Both our ProCalc calculator and our publications illustrate nozzle sizes in diameter. Hilborn FI nozzles are stamped in flow numbers. A list is provided showing:
- Hilborn nozzle flow numbers (times 0.01 equals flow in GPM)
- approximate nozzle diameter in inches (from Don Enriquez, Hilborn)
- pressure calibration value called volume area factor or VA factor that relates flow to jet or nozzle area:
The list of common Hilborn FI sizes is as follows:
Hilborn |
dia. |
VA |
number |
inches |
factor |
4 |
0.016 |
0.00076 |
5 |
0.018 |
0.00078 |
6 |
0.020 |
0.00082 |
7 |
0.021 |
0.00073 |
8 |
0.022 |
0.00068 |
9 |
0.024 |
0.00076 |
10 |
0.025 |
0.00072 |
12 |
0.028 |
0.00079 |
14 |
0.029 |
0.00067 |
16 |
0.031 |
0.00067 |
18 |
0.033 |
0.00068 |
20 |
0.035 |
0.00069 |
22 |
0.036 |
0.00064 |
24 |
0.037 |
0.00060 |
27 |
0.040 |
0.00065 |
30 |
0.042 |
0.00064 |
36 |
0.0465 |
0.00067 |
40 |
0.052 |
0.00085 |
47 |
0.055 |
0.00077 |
56 |
0.059 |
0.00072 |
61 |
0.063 |
0.00078 |
70 |
0.070 |
0.00091 |
73 |
0.073 |
0.00099 |
75 |
0.076 |
0.00110 |
81 |
0.078 |
0.00104 |
94 |
0.089 |
0.00131 |
104 |
0.093 |
0.00128 |
130 |
0.093 |
0.00082 |
210 |
0.147 |
0.00196 |
Math to generate values in the previous list:
The data result from the following math from Ref. jettingsmallblock, p. 26 or jettingbigblock, p. 35:
nozzle area = (nozzle flow) x SQRT (VA factor / 30 psi)
nozzle flow = Hilborn nozzle flow number x 0.01
nozzle dia. = SQRT (nozzle area / 0.7854)
Example for a Hilborn #12 nozzle:
nozzle area = (nozzle flow) x SQRT ( 0.00079 / 30)
nozzle flow = 12 x 0.01 = 0.12
nozzle area = (0.12) x SQRT ( 0.00075 / 30) = 0.0006153
nozzle dia. = SQRT ( 0.0006153 / 0.7854) = 0.028 inches
VA factor – Fuel pressure calibration
The VA factor is a volume area factor. It is a calibration value developed in our publications as a simple relationship between jetting flow and jetting size. Using Hilborn nozzle flow numbers and nozzle diameters, the VA factor was calculated for each nozzle. Those are shown with each respective nozzle size. The math for that is derived from above as follows:
VA factor = 30 psi x (nozzle area / nozzle flow)^{2}
nozzle area = (nozzle diameter)^{2} x 0.7854
Example for Hllborn #24:
nozzle area = (0.037)^{2} x 0.7854 = 0.0010752
nozzle flow = Hilborn nozzle flow number x 0.01 = 24 x 0.01 = 0.24
VA factor = 30 x (0.0010752 / 0.24)^{2} = 0.00060
Fuel pressure calibration for flared inlet jetting
The VA factor values throughout the list indicate the approximate fuel pressure calibration values used in the math and in our calculator for a Hilborn system with flared nozzles and bypass jets. The actual fuel pressure calibration VA factor for the Pro-Calcs calculator or a racer’s own computations can be determined from a fuel pressure gauge reading at a specific engine speed. That is further illustrated in fuelinjectionracingsecrets, p. 169. The values above can help to dial in jetting combinations when no previous pressure gauge reading is available for specific jetting combinations that are of interest.
Value of fuel pressure calibration
Fine tuning for maximum performance in a mechanical fuel injection system requires the fine adjustment of nozzles and bypass jets. Trial & error is very difficult. The right computations significantly reduce that trial & error time and expense. Fuel pressure calibration is a significant issue in computations. The previous examples illustrate how fuel pressure, flow, and jet size relate. In this case, the examples were from Hilborn flared nozzles. Flared nozzles from other manufacturers would exhibit similar VA factor calibrations for our Pro-Calcs calculator or a racer’s own computations using the math throughout our technical manuals.
Actual computations for pressure from nozzle size are quite complex. The VA factor method that we developed greatly simplifies the task.
The more time I spend with normally aspirated fuel injection technology such as this, the more I realize how much improvement can be done with jetting refinements guided by preliminary math analysis.
Footnote to normally aspirated fuel injection engines
A lot of normally aspirated FI engines are run without a high speed bypass. That is often the case with Rons FI systems for example. If the tuner in these applications is leaning down the jetting to get color into the spark plugs, it is likely that the engine is too lean at the torque peak. That is the engine speed where the fuel demand per-engine-revolution is the max. When no high speed bypass is run, the engine is richer at engine speeds above the torque peak. The high speed bypass provides the ability to trim the fuel curve for that fuel characteristic. Care should be exercised with FI when tuning is done on a normally aspirated engine with no high speed. If we leaned down jetting so that spark plugs were colored after engine loading, then we would watch engine compression closely. We would also check the condition of pistons and valves with a tear down. We would confirm that the air to fuel ratio at the torque peak is not so lean that it was damaging parts. More information about the high speed bypass is throughout Ref. fuelinjectionracingsecrets, 5000HPonmethanol, jettingsmallblock, jettingbigblock.