In recent years, Q16 has become very popular for naturally aspirated and forced induction race engines. According to NOS, however, that doesn't always make it a good choice for heavy-duty nitrous applications. "Octane is not the only consideration when choosing a race fuel, but also the formulation of the fuel is very important as well. For a nitrous application, you want to use a higher octane fuel that has a slower burn rate," says Kyle. "Oxygenated fuels, such as Q16, are generally not suggested for nitrous motors, except possibly for jet-limited classes. Nearly every fuel company these days has a fuel designed specifically for nitrous, and that is the type of fuel that should be used if possible."
Auxiliary Fuel Cells
Many street/strip nitrous applications utilize a separate fuel cell that feeds the nitrous system. In this arrangement, the engine runs off pump gas, while the nitrous system runs off race gas. In addition to a bump in octane, this arrangement provides a secondary fuel system that dramatically increases fuel volume. Some hot rodders feel this type of setup is completely safe while others think it's risky, so who's right? "While mixing different types of fuel in the combustion chamber is not ideal, it has become common," says Kyle. "That said; it's important to keep in mind that the pump gas is going to dilute the race gas and reduce overall the octane rating of the mixture. As long as you keep this in mind with the tune-up you shouldn't have any issues."
Although mild doses of nitrous oxide are perfectly safe to use in a stock engine, NOS recommends loosening up internal clearances for race engines built specifically for large volumes of nitrous. "While there are no hard and fast rules about what needs to be done, high levels of nitrous require certain concessions to be made," says Kyle." In nearly all cases where nitrous amounts are more than entry-level, we recommend wider ring gaps and increased quench clearance. How much those clearances need to be changed is directly related to the amount of nitrous you intend to spray and something the engine builder will have to take into consideration when they are assembling the engine."
Pistons and Rods
In a typical V-8 application, upgrading to a forged rotating assembly is very good idea when nitrous levels exceed 150 hp. Additionally, since the pistons are literally on the front lines of combustion, it must go beyond the specifications of a standard forging when nitrous levels exceed 300 hp. "There really is no such thing as a ‘nitrous piston' per say, but the real question is whether or not the piston is durable enough for the power level you are about to subject it to. Along that vein, you could say a nitrous piston will likely be a little heavier, have the top ring moved down a little, and have more spacing between the ring lands if possible," Kyle explains. He also points out that the proper selection of ring material will be just as important as piston design. As far as connecting rod selection is concerned, some engine builders claim that aluminum connecting rods in a nitrous motor help absorb the shock on the rotating assembly when the nitrous system activates, thus improving engine durability. While the virtues of aluminum rods in a street engine are debatable, they're very common in race engines that run nitrous. "I agree with that premise and once heard a engine builder say, ‘Would you rather hammer on your crank and block with a steel hammer, or an aluminum hammer?'"
In an all-out naturally aspirated race engine running race fuel, compression ratios of 15:1 or slightly higher are common for maximum performance. Although nitrous oxide works well in high-compression applications, NOS recommends dialing back the compression just a bit to provide a margin of safety. "In a max effort race motor, most engine builders will shoot for a compression ratio of between 13- and 14:1. Higher compression motors can get a lot more timing sensitive when the nitrous power levels get really high, so having slightly lower compression increases the tuning window," says Kyle.
Since nitrous oxide injection increases the rate of combustion, it's often necessary to retard ignition timing to prevent detonation. However, determining how much the ignition timing should be reduced is based on multiple factors, including compression ratio, nitrous volume, spark plug heat range, fuel type and compression ratio. "There is no simple formula for this based on the amount of nitrous you are spraying. The ideal timing for a given combination is dependent on many things," says Kyle. "These days, the cylinder head used and combustion chamber efficiency is one of the largest factors in that equation, which makes it impossible to come up with a one-size-fits-all formula. This is an area that most any inexperienced nitrous user is going to need help. That said, the timing chart is included with every new NOS brand systems and that is also available on our website has a very good timing chart that explains recommended timing requirements, and how you go about determining what is best for your combination."