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Two Stroke Piston - NITROUS!

> Engine Preparation:  How to prepare your engine for Nitrous Oxide Injection use.

Engine Preparation for Nitrous Injection

OK, so for small doses of nitrous and small power increases on road cars, we don't really need any special preparation...

But... (Isn't there ALWAYS a but!) There are a few very important things that you COULD and SHOULD do if you care about your motor and your success with Nitrous Oxide and want to do the job properly!


1. For road cars & big bikes with say 25BHP to 45BHP extra, a small increase, all that normally needs to be done is a few basic preparatory things. Best to do ALL of the following:

  • Make sure that it is running correctly! If possible, use a rolling road, with an experienced operator, and if not get an experienced mechanic to check it over...

  • Fit a new set of COLDER plugs. For example if your stock plugs are NGK-B7HS, for example, then use NGK-B8HS or 9 even, instead. This is one to two grades cooler. They need to be gapped about 10 to 20 percent less than stock plug gaps were originally. This has no "noticeable" effect on power, but means that when the Nitrous is operated, the standard ignition system can still cope with the higher effective compression ratio and still spark.  Also if your plugs are "projected nose" type, fit NON projected nose ones!  Ideally you want the shortest electrodes you can have! Imagine playing a blowtorch on your plugs. Which bit will glow first? Answer - the bottom earth electrode! Go for shorter ones... Or shorten the bottom earth strap and file any sharp edges and gap across the corner.
  • Use standard NGK plugs if possible. Avoid fancy expensive platinum, or multi electrode types.
  • Fit new QUALITY HT leads, Stock or preferably NGK plug leads because they are good!
  • Make damned sure your fuel pump is good enough! Drive flat out, open (operate) the fuel solenoid alone complete with its jet, and make sure the car or bike is:
    a) still performing properly,   
    b) That there is the correct amount of fuel still flowing from the fuel solenoid! - See the jetting sheet.  Pump too small equals melted engine when using Nitrous.

  • Make sure Distributor cap is clean (or new) Sparking is more "difficult" under the extra effective "compression" of Nitrous Oxide. So, if the spark can track somewhere else it probably will do! This can cause bangs, pops, backfires, and lack of power and flash backs that damage carbs and air boxes.
  • Change oil and filter - use high quality synthetic oil like Mobil 1 or Castrol RS Synthetic, or similar.
  • Check with compression tester to be sure no valve leaks exist. A leaky inlet valve can cause manifold explosions, and a leaky exhaust valve will glow and melt and can initialise the onset of detonation in that cylinder.
  • Check oil pressure against the specifications to try and rule out big end and main bearing wear or other related problems
  • Check gearbox/diff oil levels! - You should do this anyway!
  • HIGH OCTANE fuels. The best you can get. Super Unleaded, 5 star, or whatever has the highest Octane rating in your local fuel station!  This is important! Or if you want to add some additional octane booster that will also help protect your engine from detonation.
  • If a cooler thermostat and/or fan switch is available then fit them! A cooler engine will be less likely to detonate or pre ignite the mixture in the cylinders, and it should make more power too.



2. For anyone interested in going further than the 20% to 40% percent boost, like 50% upwards, up to maybe 300bhp and 500bhp+ boosts, for racing.  First read the section 1 above!!! Then consider this list.

A few other things are essential or worth considering - depending on engine and its purpose and how far you are going to "lean" on your engine...

  • Forged pistons with flat tops are the ideal for heavy Nitrous use. They should have the bore bored maybe an extra 1.5 thousandths of an inch oversized, plateau honed (bored and then only lightly honed to leave the boring tools "valleys" for the oil to sit in) Ideally you need a little extra running clearance because the pistons will get hotter than in stock applications and will expand more. Better also to choose pistons with stronger or thicker top ring located a little lower down the piston than a stock one.
  • Head Gaskets can fail due to higher pressures. The excessive pressure is often caused by the ignition timing being too far advanced when on the Nitrous, especially at low to medium RPM's. So if in doubt retard the ignition timing, at least at low to medium RPM!  If you are still having head gasket problems that are NOT related to your ignition timing then a stronger, copper or aluminium gasket may help. If not then O ringing the top of the block may be the only answer. Make sure you are using NEW studs/ head bolts where this is an issue. But its normally caused by too much advance! You want high AVERAGE cylinder pressure NOT high PEAK pressures!!!  High Peak pressures are only good for breaking engines and for causing detonation due to the rapid pressure spike before top dead centre.
  • Larger Exhaust Valves Getting the power and getting the oxygen and fuel in the cylinder is no longer a problem! So forget bigger inlet valves, big carbs etc. Stock inlet valve/cam is fine. But... Getting 3 times the amount of exhaust gas out through the same small exhaust valve is a problem...
  • Cams Stock inlet cam (or cam timing) is fine. As above we now no longer have an issue getting the charge into the cylinder. But we do have one getting it out! An exhaust cam (or cam timing) that opens faster, further, helps a little. Advancing the stock exhaust cam (as is possible on a motor with separate inlet/exhaust cams) a few degrees is also beneficial because you are opening the valve while the pressure in the cylinder is higher. This obviously wastes some of the extra power, but generally the benefits outweigh the losses - at least on the bike engines I tried this on on the dynamometer with big 140 bhp plus extra on stock engines..
  • Exhausts For serious power outputs when on BIG HITS of Nitrous Oxide, bigger is definitely better. Big headers, tapered or plain big pipe work is really better! With out any silencing. This obviously is not practical on the road!
  • Cooling is not really an issue as you will be only using Nitrous in huge amounts for short periods. But a cool thermostat, good fans and lower general block temperature will allow more Nitrous Oxide to be used before detonation becomes a problem...
  • Fuel - Use the highest octane fuel you can get. C14, C16 race fuel, Avgas 101LL, ETC. The octane rating of the fuel is a direct measure of its "anti knock" value. Knock is the onset of detonation caused by the fuel "self igniting" due to temperature / pressure in some part of the cylinder causing one flame front, and another caused by the spark plug colliding! This is "pinging, or pinking" This gets more violent and causes all the fuel to "detonate" at once. If this happens when you are using Nitrous then kiss your engine and wallet goodbye! The detonation limit is the thing that puts a very firm limit on the final maximum power output. Adding Methanol to your fuel increases the fuels effective octane rating lots! BUT IT MAKES IT RUN LEAN! So if you try this then remember to add MUCH bigger fuel jets! Its also possible to run the engine on Petrol, and the Nitrous system with Methanol... If you do, you will need jets around 2.2 to 2.5 times as big. But it will almost remove the possibility of detonation.
  • Fuel Pumps Bigger is always better here! A pump that's too small causes you motor to run lean. This is bad!  When you think the pumps big enough, get another one and connect them both together...

*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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