Fuel Injection Tuning Established by Racing Class Rules

Mechanical fuel injection (MFI) is the standard in many racing categories such as drag racing, circle racing, high speed events, and slalum. All of these have restrictive rules that affect MFI setup & tuning.

Top Fuel, Top Alcohol, ProMod, and several Nostalgia racing categories are examples in drag racing.  MFI provides the ability to modulate large volumes of fuel necessary in many of these classes.  Extensive rules have dramatic influence on MFI setup & tuning.  They profile the tuning, resulting in a relatively uniform performance level that is common to most competitors.  Narrow tuning practices because of the rules can suppress performance well below what is attainable from a better understanding of MFI science.

Amount of Air
Engine sizes, blower sizes, & blower speeds specified by racing rules determine the volume of air to the engine. Air density determines the weight of that air volume to the engine.

Amount of Fuel
Specified fuels, fuel  mixtures, and fuel pump size (in some cases) determine the weight of fuel potential.  Jetting regulates the weight of fuel to the engine.

Both of these profile MFI setup & tuning.
1. The selection of MFI parts and the initial setup are governed by many rules.
2. Tuning can be narrowed down by the rules as well.

Competitors most often all operate within a narrow range of tuning variables.  Performance levels often vary only a little between the fastest and slowest qualifiers.  As a result, most participants get by without a need to understand the science of fuel injection.  They all operate within narrow parameters with information exchanges in the pits or word to mouth that causes a natural level of performance increase. That level may be well below what is attainable from MFI science however.

Nostalgia A-Fuel Drag Racing Example — key words: normally (naturally) aspirated methanol nitromethane nitro high speed bypass high speed poppet bypass poppet
The fuel pump size and engine size are all specified by the class rules.  In that combination with a very high percentage of nitro, all of the fuel is fed to the motor at the low end with no need for any MFI main bypass jet for tuning.

Note: The main bypass is more common in other forms of racing for setup & tuning of MFI at the low end engine speed.  In fact, in most other racing with MFI, the main bypass is vital for setup & tuning.

  1. Around the torque peak (low end) of a typical Nostalgia A-Fuel drag race engine on high percentages of nitro, all of the fuel is put to the engine. For dialing in the setup, other adjustments are done such as a few degrees of ignition timing change or a small change in nitro to methanol percentage. With a rules specified transmission and clutch combination, engine RPM at the launch typically starts from the torque peak (low end).   Since there is no main bypass jet, setup & tuning is done independent of that fuel injection provision.
  1. As the engine RPM climbs above the torque peak in this normally (naturally) aspirated engine, a high speed bypass is needed to flat line the fuel curve. The high speed opening function is usually controlled by a poppet that opens at a specific fuel pressure. Accurate control over the opening pressure is vital to open that poppet just beyond the torque peak engine RPM.  Once the pressure is set properly, a minor change may be necessary for air density changes.  That changes the flat line RPM and the amount of fuel bypass from the poppet opening RPM on up through the HP peak. It is reported in one well developed setup that little tuning is needed for changes in air density.
  2. 2a. To compensate for high air density, a slightly higher pressure setting delays the opening point and increases the fuel amount for the high end.

    2b. To compensate for low air density, a slightly lower pressure setting lowers the engine speed that opens the HS bypass; that decreases the fuel amount for the high end.

This simple setup is a result of a combination of rules that limit the tuning tasks.

Opening HS too soon >>> Opening the high speed poppet too far below the low end causes a really difficult tuning task.  The amount of low end fuel would be controlled by these high speed bypasss settings. Those settings may not provide the right amount of fuel necessary for the high end.  Some tuners with this burdensome setup may try to compensate with changes to nitro percentage, spark advance, launch RPM, and/or compression to try to dial in a setup.  As a result, tuning complexity increases greatly from a high speed bypass that opens too early from below the torque peak low end RPM.

Opening HS too late >>> Opening the high speed poppet too far above the low end causes a really difficult tuning task as well.  The amount of high end fuel is increased for the entire engine speed range above the low end. Even though the fuel mixture may be correct for the launch or torque peak, it may be drunk rich for the high end with poor performance. Adjustments to nitro percentage or ignition timing may compensate for the high end enrichment but be all wrong for the low end.

Reading spark plugs >>> Trying to read spark plugs after a run could provide misleading indications. Remember, the spark plug is an indicator of the entire run. It may have one characteristic at the low end and another at the high end.

Tuning low end separate from tuning high end >>> This is one reason we recommend separating launch or low end tuning from high end tuning for a new setup.
1. Low end tuning should be done by clicking the engine off after a launch to get a spark plug reading before the reading is contaminated by a high end characteristic.
2. Once the low end is sorted out with a know spark plug reading, the high end can be sorted out with the high speed bypass independent of the low end need.

Tuning Limits in Sprint Car or Boat Circle Racing from Class Rules — key words: normally (naturally) aspirated methanol main bypass high speed bypass high speed poppet bypass poppet

In Sprint Car (or Boat) circle racing, a narrow window of fuel injection tuning results as well from a combination of restrictive rules.  Sprint cars (or boats) operate on a racing course that involves lower speeds around turns with engine setups for the torque peak at those speeds.  Engine setups are further done with higher speeds at the horsepower peak on the straight-aways.  Mechanical fuel injection setups are common with a high speed bypass that opens shortly after the racer comes out of the turn.  In this case, MFI tuning in the turns is done (or should be done) with only the MFI main bypass.  Once a good setup is achieved in the turns, dialing in the high end is done with the high speed jet size or poppet pressure.  Setups that are maintained in this manner conform to the operating mode of the race.

High Speed ‘On’ Too Soon
Setups are done by some where the high speed bypass may be open all the time from the low end around the turns.  They are really hard to tune.  Controlling engine stumble,  temperature, and power would be done with one of several MFI adjustments: main bypass jet, high speed bypass jet, or high speed bypass poppet pressure.  That is too many and tough to handle.

High Speed ‘On’ Too Late
Those setups that open the high speed bypass at the end of the straight-away would be hard to tune as well.  In this case, it is difficult to maintain a responsive fuel mixture at both the low end and into the high end as the engine speed climbs.  Trying to compensate with a main bypass adjustment would be difficult. It may either be too lean at the low end or too rich beyond the torque peak.  With a late high speed opening point, a good mixture ratio at both ends would be difficult with only a main bypass change. I saw many MFI racers lost because of problems such as this.

Enrichment for Cooling >> Power Reduction Consequence
The high speed poppet is changed by some tuners to control engine temperature.  When that poppet setting is changed, it changes the flat line RPM.  This is the engine speed where the high speed bypass opens.  If it is necessary to be raised for a richer mixture for cooling, that can reduce power.

Several setups that we examined with our calculator were on the excessive rich side of the fuel mixture.  That is needed in setups with engine cooling limits.  Those may be from undersized cooling systems to save weight.  However, extra fuel has to be carried that may offset any weight savings from a lighter cooling system.

In many, inadequate cooling system maintenance is the cause of poor engine cooling.  More extra fuel is needed to maintain engine temperature from extra enrichment to compensate for a poor cooling system condition.  Again, more fuel weight is needed.

Excessive enrichment reduces power.  That is an added consequence.  Superb cooling system maintenance is vital to minimize this power robbing alternative.  A new radiator or a radiator rebuild may be a better investment than a new fuel pump, a fresh set of cylinder heads, or new piston rings.

Top Speed Racing
Sprint Car (or Boat) circle racing engines may not work well in other forms of racing with MFI that may depend on a different range of performance.  For land or water top speed racing, a higher peak power level is better from peaky, large ports, camshaft timing, and short ram tubes.  Mechanical fuel injection may be set up without a high speed bypass with high end tuning done strictly with a main bypass.

Slalom or Road Racing
For slalom or road course racing where engines may have to pull from idle speed on up, a wider engine speed range may be better from smaller ports and less camshaft timing.  Mechanical fuel injection may be set up with a secondary bypass for a better transition from a closed throttle to snapping open the throttle.  This may be a good application for the Engler stumble valve as well.  Mechanical fuel injection may require a different mixture ratio for slalom or road race than a circle race.

More information about proper torque peak tuning, HP peak tuning, secondary bypass, stumble valve, and flat lining is throughout High Horsepower Tuning for Mechanical Fuel Injection for Normally Aspirated Racing Engines.