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Chevy Nova: Ask Anything

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The next nitrous tuning tip is to carefully set the working fuel pressure. Many newbie nitrous users make the mistake of setting fuel pressure with the pump dead-heading into a closed fuel solenoid. The problem with this approach is that when the fuel solenoid opens, the pressure drops well below the static setting. This is really bad because low fuel pressure means less fuel is delivered to the solenoid and the nitrous/fuel ratio becomes lean, which can burn a piston in a matter of seconds. Most nitrous systems are designed to have a pressure regulator feeding the fuel solenoid. To accurately set the fuel pressure, you need to measure and set it dynamically. Zex makes a very cool tool that does this, but an enterprising car crafter could replicate the design. The concept is to disconnect the fuel line that runs to the fuel solenoid. You should place an accurate fuel-pressure gauge (with a range of 0 to 15 psi) in a T that then leads to a fitting that incorporates the fuel jet you will be using. Place the end of the hose with the jet in a gas can. With all fittings tight, turn on the fuel pump and fuel will flow through the jet into the can. The fuel jet becomes the restriction in the delivery system and will produce a pressure on the gauge. Most nitrous systems are designed to operate at 5 or 5.5 psi of fuel pressure. You should do some research on the kit you are using to determine what the nitrous manufacturer recommends. Set this pressure at the regulator and then turn the system off. After reconnecting all the lines back to the plate and the solenoid and checking for leaks, you will see that the dead- head pressure will be higher. This is normal. When the nitrous is engaged, the pressure will be correct. Ignition timing is another important factor. If this is a 150hp shot, all nitrous companies would recommend retarding the timing. The general numbers are 4 to 6 degrees to prevent detonation. However, we just looked up NOS’ newly revised tuning instructions and found that for a 150hp shot with a middle-of-the-road combustion chamber like the small-block Chevy’s, it recommends only 27 degrees of total timing with a 100-octane fuel. That sounds very conservative, but you have to be careful because too much timing can cause the engine to detonate, and then it’s game over because that could cause major engine damage.

I spoke with NOS’ Jay McFarland and he told me the company recently adjusted its standard kit-jetting recommendations by reducing the size of the fuel jet, which it says will make the system work more efficiently. We found an older recommendation for a 150hp Cheater NOS system that spec’d a 0.63 nitrous jet and a larger 0.071 fuel jet. For that same 150hp Cheater system, Holley’s newest information now specifies 0.063 nitrous and 0.063 fuel jets. Note how the fuel jet is eight steps smaller. McFarland says this actually improves engine durability, as NOS’ testing indicates that excessively rich mixtures could be contributing to engine problems. This is with a typical 5-psi fuel pressure, and McFarland says the kits have not really changed other than this new recommendation, so guys with older kits might see a mild power increase by going to a leaner jet. You can find the NOS tune-up recommendations for each kit on Holley’s website. Other important points relate to the ignition system. New spark plugs, good wires, and sufficient spark energy from the distributor are all critical issues for any engine with a power-adder, because as cylinder pressure rises, it places a greater voltage load on the secondary (high-voltage) side of the ignition. You might consider narrowing the spark plug gap to perhaps 0.030 inch. This reduces the voltage requirement. With either power-adder, go with a four to seven steps colder spark plug with a short ground strap to prevent pre-ignition. Finally, when assembling the engine, make sure you add a little extra ring gap to the top ring so it doesn’t butt due to the extra heat from the nitrous and/or blower. We’d shoot for 0.022 inch on a 4.030-inch bore. Armed with all this information, you should be able to do well enough to impress your instructor—good luck!

More Info

Holley Performance Products (NOS); Bowling Green, KY; 270/781-9741;

Nitrous Supply; Huntington Beach, CA; 714/373-1986;

Zex Nitrous; Memphis, TN; 888/817-1008;

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