Read those spark plugs for good air density tuning at the NHRA Pomona Winternationals; Feb. 7-10, 2019.
Check https://airdensityonline.com/track-results/Auto_Club_Raceway_at_Pomona/ for current air density, density altitude, water grains, temperature, humidity, uncorrected barometer, wind speed, wind direction, dew point, saturation pressure, vapor pressure in imperial or metric units.
Check https://airdensityonline.com/track-history/Auto_Club_Raceway_at_Pomona/ for yesterday’s record.
Check https://airdensityonline.com/track-forecast/Auto_Club_Raceway_at_Pomona/ for tomorrow’s forecast already determined in air density units.
Check https://airdensityonline.com/free-calcs/ for free air density calculations.
Check https://airdensityonline.com/procalc/ for mechanical fuel injection jetting calculator.
Read those spark plugs for good NHRA drag racing preseason tuning; check https://airdensityonline.com/track-results/Wild_Horse_Pass_Motorsports_Park/ for current air density, density altitude, water grains, temperature, humidity, uncorrected barometer, wind speed, wind direction, dew point, saturation pressure, vapor pressure in imperial or metric units; yesterday’s record and tomorrow forecast already determined in air density units. Check https://airdensityonline.com/free-calcs/ for free air density calculations. Check https://airdensityonline.com/procalc/ for mechanical fuel injection jetting calculator.
We are seeing racers from time to time who open and close a fuel injection bypass to pulse the fuel curve. According to our numerical analysis of air to fuel ratios from our ProCalc fuel injection calculator, bypass jet sizes much bigger than 0.050 inches are too large if they are shut off or on during a power run. The change in air to fuel ratio for most fuels is too great. Either the engine is too rich before it opens or too lean before it closes.
Smaller bypass jet sizes are recommended. In our engine testing in power levels between 1,000 and 2,000 HP, we found a high speed bypass jet size of 0.040 inches diameter to cause an air to fuel ratio change of about 0.15 points. That air to fuel ratio difference kept the engine within the good power making range whether the high speed bypass was open or closed. It provided the ability to launch our engine with a bit of extra fuel for response. With a high speed bypass pathway located after the barrel valve, it also provided an extra safety factor of fuel if the throttle was lifted for traction loss or drifting. Then, when the throttle was hammered back on, the engine came from a rich state before leaning down to the ‘best power’ air to fuel ratio. This method reduced the chances of backfiring from air surges caused by throttle manipulation.
After that, you repair all the mechanicals that are broken. You review what went wrong. You revise your tune-up to try to avoid it again. For fuel system adjustments, try precise numerical control over tuning using air/fuel ratio!
Without knowing it, this is something you already do:
- When you adjust fuel injection jetting from plug readings, you are changing air/fuel ratio.
- When you adjust fuel injection jetting from exhaust temp, you are changing air/fuel ratio in each cylinder.
- When you adjust the blower overdrive in blown engines from spark plug reading or exhaust temp., that is air/fuel ratio tuning.
- When you adjust the boost in Turbo cars from spark plug reading or exhaust temp., that is air/fuel ratio tuning.
You can control these situations with precise air/fuel ratio numerical control. For a more finely tuned setup, you can find out what your ideal air/fuel ratio is and adjust your setup to maintain that ratio. When you don’t know the ideal air/fuel ratio for your engine setup or what it is after you make a tuning change, tuning is going from one result to a new one without being fully in control of the outcome.
Some setups are overly rich or overly lean. Spark plug readings and exhaust temperatures are misleading. Some setups have momentary overly rich or overly lean stages in an otherwise good setup. Here are a few scenarios that may occur:
- forward facing scoop increases air into an engine with speed (leaning out the air/fuel ratio without suitable enrichment)
- air density changes
- temperature changes
- barometer changes
- humidity changes.
Air/fuel ratio can be measured with an O2 sensor, however, readings are probably only accurate for fuel mixtures near the stoichiometric AFR ratio. In high output racing engines that are run rich:
- excess fuel enrichment to retard detonation can cause O2 sensor error
- after burning in the exhaust can cause O2 sensor error
- excessive camshaft overlap causing intake dilution into the exhaust can cause O2
The best way to determine AFR is to calculate the ratio of the weight of fuel to the weight of air. That can be determined from the weight of air going through the intake system and the weight of fuel going into the engine. A good air/fuel ratio has good performance and good spark plug color. Adding air/fuel ratio numbers to your racing records is a key to long-term success.
Learn more about this and other factors that make a winning engine combination in our online book store.
Setting up a new MFI fuel system:
- main bypass adjustment to control the air to fuel ratio
- adjustment of the combination of the main bypass and nozzle sizes to maintain an optimum fuel pressure range from the lowest to highest engine RPM.
Changing the main bypass jet for tuning affects the air to fuel ratio and fuel pressure. As long as the jetting combination is maintained for a fuel pressure operating range and the air to fuel ratio, the engine should run very well. The fuel pressure operating range should be over 50 psi on the low end and not to exceed the fuel pump pressure rating at the high end. Many tuners determine all of that with trial-and-error testing. Our writings show how to do that numerically as a quicker way to the best performance.
Simple (neural network) Math for Fuel Pressure vs Flow
fuel pressure in psi = multiplier x [flow in GPM / jet area in sq. in.]2
Multipliers from some popular fuel systems
fuel system examples multiplier
Kinsler (radiused nozzles & bypass jets) 0.00055
Hilborn (flared nozzles & bypass jets) 0.00065
Enderle (square edge nozzles & bypass jets) 0.00075
nostalgia nitro (16 misc nozzles) 0.00120
professional top fuel (misc nozzles @ real high pressure) 0.00140
flow = 12.53 GPM
jet area = 0.029 square inches from our simple blown alcohol fuel system
- hat jets: 8 x 0.053 in. dia.
- MBP: 0.115 in. dia.
- HS: 0.040 in. dia.
Enderle multiplier = 0.00075
fuel pressure in psi = 0.00075 x [12.53 GPM / 0.029 square inches]2
fuel pressure = 140 psi
Throughout our books & ProCalc MFI calculator, this simple multiplier is called volume area (VA) factor. It is very easy to determine for a specific fuel system or jet or nozzle, and it remains a stable value for different tuning changes. With it, tuning is easier!
Note: The actual mathematics for pressure vs. flow is very complex involving calculus. Most racing engine tuners who try to get into that mathematics alternative become overwhelmed.
*Corrected 8 PM PT, 12/05/2018; if you read or downloaded this example prior to that, please read & replace that copy with this corrected version; in the previous copy, an error was noted by Earl Miller, Kinsler Fuel Injection; we appreciate Mr. Miller’s edit & correction.