Calculate water-methanol injection flow from crank horsepower, split it across nozzles, adjust nozzle flow for pressure and estimate how long a tank lasts.
Water-Methanol Injection Flow Formula
The calculator keeps four planning tasks. The horsepower route applies an explicitly assumed coefficient or the named ProMeth starting-point guide; engine power alone does not establish a validated injection requirement:
Variables:
- WMI is the scenario mixture-flow target, in cc/min
- HP is crank horsepower
- K is an assumed cc/min-per-crank-hp coefficient. ProMeth publishes 0.5 for water, 1.0 for nominal 50/50, and 1.5–2.0 for methanol as starting figures that omit other variables. The methanol midpoint assumption is displayed as 1.75. Custom mixtures use a supplied coefficient; no unsupported intensity multiplier or composition interpolation is applied.
If you already know your fuel flow, the flow is set as a percentage of that fuel flow instead:
- FF is total fuel flow, in cc/min
- P is your chosen injection/fuel volumetric ratio in percent, not a mass ratio. No universal tuning percentage is selected. Injector-derived fuel flow uses rated flow × square root of actual/rated differential pressure × duty fraction × injector count; static-flow scaling omits opening offsets and short-pulse nonlinearities.
For the same fluid and discharge behavior, a nozzle’s flow approximately scales with the square root of pressure difference across the tip. Use measured nozzle-inlet pressure under flow, rather than a pump’s advertised pressure:
- Q is the actual delivered flow
- Qr is the rated flow at the rated test pressure Pr
- Pnet is nozzle-inlet liquid gauge pressure minus manifold boost gauge pressure, using a common reference. Pr is the manufacturer’s rated differential pressure. Injector calculations also need pressure across the injector, not rail gauge pressure alone; see Injector Dynamics.
The last mode turns a flow rate into run time from a reservoir:
- T is modeled elapsed time to deplete usable reservoir volume under the stated average-flow assumption
- V is measured usable finished-fluid volume, excluding pickup/dead volume; blending component volumes can contract
- Q is the nozzle flow rate
- DC is the effective average-flow fraction. It equals time spraying only for constant on/off flow; it is not automatically pump PWM duty or controller command. Zero average flow gives no finite modeled depletion time; an empty reservoir is reported explicitly.
Use horsepower mode for a labeled starting-point scenario, fuel mode for an externally selected volume ratio, pressure mode for approximate rating conversion under unchanged fluid conditions, and reservoir mode for a constant-average depletion scenario. Per-nozzle flow assumes equal sharing; the calculator does not select a part or replace a calibrated system, low-level protection or flow failsafe.
Injection Flow and Nozzle Size Reference
This table illustrates HP × three assumed coefficients. The column values are arithmetic scenarios, not validated engine requirements or fluid recommendations. Manufacturer-specific starting guides omit important operating variables.
| Crank HP | Assumed K = 0.5 | Assumed K = 1.0 | Assumed K = 1.75 |
|---|---|---|---|
| 200 hp | 100 cc/min | 200 cc/min | 350 cc/min |
| 300 hp | 150 cc/min | 300 cc/min | 525 cc/min |
| 400 hp | 200 cc/min | 400 cc/min | 700 cc/min |
| 500 hp | 250 cc/min | 500 cc/min | 875 cc/min |
| 600 hp | 300 cc/min | 600 cc/min | 1050 cc/min |
| 750 hp | 375 cc/min | 750 cc/min | 1312.5 cc/min |
| 1000 hp | 500 cc/min | 1000 cc/min | 1750 cc/min |
The second table is a US-volume unit conversion reference, not a nozzle catalog or horsepower rating. A real device’s flow must be compared at its specified test pressure and fluid. For example, the cited ProMeth extended-tip nozzle ratings are stated at 200 psi; pressure scaling alone does not correct a different fluid or spray behavior.
| US gal/h | cc/min | mL/s |
|---|---|---|
| 1 | 63.0902 | 1.0515 |
| 2 | 126.1804 | 2.103 |
| 3 | 189.2706 | 3.1545 |
| 4 | 252.3608 | 4.206 |
| 5 | 315.451 | 5.2575 |
| 7 | 441.6314 | 7.3605 |
| 8 | 504.7216 | 8.412 |
| 10 | 630.902 | 10.515 |
| 12 | 757.0824 | 12.618 |
| 14 | 883.2627 | 14.721 |
Example Problems
Example 1: Size a nozzle from horsepower.
For an illustrative 500-crank-hp scenario, choose the ProMeth nominal 50/50 starting guide coefficient of 1.0 cc/min per hp. This assumption is not a validated engine requirement:
WMI = 500 × 1.0 = 500 cc/min, or about 7.9252 US gal/h. For two equal nozzles, the arithmetic target is 250 cc/min each. Nominal 50/50 volume bookkeeping gives 250/250 cc/min allocations, ignoring blending contraction; this is not a mixing recipe. Compare actual device flow at the relevant pressure and fluid rather than automatically selecting an 8-GPH part.
Example 2: Find how long a tank lasts.
You feed one 500 cc/min nozzle from a 3 liter reservoir and spray about 10 percent of your driving time. At full spray the tank would empty in 3000 / 500 = 6 minutes. At a 10 percent duty cycle it stretches to 3000 / (500 * 0.10) = 60 minutes of driving before a refill.
Frequently Asked Questions
How do you size a water-methanol injection nozzle?
Apply a documented manufacturer starting guide or a supplied coefficient/volume ratio to create a planning target, then compare the actual nozzle’s measured delivery and test conditions. Horsepower, a nominal GPH label and simple square-root pressure scaling do not establish a safe engine calibration, spray quality or suitable fluid concentration. Final system selection and calibration require the applicable manufacturer guidance and verified operating data.
Should you run more water or more methanol?
Fluid concentration must follow the actual system’s limits; chemical compatibility is not operating approval. For example, AEM advises no more than 50% methanol for its systems/components. A vendor’s pure-methanol sizing scenario is not a recommendation for other equipment. Methanol is toxic and flammable. The calculator does not select a safe concentration, predict freeze protection or approve a fuel/timing change; use the specified premix and rated temperature limits for the application.
Does boost pressure change nozzle flow?
Yes, through pressure difference across the nozzle. If nozzle-inlet liquid gauge pressure under flow is 200 psi and manifold boost is 20 psi with the same reference, the differential is 180 psi. A device rated 500 cc/min at 100 psi would approximately flow 670.8204 cc/min under unchanged fluid/discharge assumptions. Check actual delivery, fluid, valve behavior and operating limits; the model is not a pump or spray-quality prediction.
