Nitrous Oxide Jet Calculator
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Air/Fuel RatioWhat constitutes correct A/F ratio?
Most recommend running slightly rich (generally, between 11.5-12.5:1), but agreement on this point is by no means universal, and some sources recommend going even richer under some circumstances.
At 250 hp and under, you can run the same as the engine's air/fuel ratio. But at 450-500 hp run 11.5:1 as a safety factor.
Try adjusting the exact air/fuel ratio according to engine displacement. A 400hp kit delivers about the same 400hp boost on a 400ci engine as it does on a 600ci engine, but because the larger engine has much more total cylinder volume, the mass fraction of nitrous inducted into that engine as a proportion of that volume is less, so it is more easily absorbed.
The big engine may be happy with an 11.5:1 A/F ratio, but you may have to go as rich as 10:1 on the smaller motor. Finding the exact A/F ratio to run can be somewhat tricky because nitrous carries its own oxygen. The ratio in effect becomes a variable of how much nitrous you put into the engine.
The following equation to work well as a starting point:
Target A/F Ratio = [(engine hp / total hp) x 13] + [(nitrous hp / total hp) x 7]
Actual A/F ratio tuning is accomplished with either a wideband O2 sensor, or by reading the spark plugs. Usually a light tan porcelain color on the plug is an indicator of the right timing. You need to read all of the plugs. If tuning with a wideband O2, the higher the ratio of nitrous power to engine power, the less accurate the O2 sensor will be. That's why most hard-core guys look at the plugs and exhaust-gas temperatures. The plugs won't lie to you.
Bottle Temp to Bottle Pressure ratio
Raising nitrous-bottle pressure makes the system hit harder off the line at the expense of inconsistent taper-off on top. To avoid problems, most experts advocate sticking to the system manufacturer's recommended bottle operating pressures. Although this can vary slightly among different manufacturers, it's usually 900-1,000 psi. "The higher the bottle pressure, the greater the drop-off." If you start out at 950 psi, you'll finish at 750; if you start out at 1,100, you'll finish at 600. The larger the bottle, the less the drop-off." Whatever pressure you run, it should always be consistent, because it affects the rest of the tune-up. There is a direct correlation between bottle temperature and the bottle's internal pressure, so keeping the bottle at a specified temperature will also maintain the desired pressure. For example, if you maintain the bottle at 90-92 degrees F, there will be about 950 psi at the start of the run. The pressure will drop during the run, but you should still have 880-900 out the back. Depending on local climate conditions, this may require a bottle blanket and electric heating device for warming the bottle, or a cold towel and ice to cool it.
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