Tuning Formulas

Linear Scaling

This is the most basic calculation you’ll ever use tuning fuel injection. Take the difference between two values and use it to scale another value.


Also remember your elementary school math:


For example, if you have a block learn mode value of 122, and your target is 128, you have a mutiplier of 0.953125. If you find a value such as fuel pressure, volumetric efficiency, maf calibration, etc. you can multiply it by 0.953125 to get fairly close to your target.

Looking at it a different way (times 100), approx 95.3% of the currently calculated fuel would be required to achieve the target. In theory, of course.

Another example would be AFR on a wideband. If you’re shooting for 13.3:1 AFR in power enrichment, and your wideband measures 12.5:1 AFR, you can multiply the appropriate PE target tables by 0.9398 to get close to your target.

When scaling real-world tables using these values, it’s best to “soften” the change to avoid overshooting it due to imprecision or unknown variables. So in the above example concerning power enrichment, rounding up to 0.95 would be a good idea (producing slightly less of a change)

This works for many aspects of tuning, as long as things are fairly linear.

Always use common sense checks when scaling, make sure if things are lean, your math ends up increasing fuel, and vise versa.

Scale Injector Flow vs. Fuel Pressure

Altering fuel pressure at the fuel rail will change the injector flow rate. Also, when installing aftermarket injectors, they may be rated at a rail pressure different from your target vehicle.

To calculate a multiplier that can be used to determine the new injector flow rate:

Square Root ( New Fuel Pressure / Old Fuel Pressure )

Multiply the result of this equation by the original injector flow rate to yeild the corrected flow rate.

For example, if we have an injector rated at 33lb/hr of flow at 22psi of rail pressure, and we are running that injector at 28psi, sqrt(28/22)*33=37.22. The increased pressure will make the injector flow at 37.22lb/hr.

Conversion for Injector Test Fluids to Gasoline

If an injector was flow tested with n-Heptane test solvent:

Multiply the flow rate by 1.035 (n-Heptane flows 3-4% less in common injectors)

Stoich AFR for Ethanol Blended Fuels

To determine a stoich AFR for an ethanol blended fuel:

( -0.057 * BLEND PERCENTAGE ) + 14.7

For example, 85% ethanol would be (-0.057*85)+14.7 = (-4.845)+14.7 = stoich 9.855

This is becoming very important, as many areas only have 5% or 10% ethanol blends available. Plugging in some numbers, we find a stoich AFR of 14.42:1 for E5, and 14.13:1 for E10.

Referring back to our linear scaling, we find that to be approximately a 2% lean error for E5 and 4% for E10. It’s not much, and that’s well within the range of correction for a common oxygen sensor, so in closed loop it will simply show up as 130-134 trims. However, it will lean out open loop and WOT a bit, so it’s worth taking into account.