Ford 351 Engine Induction Guide: Nitrous Oxide

The first thing to remember with nitrous oxide is that it makes fuel burn faster. This means you must be mindful of what it can do, both productively and counterproductively. All that instantaneous power comes for a reason. As a result, extraordinary attention to detail must be paid on the road to power.

Using nitrous oxide generates greater amounts of power from the air/fuel charge you introduce to the combustion chambers. Think of nitrous as a very simple gas composed of two nitrogen atoms attached to one oxygen atom. Chemists know it as N2O.

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Nitrous oxide, or “squeeze,” is popular today for those looking for quick and easy power (50 to 150 hp) on demand. And it makes boatloads of power at the touch of a button. But nitrous can be very harmful to an engine that isn’t properly prepared and tuned. Nitrous severely damages pistons, rings, and bearings if not properly executed. It can and does hammer rod bearings, resulting in severe wear. It is also hard on main bearings due to the severe loads. And no matter what the nitrous crowd tells you about “laughing gas,” nitrous shortens engine life. So don’t be drawn into believing it’s a magic horsepower pill without consequences. If you use nitrous oxide, be prepared for shorter engine life. Contrary to what you may believe about nitrous oxide, it is not a poisonous gas nor is it harmful to the atmosphere. This doesn’t mean you should breathe it, however. Because nitrous oxide is an asphyxiant, it can suffocate you if inhaled in heavy quantities. It would have a similar effect as inhaling carbon dioxide: oxygen deprivation.

 

The Science of Nitrous Oxide

Nitrous oxide is available in three basic grades: commercial, medical, and high-purity. Commercial-grade nitrous oxide is what you use in your engine for performance gains. The medical grade is commonly known as laughing gas used by dentists and surgeons. It has to be very pure for human consumption. You must be licensed as a medical professional to get it. High-purity is also a medical grade nitrous oxide that is extremely pure, and priced and controlled accordingly.

Commercial-grade nitrous oxide is marketed as Nytrous+ and sold by the Puritan-Bennett Corporation. You can find it all across the country. It is a mix of 99.9 percent nitrous oxide and .01 percent sulfur dioxide. Puritan-Bennett adds the sulfur dioxide to give it a gas odor just as you experience with natural gas.

When you buy nitrous oxide, it is pumped into a storage tank that you provide the supplier. You need an appropriate tank capable of holding at least 1,800 pounds per square inch (psi). To be safe, your tank(s) must have a visible certification date within the past five years.

In nature, nitrous oxide is a gas. Inside a pressurized tank, nitrous oxide is in liquid form. When it leaves the tank, it’s very cold, similar to refrigerants and propane. The use of nitrous oxide to make power is nothing new. During World War II, it was used to help aircraft engines make power. Nitrous oxide stored under pressure must be anchored securely. I stress safety because a carelessly handled nitrous oxide bottle with nearly 1,000 psi of pressure behaves like a bomb if the bottle fails. It can explode, maiming or even killing you. To get the nitrous needed for performance use on demand, the gas is metered from the bottle via electrical solenoids that are fired when you press the button. Nitrous oxide should be administered on demand at a time when it is safe to do so. Too much nitrous oxide and not enough fuel can destroy an engine in nanoseconds. For one thing, nitrous oxide should never be administered to the intake ports unless the throttle is wide open. Set up properly, the throttle should close a nitrous oxide solenoid switch when in the wide-open position. Closing the switch activates the nitrous oxide solenoid, releasing the nitrous oxide into the intake manifold.

Nitrous oxide gets into the intake ports a number of ways, depending on how the engine is set up. Carbureted engines get their nitrous oxide diet through a fogger plate located beneath the carburetor. If you pin the butterflies, the nitrous oxide “fogs” the intake plenum, assisting the air/fuel mixture en route to the chambers.

Carbureted engines may also use nozzles at each intake port to administer the nitrous oxide. The nice part about this design is being able to tune each cylinder bore based on the individual needs of each cylinder. The center ports typically receive more fuel and air than the perimeter ports. The outers tend to run leaner than the centers, which is critical when you are running nitrous oxide.

Port fuel-injected engines also use nozzles off a common tube manifold to administer nitrous oxide at each port. Like its carbureted counterpart, the portinjected nitrous oxide arrangement can be port-tuned for better performance. This is especially true when you think of your V-8 engine as eight separate engines operating on a common crankshaft. One popular misconception is that you get power from the nitrous oxide itself; this isn’t true. Nitrous oxide works hand-in-hand with the air/fuel mix to make power in each cylinder bore. Nitrous oxide brings out the best in the fuel. Not only is the nitrous oxide mist cold (good for thermal expansion), it is also loaded with oxygen, which gives the igniting air/fuel mix a bad attitude. It makes the air/fuel mix burn faster, which creates a powerful thermal expansion experience in each combustion chamber.

Be careful with nitrous oxide in how you feed it to your engine. Perhaps this isn’t the best parallel, but using nitrous oxide can be thought of in the same way as you would use cocaine, crystal meth, or nicotine. The more powerful the experience, the more you want. So you keep feeding your engine more nitrous oxide in your quest for power until it fails under the stress. You must recognize your engine’s limits before even getting started on a nitrous oxide diet.

 

Engine Tuning

Administering nitrous oxide to the combustion chambers should not be done with reckless abandon. Too much of it burns pistons. You have to think of nitrous oxide and your air/fuel mixture just as you would oxygen and acetylene. When you’re using oxygen and acetylene to weld or cut steel, you use lots of oxygen to blaze a path through the steel. It’s the same inside your engine when you use too much nitrous oxide: You burn right through the piston like a cutting torch. And aluminum pistons aren’t as forgiving. They melt at 1,300 degrees F.

When tuning a small-block Ford to run on nitrous oxide, air/fuel mixture and spark timing must be just right or you face certain destruction. So how do you get there? First, you have to control fuel delivery to where it jibes with the fl ow of nitrous oxide. Too much nitrous oxide and not enough fuel overheats the chamber and melts the pistons. This means controling fuel and nitrous oxide fl ow to a finite point so you get the most power possible without engine damage. This takes practice.

 

Light-Off

The key to getting the most power from nitrous oxide is getting spark timing, fuel delivery, and peak cylinder pressure going at the same time. Ideally, the air/fuel/nitrous mixture lights when you have peak cylinder pressure, making the most of the incoming charge. When everything is working well together, you get a smooth, firm light-off that nets a lot of power. Things go wrong when the light-off resembles an explosion, exerting a shockwave on the top of the piston. This is the spark knock you hear as a multiple “rapping” under acceleration.

When fuel, air, and nitrous ignite violently, why don’t you net more power from the explosion? The answer is simple. When an engine is running smoothly, you get that “quick-fire” discussed earlier in this chapter. A smooth light-off applies pressure to the piston dome, forcing it downward in the cylinder bore, turning the crank and completing the power stroke. Detonation is what occurs when you get a spontaneous light-off, especially from two points in the chamber. The two waves of power collide, causing spark knock or pinging under acceleration. The problem with this kind of light-off is violent combustion spikes that don’t really yield much power.

 

 

Fuel System

So how do you safely make the most of nitrous oxide? First address the fuel system because you need to have enough fuel to meet the demands of nitrous oxide. Without enough fuel, the engine gets toasted. Another issue is fuel octane rating. What octane rating do you expect to use? Next is ignition timing. Where does yours need to be? And finally, what is your engine’s compression ratio? Too much compression with nitrous causes destructive detonation where damage happens in a nanosecond. Getting each of these elements dialed in is crucial to productive performance.

 

Compression

Compression has to be thought of two ways: static and dynamic. Static compression is the swept volume above the piston, with the piston at BDC, versus the clearance volume left when the piston is at TDC. If you have 100 cc of volume with the piston at BDC and 10 cc left with the piston at TDC, then you have a static compression ratio of 10.0:1, 100 to 10 cc.

Dynamic compression is the kind of compression that happens with pistons, valves, and gasses in motion through the engine. Dynamic compression comes from huffing lungfuls of air through the engine during operation. With the engine running, more volume pumps through the cylinders and chambers than when simply hand-cranking the engine. This actually increases the compression ratio, which means dynamic compression is higher than static compression.

So what does all of this mean for your engine? It means you need to consider the dynamic compression ratio as your engine’s actual compression figure when you’re planning nitrous oxide.

Nitrous-burning engines need different camshafts than those that are naturally aspirated or supercharged. Dynamic compression ratio is affected by camshaft profile. A camshaft profile with a short duration yields greater dynamic compression. When you lengthen the duration, dynamic compression is lost. On the exhaust side, duration is a very important component with nitrous. Because the air/fuel/nitrous charge coming in expands with fury during ignition, it needs a way to escape when the exhaust valve opens. You need a longer exhaust valve duration with nitrous for good scavenging and thorough extraction of power, which means nitrous cams must be ground differently.

While you’re thinking about exhaust valve duration, you must also consider overlap. Less overlap, more dynamic compression. More overlap, less dynamic compression. Overlap is the process in the power cycle where the exhaust valve is closing and the intake valve is opening. The incoming charge helps scavenge the outgoing hot gases through the overlap process. This means the exhaust valve needs to open earlier in the cycle and stay open longer for adequate scavenging. Fuel octane plays into the power process because you need to understand when and how the fuel ignites. The higher the fuel octane rating, the more slowly it ignites and burns, which reduces the chances of detonation and pre-ignition. A higher octane rating produces a smooth, more predictable lightoff in the chamber. A lower fuel octane rating produces a more unstable fuel that lights quickly and causes pinging. When you throw nitrous oxide into the equation, you can count on a quick-light that can be violent in nature. This is why a higher octane rating is so critical to a cohesive performance package.

 

Air/Fuel Ratio

Now let’s address the all-important topic of the the air/fuel ratio. You change it by adjusting jet size in the carburetor or controlling fuel injector pulse width. Jet size and pulse width both determine how much fuel enters the chamber. If your tuning effort involves a carburetor, you have to get jet sizing down to where your engine can live on nitrous oxide. Jetting needs to be richer to compensate for the abundance of nitrous and higher combustion temperatures. As a rule, carbureted engines live happily with an air/fuel ratio of 12.5:1 to 13.0:1. This is where you have just the right amount of air and fuel to make power. Going too lean can cause engine damage and lost power. Too rich and power can be lost as well.

When you’re working with fuelinjected engines, you can control fuel mixture by reprogramming the ECM or changing injector size. With nitrous, you typically increase injector size and fine tune from there. Too large is better than too small. Factory fuel injection systems run a fuel manifold pressure of 30 to 45 psi. If you’re running nitrous oxide, you need a lot more fuel pressure to get the job done safely. Around 80 psi is considered the norm for nitrous oxide and electronic fuel injection. This is when you need to step up to high-pressure hoses and fittings.

There is a formula that helps you prepare a fuel system for nitrous operation. The air/fuel ratio in a naturally aspirated engine should be between 12.5:1 and 13.0:1. This is a range where engines are happiest and make the most power. Things change dramatically when nitrous oxide is introduced to the air/ fuel mixture. More fuel is needed to both make power and prevent engine damage.

Most nitrous experts suggest a nitrous/fuel ratio of 5.0:1 as a starting point for engine tuning. Starting here means going decidedly rich, but it’s the safest approach. Begin at 5.0:1 and steer your tuning toward 6.0:1 for optimum results. If your power goal is, for example, 500 hp on nitrous oxide, you need 37.94 gallons per hour, or .63 gallons per minute, to run happily on nitrous oxide. Obviously, you’re not going to stay on nitrous oxide for an hour, but it gives you a good idea of how much fuel you need.

 

Ignition Timing

Ignition timing is the next big hurdle because it can kill an engine as quickly as a lean fuel mixture or too much compression when you’re running nitrous. You want the spark to occur in advance of peak cylinder pressure because it takes time for the air/fuel/nitrous mixture to ignite. Under normal circumstances, without nitrous, you want full spark advance around 36 to 38 degrees BTDC. Exactly where the spark occurs depends on how the engine is equipped and how it performs at full spark advance at 3,500 rpm. Because every engine is different, full advance is going to vary from engine to engine.

Total spark advance and its effect on an engine is determined by taking the engine to 3,500 rpm with vacuum advance connected. It’s a good idea to goose the throttle and watch your timing mark just to make sure it isn’t going any further than 36 to 38 degrees total. In other words: engine at 3,500 rpm, watch mark, and goose throttle. Watch the mark; if it moves any further, you haven’t reached total advanced timing.

The acid test is to load the engine either in a car or on the dyno. With a load at wide-open throttle, you should see crisp acceleration without spark knock. If there is spark knock, retard timing 1 degree and try it again. There is also fuel mixture to consider, which may be too lean.

When you throw nitrous oxide into the equation, you have an air/fuel/ nitrous mixture that is going to ignite more rapidly than the conventional air/ fuel mix. The pros suggest retarding the ignition timing to approximately 12 degrees BTDC with the throttle open because the air/fuel/nitrous mixture ignites much more quickly. With the full spark advance at 36 to 41 degrees BTDC, you would waste the engine in short order. Retard timing to 12 degrees

BTDC total timing and go from there. Twelve degrees BTDC at 3,500 rpm needs to be your baseline, then slowly advance ignition timing from there. Test it out at wide-open throttle under a load, beginning at 12 degrees BTDC, then advance from there 1 degree at a time.

 

Atomization

There are other points to consider when running nitrous. To be effective, fuel has to atomize (vaporize) properly. This means the fuel has to “mist” as it enters the intake manifold and, ultimately, the combustion chamber. The problem here is, nitrous comes out of the fogger or nozzle at a frigid –100 degrees F. This makes it very difficult to atomize the fuel effectively. At that temperature fuel tends to exist as large droplets, rather than the mist you need for good ignition and combustion.

Nitrous oxide system manufacturers have dealt effectively with the issue of fuel atomization by designing systems that allow the gasoline to atomize with the nitrous oxide fog or mist. The finer you can get the mist, the more power you’re going to make.

 

Written by George Reid and Republished with Permission of CarTech Inc