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> Fuel Pressure Regulators:  Pressure Regulators. Different types, why we need them, how they work!

Fuel Pressure Regulator.

What is a regulator?

A very simple fixed pressure non adjustable regulator pictured.
This is a 3 port type. These have a connection to the inlet tract via the small pipe fitting on the right, to keep fuel pressure at a fixed value ABOVE inlet manifold pressure. >>>

A fuel pressure regulator is a small device with a diaphragm inside that maintains a constant pressure (or tries to) regardless of the rate of flow. Basically you can put any fuel pressure in, and get a constant "set" pressure out - however much the input pressure might vary. So a Fuel injected car for example has a pump that might run at anything between 150psi static, and 50psi under flow conditions in use. This is "regulated" by your fuel pressure regulator to (usually) 45psi constant (3 bar). 


There are a few different types of regulators. And fuel systems.

We will consider the types and how they work and why they are used. This stuffs important! You need to know it so try to keep up!

Carb Fuel systems
Fuel Injection Systems

An adjustable one...                   simple 2 port adjustable regulator                        Adjustable 2 port

Facet "Red Top" fuel pump

Carb Systems

  • Old style engine driven mechanical pumps without a regulator. Do yourself a favour, throw it away! They are subject to wear, lack of flow, and with modern fuels, "vapour locks" at least in summer... Or at least disconnect it, and add a modern cheap powerful electric pump instead. Reliable fuel flow is VERY important with Nitrous! And add a simple two pipe (two port) pressure regulator. There are lots of brands, like Filter-king, Morosso, etc. Set the pressure regulator to 3 psi. Those old mechanical standard pumps are NOT suitable for Nitrous unless you are sadly desperate.

  • Electric fuel pump with no regulator. This may well be suitable if its a bit larger than the engine alone requires. It is best to fit a simple two port fuel pressure regulator like Filter-king, Morosso, etc. Set the pressure regulator to 3psi maximum as this is what most carbs require and the pump if it worked ok may not be able to keep a higher pressure than this under high  flow conditions.


  • If your bike/car is turboed with a DRAW THROUGH carb, then you will do the same as the above. The turbo in this instance places the same "pressure" demands on the fuel system as a naturally aspirated motor. In this case you will need to inject the fuel/nitrous before the turbo!

  • If your bike/car is turboed with a blow through system then you must use a 3 pipe (3 port) regulator and a much higher pressure pump. A three port regulator maintains its set pressure (usually 3 to 5 psi) ABOVE the inlet manifolds pressure. It does this because the third smaller pipe that is connected to the TOP of the diaphragm is also connected to the inlet system after the turbocharger. Now, since the carbs float bowl, and the top of the pressure regulator are connected to the engine side of the turbo they both see the boost (typically 1 to 2 bar) as the turbo spins up. To feed fuel via the carbs, and via the Nitrous system this has to be the case. Its not possible to feed fuel to a carb, at 2 bar (45psi) plus three 3 to 5 psi extra because the carb's needle/float system couldn't cope with this pressure! This means that any correctly working blow through system, already has the required Pressure Regulator fitted. All you need to do is ensure that the pump can cope with the extra demands.

  • In ALL the above cases for BIG boosts, a separate pump and regulator just to supply the nitrous system alone may be needed. To find out run it flat out on the track or the dyno, with the bottle turned off, but operate the nitrous system. Route the fuel pipe into the car and collect the fuel, noting the engines performance and the fuel pressures!

Fuel Injection Systems

With fuel injection most of the worries about correct fuel pressures are over! This is generally well sorted out for you already. Below is a description so you can see how the fuel supply bit works! At least for small moderate non "race" applications.

  • Normal modern computerised fuel injection non turbo. Some vehicles vary from this "convention" but very few! Most use common fuel injection components nowadays, so its possible to give an easy overall description. The fuel pump provides around 100psi and adequate flow for the "biggest engines" in the range on that specific model of your chosen car. It lives inside of / right next to the fuel tank! It supplies fuel to the fuel rail that feeds your fuel injectors. It does this while the engine is running, continually, and the pressure on the fuel line that feeds the fuel rail is around 3bar or 45 psi. This is because there is already a regulator on the fuel rail, and it simply returns the excess fuel at low pressure back to the tank via a second fuel pipe. Good eh?

    So all you need to do to get a 3 bar fuel supply is to tap into the supply line that feeds the fuel rail. Do this with care because at 45psi a small leak can be a big fire!  This regulator is one of two types. It CAN be 2 port, meaning it sets 3 bar pressure against atmospheric pressure. Or it can be 3 port type that has the top small pipe connected to the inlet manifold. This means it gives 3 bar of pressure compared to the inlet manifolds "vacuum" so to speak. It does not matter to us! Since Nitrous is only used at full throttle the inlet manifolds depression will be not far removed from atmospheric pressure anyway.

    A turbo engine will have to have a 3 port compensated type regulator anyway though, or as boost increases the fuel supply will fall! So there is little difference fitting nitrous to a turbo car compared to a naturally aspirated one.

  • If you want big power increases, then a separate pump and regulator may well be required. The stock system is not designed to cope with the demand for fuel, and pressure will fall with disastrous consequences. To find out run it flat out on the track or the dyno, with the bottle turned off, but operate the nitrous system. Route the fuel pipe into the car and collect the fuel, noting the engines performance and the fuel pressures!

Diesels and Turbo Diesels

  • A nitrous system for a Diesel is a special case. You DO NOT need or want a Fuel solenoid... If you fire any fuel into the intake system you will probably drive over your crankshaft. You fire only Nitrous into the intake. Then you need to figure out a way to fool your stock fuel system to give you more fuel! Normally this will cause lots of black smoke but little if any extra power. Because the Oxygen needed to burn it was not there. But the Nitrous provides the Oxygen... Don't forget you may need to retard you timing!  In addition it may help to add some Propane.

Diesel Nitrous System

*Technically the term NOS is incorrect as this refers to an specific company (called Nitrous Oxide Systems, in the US)





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