuel pressure are still the same. You may have to change the way you test, but ensuring that the fuel delivery system is fully functional remains the ultimate goal. Most nitrous system users tap into the existing fuel delivery system so that both the engine and the nitrous feed off this one system. Slightly more sophisticated is a dual system where one pump feeds the engine and a separate pump and regulator feed the nitrous system. We will deal with both, but let’s start with how to test fuel pressure on a dedicated fuel delivery system. Then we’ll come back to a single fuel system and how to test that.

Most nitrous fuel delivery systems are designed as dead head systems. This is where the pump picks up fuel from the tank, preferably through a pre-filter, through a second filter, and up to the fuel pressure regulator before it reaches the fuel solenoid. This is a very simple system that is what the ZEX tool was designed to test. Merely turning on the fuel pump and measuring the pressure with the pump pushing against a closed fuel solenoid tells us very little. This will produce the maximum amount of pressure that the pump can provide but this pressure will drop dramatically as soon as fuel flow begins when the fuel solenoid opens. The ZEX tool simulates this action of an operating fuel solenoid and flowing fuel through the jet.

The ZEX tool consists of an accurate 0-15 psi pressure gauge connected to a running tee. One end of the tee is connected to the outlet from the regulator to the fuel solenoid. The other end of the tee run uses a braided hose with a fitting on the end that will accept the fuel jet which will exit and be dumped into a catch can. The idea is as simple as it is elegant. The best way to determine the dynamic fuel pressure is to flow the same amount of fuel as the nitrous system will demand. All we have to do is install the fuel pressure gauge in between the pressure regulator outlet and the nitrous jet. With the fuel pump delivering fuel, it is a simple process to adjust the fuel pressure to the desired spec.

Voltage Variables

Once we sat down to write this story, we realized that we had inadvertently omitted an important variable. All this testing with the car static and engine not running left out the minor fact that the charging system was not operating. This means that when we ran the pump at 12.5 volts isn’t the same as 14.0 volts that the battery sees when the engine is running and the alternator charging. We went back to Greg’s car and hooked up a battery charger that put 13.9 volts into the battery and then ran our dynamic pressure test again with identical results. The best thing is to measure the battery voltage while performing your test to ensure you have sufficient voltage. Otherwise, low battery voltage could cause a reduction in pump capacity and slightly higher nitrous fuel pressure when the alternator is supplying 14 volts of system voltage. Of course, if your race car is using a 16-volt system, you will need to test at that voltage since an electric fuel pump will supply substantially more fuel at 16 volts than it does at 12.5.

Dynamic Testing

We decided to test this system on a friend Greg Smith’s 434ci small-block ’55 Chevy. The small-block makes 680 hp on pump gas and the car runs deep in the 10’s but for a little added kick, Greg also has a 150 hp nitrous kit. The engine is fed by a full return style fuel system using a sumped tank and an Aeromotive A-1000 pump running 6 psi of fuel pressure. Rather than tap into that system (which would have worked fine) for the nitrous, Greg built an entirely separate fuel system using a small, Holley (70 gph at 9 psi) pump and a pressure regulator. The pump has sufficient capacity to deliver over 750 hp worth of fuel at 9 psi so 6 psi worth of fuel for a 150 hp nitrous system will be no problem.

Once we connected the ZEX pressure gauge to check the pressure, we expected to see 5 to 5.5 psi. But instead, the gauge did not register. We had fuel flow under what appeared to be more than sufficient pressure, but the gauge indicated zero pressure. Then we remembered reading that if the pressure gauge had been subjected to high temperatures – above 100 degrees F, the liquid inside the gauge expands and exerts a force on the mechanical movement in the gauge. We noticed that the normal air bubble in the gauge was no longer present. So we removed the sealed rubber inlet where the glycerin is added, which equalized the pressure. We also drained a small amount of glycerin to create an air bubble. We then reconnected the gauge and it was now reading properly. Greg had been running slightly more than 6 psi of fuel pressure but by adjusting the pressure regulator, we dropped it down to 5.5 psi.

In our second system test, we decided to look at the fuel pressure for our 4.8L LS motor in our ’66 Chevelle. This fuel delivery system is designed a little differently. The car uses a Weldon Racing fuel pump mounted at the rear of the car with a complete return system using TechAFX PTFE fuel lines. This delivery system is a bit different in that it uses two regulators. The first regulator sets the system pressure at 12 psi, returning unused fuel to the tank. The outlet of this main regulator is connected a second, dead head regulator that feeds both the carburetor and the nitrous system. Since the pump has sufficient capacity to feed over 800 hp, we did not see the need to create a second delivery system when we added the nitrous system. The way the system is currently configured, we used our ZEX gauge to check the accuracy of our Auto Meter gauge to make sure that our dynamic fuel pressure was accurate. Unfortunately, this test isn’t completely accurate since the engine is not under load, only the nitrous portion is flowing fuel. Another limitation to this configuration is that adjusting the nitrous fuel pressure also lowers or raises the pressure to the carburetor. This is a minor issue that has not caused any problems while running 5.5 psi pressure for the nitrous. We could have configured this system where the return regulator would control the fuel pressure for the carburetor while the second regulator would manage the pressure for the nitrous. This way we could manage the nitrous fuel pressure independently of the fuel pressure to the carburetor.

Conclusion

While this description may sound a bit complex, once you’ve run through the test once you will see how easy it is to complete. As with many processes – it takes longer to set it all up than it does to execute. But if you run nitrous, the tune has to be as accurate as possible and that means setting dynamic fuel pressure is the only way to get there. Your engine will thank you.