An Advanced Method in fuel injection tuning for certain normally aspirated (NA) engines is to flatline the fuel curve. Flatlining is not always possible, especially in very high flowing cylinder heads or efficient pent roof heads on small displacement engines. However, in race engines with common wedge heads or early model Hemi heads, volumetric efficiency normally falls off quite a bit with engine speed beyond the peak torque RPM. Flatlining is a method for max power fuel control in this case. Flatlining is further explained in JFIBB, p. 38. And in JFISB, p. 31.

Note the following Example:

• Volumetric efficiency at horsepower peak for wedge & early Hemi — drops from 100% at the torque peak from the previous example; down to 80% at the horsepower peak
• Air to fuel ratio at horsepower peak — 5.26 to 1 (similar value as before)
• High speed jet — big increase from 0.049 inches diameter to 0.092 inches dia. for engines with 80% volumetric efficiency at horsepower peak; even a larger size or no jet at all in some fuel systems.
• Max. fuel pressure @ 8,000 RPM with the larger high speed open — 55 psi.; only a slight rise from the high speed opening point previously determined.

NA Volumetric Efficiency @ Horsepower Peak Explanation
The volumetric efficiency of the engine at the horsepower peak is lower than it is at the torque peak for these engine designs. At the horsepower peak, intake port flow becomes limiting for the high speed engine demand. That reduces the amount of air in an intake pulse as engine speed goes up. However, the number of power pulses per second increase with engine speed. So horsepower is commonly higher for a range of engine speeds above the torque peak. That is the case up to an RPM limit. Above that, there is a significant reduction in power from engine friction & intake port flow limits. That power reduction is more than the increased power pulses per second at the higher RPM. And horsepower falls off. That all assumes the engine valve train, short block, and ignition are capable of high RPM to a horsepower peak value.

NA Volumetric Efficiency Lower @ HP Peak Explanation
In this example, volumetric efficiency at the horsepower peak was 80%. That is a common value for volumetric efficiency at the horsepower peak RPM of a big block and useful to set up flatlining using Pro-Calc.

No High Speed Jet in Some NA Fuel Systems
Flow bench services often find that the high speed bypass flow through a spring loaded poppet or regulator causes a flow restriction that is adequate for flatlining even with no jet installed. So these systems are delivered that way. High speed poppet spring rate and number of shims can significantly affect the flow restriction of this component. For example, the amount of opening may be restricted by a lighter spring with a lot of shims verses a heavier spring with no shims. A limited opening from the shims is more restrictive and fuel flow is reduced by the poppet alone. In this case, flat lining is limited or cannot be achieved. A stronger spring with fewer or no shims may be less restrictive to fuel flow and flat line better.

No High Sped Jet In Flowed NA System vs. Large High Speed Jet In Calculated NA System
Several flowed systems were analyzed that were set up with no high speed jet installed. The restriction in flow through the spring loaded poppet or regulator was equivalent to a larger high speed jet such as 0.092 from the previous example developed from our math section in “Fuel Injection Racing Secrets“, Appendix 3, and in the Pro-Calc calculator. When a high speed jet is put into the calculator in that size with a volumetric efficiency of 80%, good air to fuel ratio numbers are produced in the 5.26 to 1 range in this example. The calculator can be used to assist a flatlining fuel curve. Setting up a normally aspirated FI fuel system starting out with no jet in the high speed is not recommended without first tiptoeing during testing. And ProCalc makes it easy to keep numerical control over the flatlining setup.

Maintain Fuel Pressure
Fuel pressure should be at an engine speed just above the torque peak RPM. It should also be at or above 50 psi to maintain engine response. If the fuel pressure drops below 50 psi at the torque peak opening point, the nozzles should be reduced in size to bring the fuel pressure back up over 50 psi at the torque peak opening point. Several fuel injection manufacturers report also that there is no horsepower gain from high speed fuel pressures above 100 psi in most NA fuel injection systems. That is from using the fuel injection parts as they are manufactured today.

Testing For Flatlining An NA High Speed Bypass
With a system that is not flowed, a high speed bypass jet can first be installed according to the values in our jetting books. Over several test runs, it can be slowly increased in size monitoring spark plug color after loading, engine temperature, and engine power throughout the interim.

Vital Main Bypass Maintenance for Torque Peak AFR
The main bypass jet must be adjusted for air density changes during the interim testing as well. This is necessary to keep the same air to fuel ratio at the torque peak. For dramatic changes in air density, the nozzles may need changing to keep the total nozzle & jet area the same. That is necessary:

• to keep the system pressure the same
• to keep engine response the same
• to keep the high speed poppet or regulator opening at the same engine speed.

Adjusting for Air Density Changes During Interim Testing
Pro-Calc makes it easy to determine the main bypass and nozzle changes necessary for that interim testing.

• When the main bypass is the only adjustment for different air densities, the proper size to control the AFR can be determined. Changes to fuel pressure result from this. Fuel pressures can also be determined from the calculator to make sure they are not too low for poor response or too high for fuel pump damage.
• If both nozzle & main bypass adjustments are made for different air densities, the proper sizes of both can be determined to control the AFR & maintain the fuel pressure.

The web site www.airdensityonline.com/free-calcs/ can calculate air density values from known temperature, pressure, and humidity forecasts. And for most USA & some Canadian race track locations, www.airdensityonline.com/tracks/ can provide air density forecasts without computation.

Unfortunately changing the nozzles often requires a lot of work & expense of added nozzles. As a result, tuners will more often change only the main bypass jet size.

Consequence Of Changing Only Main Bypass Jet Size For Air Density Changes
When the main bypass jet is the only change for air density corrections, the total system nozzle & jet area will change. For example, for an air density on a hot day that is lower from the previous 100% value, the main bypass jet size needs to be increased.

• This is to reduce the amount of fuel to the engine to maintain the same air to fuel ratio.
• The total system nozzle & jet area increases from this change.
• The fuel pressure drops for the new setup.
• Engine response may be less because of lower fuel pressure.

There are several variables that occur from changing only the main bypass for different air densities:

• engine response from air to fuel ratio
• engine response from fuel pressure
• high speed bypass opening point.

Which ones are affecting engine performance are often not known. As a result, the tune-up is wandering in the dark.

Determination of New Fuel System Pressure
Unless the high speed poppet or regulator is re-adjusted for different main bypass jet sizes, it will open at a differing engine speed. In this case, the engine speed will be higher. That introduces another variable into the testing interim that makes tuning difficult. Pro-Calc again makes it easy to determine a new fuel pressure value for a differing system nozzle & jet area total. That new value can be determined at the target high speed opening point. The poppet or regulator can be reset to open at a consistent engine speed, 5,600 RPM in the above example.