Calculate the horsepower gain from any nitrous jet size, or the jet needed for a target HP shot, plus fuel jet size, nitrous flow, and bottle spray time.
Nitrous Jet Horsepower Formula
HP = D^2 * 70 * N * sqrt(P / 900)
D = sqrt(HP / (70 * N * sqrt(P / 900)))
PPH = HP * 3.6
t = 3600 * W / PPH
- HP is the estimated horsepower gain from the nitrous shot
- D is the nitrous jet orifice diameter in millimeters (thousandths of an inch times 0.0254)
- N is the number of nitrous jets or nozzles flowing at the same time
- P is the nitrous bottle pressure in psi (the 70 constant is calibrated near 900 psi)
- PPH is the total nitrous flow in pounds per hour
- W is the bottle capacity in pounds and t is the full-throttle spray time in seconds
The first formula drives the default mode of the calculator: enter a jet size in thousandths of an inch and it converts the orifice to millimeters, squares it, and multiplies by 70 and the number of jets to estimate the horsepower gain. Because orifice flow scales with the square root of supply pressure, the estimate is corrected by sqrt(P/900) when you change bottle pressure in the advanced options. The second formula is the same relationship solved backward, which is what the jet-size mode uses to find the jet diameter for a target shot. The flow formula converts horsepower to nitrous consumption in pounds per hour, and the last formula divides your bottle capacity by that flow to tell you how many seconds of wide-open spray a bottle holds. In wet mode the calculator also sizes the fuel jet by balancing orifice areas against the pressure difference between the bottle and your flowing fuel pressure at a chosen nitrous to fuel mass ratio, with 5.8 as a safe rich baseline.
Jet Size to Horsepower Chart and Bottle Spray Time
The table below shows estimated gains for common single-jet sizes at 900 psi bottle pressure. Multiply by the number of nozzles for direct port setups. Manufacturer charts often rate the same jet slightly lower because they build in a safety margin, so treat these as the upper end of the realistic range.
| Nitrous jet (in) | Est. HP gain | Nitrous flow (lb/hr) |
|---|---|---|
| 0.020 | 18 | 65 |
| 0.030 | 41 | 146 |
| 0.040 | 72 | 260 |
| 0.050 | 113 | 406 |
| 0.062 | 174 | 625 |
| 0.073 | 241 | 867 |
| 0.082 | 304 | 1093 |
| 0.093 | 391 | 1406 |
This second table answers a question most jet charts skip: how long a bottle actually lasts. It shows full-throttle spray time in seconds by bottle size and shot size, using t = 1000 * W / HP. A hard quarter-mile pass sprays for roughly 8 to 12 seconds, so divide the seconds by about 10 to estimate passes per fill.
| Bottle size | 100 HP shot | 150 HP shot | 250 HP shot | 400 HP shot |
|---|---|---|---|---|
| 2.5 lb | 25 s | 17 s | 10 s | 6 s |
| 5 lb | 50 s | 33 s | 20 s | 13 s |
| 10 lb | 100 s | 67 s | 40 s | 25 s |
| 15 lb | 150 s | 100 s | 60 s | 38 s |
| 20 lb | 200 s | 133 s | 80 s | 50 s |
Example Problems
Example 1: You have a single fogger nozzle with a 0.062 in nitrous jet and the bottle sits at 900 psi. Convert the jet to millimeters: 0.062 * 25.4 = 1.575 mm. Square it and multiply by 70: 1.575^2 * 70 = 173.6, so the shot is worth about 174 HP. Flow is 174 * 3.6 = 625 lb/hr, which drains a 10 lb bottle in 3600 * 10 / 625 = 57.6 seconds of wide-open spray, or roughly five to six hard passes.
Example 2: You want a 250 HP shot on an 8-nozzle direct port system. Solve for the per-nozzle jet: D = sqrt(250 / (70 * 8)) = 0.668 mm, and 0.668 / 25.4 = 0.026 in. Each nozzle needs about a 26 thousandths nitrous jet. The calculator also suggests roughly 10 degrees of total ignition retard at this power level, following the common rule of about 2 degrees per 50 HP of nitrous.
Frequently Asked Questions
Why does my kit’s jet chart show less horsepower than the formula? The D^2 * 70 relationship describes ideal orifice flow at roughly 900 psi. Real systems lose flow to solenoids, line length, and filters, and manufacturers rate their charts conservatively so a customer’s first pull is safe. If the chart that came with your kit disagrees with the formula, follow the chart and treat the formula as an upper bound for comparing jet sizes.
What bottle pressure should I run? Most manufacturers recommend 900 to 950 psi at the start of a run. Pressure tracks bottle temperature, so a bottle held near 85 to 92 degrees F sits close to 950 psi. Running higher pressure makes the hit harder but drops off more during the run, and every change in pressure changes the effective jetting, so consistency matters more than the exact number.
Do I need to change timing or spark plugs for nitrous? Yes. Retard ignition timing roughly 1.5 to 2 degrees for every 50 HP of nitrous, starting from your best naturally aspirated tune, and step to a colder plug heat range with a smaller gap for shots above about 100 HP. Verify the tune with a wideband air/fuel reading or plug inspection before stepping up to a larger jet.
