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> Nitrous Oxide Solenoid Valve:  Build your own Nitrous Solenoids from cheap air solenoid valves!

Nitrous Solenoid DIY modifications and why you need them! There's a lot of serious detail on this long page that's essential if you plant to build your own systems.
(Serious DIY Build and Modify details a long way down this page!) 




First a very rough schematic so you can see what goes where!


Controlling the fuel and nitrous flow
The on / off control is taken care of electrically, as both solenoids (the on off valves) will be automatically 'off' unless an electricity supply is turned on. On a bike this can be the horn or starter button, or a small micro switch on the throttle so as to activate only at WOT (wide open throttle) conditions. An arming switch on the dash, or anywhere accessible is also required so the system only works when you decide to "arm" it..

Actual power increase provided depends on the amount of Nitrous Oxide and additional fuel delivered to the motor, and this will be a fixed constant amount regardless of engine's rpm.

The amount of Nitrous Oxide delivered depends on the size of the 'jet' fitted into the Nitrous Solenoids outlet. This jet is fitted into the back of the 1/4" BSP to 4mm OD nylon fitting required to connect the 4mm OD blue nylon pipe to the Solenoid Valve.

The best jets to use are 'Weber' carburettor jets available from many places. These are like cheese head screws, with a 5mm metric thread and a hole down the middle! (So if you can drill a hole down a screw you can make your own, as I do now. Actually M5 short stainless cap headed (Allen heads) are ideal jets once drilled. In all cases these jets like Japanese Mikuni jets / Weber jets,  are all marked in Millimetres ID, or bore.  So a 125 main jet on a bike is 1.25mm bore.

These cheap plastic 1/8th BSP to 4mm nylon tube push and lock fittings are also available in brass or plated brass and with nuts and olives for greater security, but these work just fine if you keep your eye on them!

These screw into the outlets of the two Fuel and Nitrous solenoids One is shown drilled and tapped to accept the nitrous flow control jet  - Actually its a Weber carburettor main jet from the local ford dealership...

Now the fun bit..  Nitrous can give literally as much extra power as you want, with the limiting factor being only "detonation" or physical strength of gearbox or whichever bit breaks first!  THIS IS THE ONLY REAL DANGER! TEMPTATION!

Most healthy modern engines can cope easily with a 25% to 40% increase without any other changes.

So an average Japanese bike that makes 120 BHP at the back wheel will (should!) take a 40BHP increase easily, (and usually, much, much more!) Lean mixtures and over advanced ignitions are what does the damage.  So retard the ignition by a good few degrees to begin with, and make very sure your nitrous system is set over rich to start with. The CORRECT timing and the one that gives the best power will be more retarded than stock settings because you have more of everything in the combustion chamber!

This means effectively higher compression.  This also means that less initial advance will be needed as the oxygen rich, more densely packed cylinder also burns much faster.  The faster pressure rise means a more retarded ignition will be needed, to prevent detonation. Also use smaller plug gaps with stock ignition systems, colder plugs, and higher octane fuel all to prevent the possibility of detonation. Just in case!

How much power will a given jet give? 
Click HERE for PDF document

You will need this chart if you plan to build your own systems...

Buy a cheap small drill set! And a handheld "pin" drill. Now you can drill ANY jet you might want in a couple of minutes!

Tiny M5 Alloy anodised Allen screws make excellent jets! (If you cannot buy the Weber ones locally.  Remember a "jet" is simply a screw with a small hole in it, that's all!  Nothing technical here!

Now using my chart you can see that its easy to work out how much fuel you will need for any given nitrous jet size. But remember that this is a safe/rich guide only on most non turbo motors. Weather you get this fuel volume delivered by low pressure by a big fuel jet, or by using a very small jet and tapping into the fuel rail on a modern fuel injection car (3 bar) makes no difference!

The fuel flow is controlled via a similar jet in the same position in the fuel solenoid. At 10 psi regulated fuel pressure the correct fuel jet is about the same size as the nitrous jet used - but every installation is different. Mixture setting : Read this carefully!

A single injection point usually gives better distribution, as flow is constant and each cylinder inducts at different times. No jets are needed at the injection point only the ones fitted at the solenoids are required. If more than 1 point of injection is used then a distribution block or several tee joints will be needed. All pipes going to the injectors must be the same length.

Inline X2 X3 and X4 plastic push lock fittings are available for the 4mm plastic / nylon tube and these are ideal, but several tees are also ok - see pic! All compressed air/pneumatics dealers keep these kinds of fittings for industry.

Pipework from Solenoid Valves to Engine
Inlet manifold or cylinder head or carb rubbers must be tapped M5 to allow the fuel and nitrous injection points to be added.

Fittings with a nut and olive or push lock fittings can be used, available for industrial airline or hydraulic system use. These fit the ends of your 4mm OD nylon pipes and have an M5 male thread. The Nitrous ones must be fitted in such a way as to atomize the fuel and carry it into the engine. This may require a little thought before you drill any holes!! It may be possible to use just one nitrous and one fuel injector for the whole motor or a pair of each, if fuel injected or turbocharged, depending on layout and throttle bodies etc. Bikes will normally need to have one Nitrous and one fuel injector per cylinder, unless turboed.

A single injection point usually gives better distribution, as flow is constant and each cylinder inducts at different times. No jets are needed at the injection point only the ones fitted at the solenoids are required. If more than 1 point of injection is used then a distribution block or several tee joints will be needed. All pipes going to the injectors must be the same length.

Inline X2 X3 and X4 plastic push lock fittings are available for the 4mm plastic / nylon tube and these are ideal, but several tees are also ok - see pic! All compressed air/pneumatics dealers keep these kinds of fittings for industry. See below...


M&H Pneumatics in Grimsby (Where I live) keep all the pipe, air fittings, solenoids etc that I use.

Why jet it at the solenoids outlet instead of the "Foggers" ???

Lots of people keep asking why it is jetted at the solenoids themselves rather that at the point of injection (as all US based commercial systems are). HighPower Nitrous systems are also jetted at the solenoids as we all understand why this is far superior. HighPower did this first, well before anyone else did. Me, I just looked and saw all the advantages!

THE REASONS? Basically as follows:

Fuel 1st ?  Firstly there is no air or anything in the fuel line before the solenoid. So it does not atomise at all here - JUST SOLID LIQUID FUEL - the jet just controls the flow.  It does this equally well if at the point of injection, (Like NOS etc) or at the solenoid's outlet. 

If there is any air in the lines after the fuel solenoid due to the pulsing from the motor as different cylinders induct at different times, - (and there will be) - then the fuel cannot reach the motor until it has pushed all this air through the small fuel jet at the inlet manifold end (nozzle?) on US style systems. This increases the time it takes to for it to arrive at the motor. This means it can go lean for an instant, when you hit the button on US style systems. This can start destructive detonation off nicely even if you don't notice the delay. Or it can result in an instant intake backfire as you hit the Nitrous button off the line, due to the initial weak mixture. 

If it's jetted at the solenoid's valve outlet instead, then the correct flow of fuel starts straight away as there is no restrictive jet at the end of the line that you have to first push all the air through.  It ALWAYS takes the same time to reach the engine. That is its more consistent.

Nitrous 2nd ? If you are trying to get reliability and consistency then the nitrous too must be jetted at the solenoid!  Here the jet is controlling fully dense liquid Nitrous Oxide direct from the bottle. It is still liquid as the solenoid is supplied by a larger pipe than its rated flow and it is cool, not attached to the hot engine or going through hot areas... If the metering flow jet is installed here then you are measuring a known quantity and density of Nitrous.  Every time.

Alternatively if you fit these metering jets near the hot (especially at the strip in summer!) motor / engine bay then the pipe work after the solenoid is both long and hot due to the engines heat, and the hot engine bay area.

This is much worse when sat on the start line due to zero airflow.  Nitrous (no longer sat as a liquid at the correct pressure to keep it liquid in the bottle) is in a long pipe hot pipe! So it boils, expands and "foams" as it travels towards the metering jets on the engine. So you are now metering what? A liquid? Or Gas? Or a VARIABLE  mixture of both!  In reality in unknown and varying proportions depend on temperatures and length of the pipe work etc.

Eventually (a few secs with low thermal mass nylon tube) the pipe is cooled so the liquid now flows all the way to the metering jets at the nozzle, and you get to meter a much more dense liquid so it runs leaner later on... So it is far more accurate to meter it at the solenoids outlet where it is still a liquid. Here the density remains pretty constant at all times.

Fitting the metering jets at the "end of the line" will obviously work, as all the US based NOS / Nitrous Express etc; systems show, but it is simply not as good a solution. I know I tried it both ways and did lots of testing. Plus... More importantly still:

If you decide to use a Nitrous controller that pulses the solenoids to control the power, with the jet fitted directly to the solenoid the amount of nitrous you get is proportional to the length of the pulse width. So 50 percent open time will give half the Nitrous flow..
If the jetting is at the inlet manifold end / port, then the size of the solenoids seat is the limiting factor - not the jet, as the pipework becomes a Nitrous "reservoir" that the solenoid just keeps topping up...

This means that for example a solenoid rated at 150bhp maximum flow would flow around 75bhp of Nitrous at a 50% pulse width. If the jet on the end of this pipe is a 25bhp jet for e.g., then the solenoid just keeps topping up the reservoir, (the pipe work) so you would get about 20bhp at a 50 percent setting! This is not too serious unless you consider that the fuel is not compressible so it will be giving a true 50 percent... Melted pistons anyone??

Another quick description!

This was in reply to an email question.

Because in the beginning there was NOS...

Then came a massive multitude of clone systems designed by someone without a brain!

Trevor Langfield (The Wizard of NOS  or High-power Nitrous Systems) also jets at the solenoid - he did it first.  Here is why...

1) Fuel line gets full of air as the fuel drains out of the (oversized) line. Now when the solenoid opens it has to push out the air first, sometimes taking a full second!  During this time you are metering only air.  And because of the often hot (engine bay in summer!) lines it gets worse!  The fuel can boil and vaporise along the hot line and some jets get fed only fuel vapour for some time!  And the higher the thermal mass of the often metal braided lines the longer this happens for.  So apart from the air being in the way, the temperature also makes a difference! Sometimes a very expensive (nitrous only = backfire) one!

2) The fuel line isn't the only problem!  The nitrous line also has problems.  Now on a cold day the liquid nitrous will most likely stay liquid (mostly) all the way to the jet.  Except that it gives off gas (boils) as it goes from the solenoid into the lines and boils off further as it goes through distribution blocks etc.  Add to this the fact that the lines are hot in summer under the bonnet, and you can see that you have no real idea quite how dense (how "liquid") the foaming nitrous oxide now is!  Its density varies between pure dense liquid, and pure gas.  And everything in between.  Remember you are trying to meter the weight of Nitrous delivered accurately!  But are you metering vapour, gas, or a mixture of the two?  Who knows. It depends on Pipe volume, Pipe thermal mass, pipe temperature. Pipe internal volume.  Now to keep accurate control of the mixture from when you hit the button.  After the nitrous cools the line down (by the very act of boiling off and reducing density) it will eventually reach the jet as a say 90 percent dense liquid.  All depends on too many things!

Typically what happens on a cold day / cold engine bay is this.

Press button LIQUID Nitrous reaches the engine in say .3 of a sec, because its all cool.  You meter the full 100bhp of nitrous.  The fuel though takes up to a say half sec (or more) to push all the air out of the oversized lines through the jets before it arrives.  So initially very weak mixture that can start off detonation that never goes away! Or could cause a weak intake "backfire"...

Hot day/engine bay.  Hit button.  Fuel gets there and is flowing properly in 0.5 but the nitrous takes ages to "get going" because you are only metering gas or vapour (thousands of times less dense) for anything up to 3 secs!  You see this a lot.  Recognised by a puff of black smoke at launch.  It just costs performance.

Solenoids - Modifying and Buying Details..
The Solenoid valves that I use are available from the same type of industrial compressed air / hydraulic suppliers as the other fittings and nylon tubing. They are quite common in factories. They have an inlet at one end and an outlet at the other so are axial rather than the NOS type. This is neither an advantage or a disadvantage as I see it.

The solenoids that I use, are very simple pneumatics (compressed air) industrial ones. They are available in different makes, materials and styles but are rated at only 140psi. This is fine and can be used unmodified for Petroleum, Methanol etc, but MUST be modified to work at much higher pressures of Nitrous Oxide. For this reason, to work with 12V dc automotive systems, a stock 12V is fine for fuel, but a 6V dc one must be obtained for Nitrous use. A 12V one will work but only up to around 70bhp extra, where a 6V one will allow a larger seat area to be used and is completely reliable even on 12V supply (because cold liquid Nitrous Oxide flows through it and cools it) so can go up to 140 brake horsepower extra. A 12V and 6Volt electromagnet is shown here, both fit the same actual valve body.

These solenoids came from a small factory unit in Grimsby in the UK called M&H Pneumatics, call them on +44 (0)1472 241370 and fax on same codes but 346402. They are not on the net... 

Although these people may be able to help http://www.kvautomation.co.uk/ even with a ready to go solution! At a price. But you will need to order large quantities.

Or maybe here http://www.aeroconsystems.com/plumbing/solenoids.htm )

Similar ones are made by many manufacturers around the world, and have seen a few that could also be easily modified. Nitrous ready ones can also be bought, according to a recent email, contents here.


The fuel and the nitrous are both "jetted" to control the amount of power increase given. The jets are in the form of a simple carb type jet, in this case a Weber carb jet, and these are fitted into the rear part of the pipe fitting that screws into the outlet of the fuel and nitrous solenoids. All three shown serve the exact same purpose.

All are drilled and tapped to take a "control jet" and are standard easily obtained fittings. The all plastic one is cheapest and cannot corrode, and has low thermal mass but the brass one is safer as it uses nuts and olives to hold the nylon pipe rather than simple push/lock fittings. All accept 4mm nylon tube, and are male 1/8th BSP thread, to fit the outlets of the solenoid valves.

This is a Weber carburettor jet (1mm bore or 100 jet size, so limits the nitrous flow to 70bhp). The rear of the plastic fitting is drilled and tapped M5 thread and countersunk to allow the jet to fit flush.  
Half unscrewed...  
Completely unscrewed! This "jet" can be a brass M5 grub screw, or M4 Alloy Allen headed set screw, or whatever, but drilled to form a jet. Small drill sets are much cheaper than buying bucketfuls of ready made jets!  
The fitting here is about to be fitted to a standard solenoid for the enrichment fuel. This is jetted in exactly the same as the nitrous one. In the outlet.  
1/8th BSP 1/4 inch "tail" for the petrol pipe goes in the end inlet end of the fuel solenoid..  
The internal "bore" of a stock solenoid is 1/8th inch, or 3mm - you can see this by looking down the end of the valve.  
Here is the same view of my modified one, with smaller new seat fitted.  
Here is a modified one (similar view of unmodified one 9 pictures above!) to make it work at up to 1100psi, and with liquid Nitrous Oxide) It has to have a much smaller seat area. The old one needs to be drilled out, and a much more accurate and much smaller one machining up, and soft soldering in place. The plunger or piston on a stock one also has to be modified. The stock rubber type material is fine for fuel, or air, but will not work with nitrous! Check out the small nylon or PTFE black "Bic Biro" pen ends! there are two spare ones in the picture. Compare this to the photograph 9 places above, and also the new smaller seat area... (The ENLARGED bit!)  
Another picture of the new seat.  
Also detail of the modified Piston/plunger. The one with a "blob" sticking out is the modified one. PTFE works just as well, if no suitable pens can be found!  
This is the other end of the piston. The modified one has had to have about 3mm machined off the end to give the correct (1.5 times the bore of the new seat) lift.  
Ready to fit, tested at 1100psi down to 10 volts, and works reliably and never leaks. If it will not open, then your seat area is too large or the area of the "seat rim" is too wide. The pressure will then hold it closed even when you apply power.  
A simple nitrous injector/or nozzle. It has no jet. That lives at the solenoid outlet, this simply has to direct the flow in such a way as it collects and atomises the fuel as shown in the diagram. The nitrous must atomize the extra fuel well. The shiny bit is just a bit if brass tube from a local model shop, with a small 2mm hole drilled in the side, and the end soldered up. The plastic fitting has a M5 thread tapped in the bottom, and a 5mm grub screw screwed into it. This was drilled to accept the brass tube, and was all soldered together and tinned to make it silver at the same time.
Ok, for a clearer idea here is a fake inlet port or throttle body I made earlier! See the small hole in the side of the brass tube? it simply directs the nitrous down the port towards the engine. The fuel is added in front of it, so the nitrous collects it and atomises it. These fittings are a bit big, but I made them just for the photograph, for clarity. In reality I use various different methods to achieve the same result, and in any case small M5 fittings.  Often two small fittings can be fitted angled together slightly to achieve the same effect without any brass tube...
Pair of solenoids ready to fit to my V8 Sierra car. This was to begin with, so only 70 BHP extra. This meant a 1mm Nitrous Jet fitted into the fitting where the blue pipe is, and a 1.3mm jet fitted into the outlet of the fitting where the black pipe goes in the fuel solenoid valve. This was at 4 to 5psi of fuel pressure that also feeds the twin SU carburettors. The fuel solenoid was simply teed into the supply pipe. The nitrous solenoid obviously connects to the bottle in the back! (In this case with braided brake pipe.) Both of the 4mm pipes seen here are teed off (split two ways) to go to each one of the two carbs. In this installation the carbs were tapped and drilled in the bottom of each "mounting flange" so nothing could be seen! The solenoids were mounted near the cars fuse box and were not noticeable...
It really IS this simple!


A typical example for a bike motor drawn but same principles apply to any engine!



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


Web design & contents, Nitrous / fuel Jet Sizing, technical information, pictures, charts are all 1987 and on. Any theft will be pursued vigorously..



















































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