Nitrous Part II

More -----> TUNING TIPS

Fuel Pressure:
The majority of racers fine-tune air/fuel ratio by changing the fuel pressure. Because the fuel side operates at much lower pressures than the nitrous side, very slight changes in fuel-system pressure can significantly alter system performance. Raising the fuel pressure richens the mixture; lowering the fuel pressure leans the mixture. It's also quick and easy out in the field, and you don't have to carry a bunch of different jets with you. Tune off fuel pressure once the SPREAD is established. However,plate systems seem to like low pressures. With modern nozzles, 6 psi is a mean starting point. You have about 1 psi to play with either way. Find out what it likes: Get the pressure right, then play with the jetting. The low-pressure fuel side is so sensitive to pressure variations that any changes should be made in 1/2- or even 1/4-psi increments. Most dead-head (non-return) pressure regulators tend to creep at idle, so it is common practice to test actual fuel pressure under simulated running conditions, or to use a data-logging computer that records the actual pressure during the run.

Ignition Timing:
Nitrous burns faster, so it requires less timing, but how much to take out initially and how fast to put some of it back in are key fine-tuning elements. Timing is a great way to control the system. Every outside thing affects the nitrous, affects how it loads the engine--vehicle weight, converter, clutch, gearing. The exact amount of timing to pull out varies for every combo. Start off conservatively, and put timing back in gradually. On a nitrous system, even a 1-degree change in advance can make a huge difference. A safe rule of thumb with good race gas is to pull out 1.5-2 degrees for every 50hp increase. If you push timing to the ragged edge there's not much of a performance improvement, but the engine will be much more prone to detonation. The timing can get down as low as 10 degrees with both stages on a multistage system activated. If you run a datalogger and have a window of tuning available try taking a few degrees out before and after peak torque. Do Not use timing as a crutch for trying to run too big a system. It's better and safer on parts to run a smaller nitrous hit with just enough timing. The exact timing may vary per cylinder. For example, cylinders 2, 4, 6, and 8 tend to run hotter on a big-block Chevy. Torque converter-equipped cars may require even more retard. With the timing so retarded, the distributor rotor phasing gets out of whack. One of the most overlooked contributors to nitrous engine failures and ignition problems when retarding timing electronically is rotor phasing. The rotor must always be in phase to the lowest timing that the engine will see during nitrous use. If you will be retarding more than 8 degrees electronically, we recommend that you use a crank trigger so that the rotor may be phased properly.

Finally, there are the nitrous and fuel jets. On prepackaged systems, it is standard practice to run the jet combos recommended by the system manufacturer for the power level desired. If you're not making the power you think you should, majority opinion says you should play with the jets only as a last resort once the car fails to respond to fuel-pressure and timing changes. Others recommends leaving pressure and fuel jets alone. "Keep fuel pressure at 53/4-61/4 psi, and just change the nitrous jet" to compensate for a bad-air day. The nitrous side pressure is so much higher than the fuel side that going up or down one nitrous jet size is a much finer (and safer) tuning tool than changing fuel jets. However, some tuners disagree, recommending size changes to the fuel jet in the nitrous system to get the engine to perform at its best. It's just like jetting a carb. Either way, be sure to keep careful records of any changes and carefully read those plugs! If you have to change jets more than two sizes, look for a restriction somewhere else in the system.

Failure Modes:
The two most common nitrous-motor catastrophic failures are blown head gaskets or burnt pistons. A blown gasket can be hard to run down because it can be caused by either the wrong air/fuel ratio or detonation (or both, since the two can be related). If you just treat the symptom with improved gasket sealing, there's a good chance the next failure will be with the pistons. Piston failures are easier to diagnose. You'd think going too rich is safer than too lean, but if liquid fuel trickles down past the rings, the resulting distress can lift the rings upward. Many nitrous failures are caused by running the engine excessively rich. Contrary to popular belief, richer is not necessarily safer. On the other hand, burning a trough down through the rings into the skirt usually means you are both too rich and have too much timing in that cylinder. If you burn a hole through the top of the piston, you are running too lean. If you start the burn and there's nothing left to burn, everything's gone--you'll just burn the aluminum.
Dialing in a sophisticated nitrous system requires a patient, methodical approach. Do not get greedy. When you're going from a compressed liquid to a gas, the density changes constantly, and you don't have as much control as you'd like to.
Go conservative on timing.
Be careful not to run too rich or too lean.
Use really cold plugs.
Learn how to read the plugs.
Institute a nitrous-system maintenance schedule. And hit the nitrous at the top of each gear, not the bottom. If building from scratch, design for high rpm--it's easier on a nitrous motor. Nitrous only magnifies existing problems. Get the engine running right on the carbs before you hit the bottle.

On a sheetmetal type intake. Change the jets in the carbs about 4 numbers leaner when you switch to the nitrous. Then dial-in the nitrous oxide on one of the recommended lower-power tune-ups. If it doesn't perform as it should, don't step up to the next level until you've sorted out any existing problems. Change only one thing at a time.

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