Two ways to express the same fuel map
ESP offers two tuning methods, and the base fuel table means something different in each.
| Injection Time | Volumetric Efficiency (VE) | |
|---|---|---|
| The table holds | A raw injector pulse width for each load and RPM site | How well the engine fills its cylinders from the air available in the manifold, as a percentage |
| Target mixture | Built into the numbers you enter | Set separately in the Target AFR table |
| To change mixture | Change injection time until the AFR is right | Change Target AFR; the ECU recalculates fuel |
| Also called | Pulse width tuning | Speed density |
With injection time, you're tuning around both the engine's efficiency and the mixture you want, all in one number. With VE, those two things are separated. The table describes the engine's breathing, and the Target AFR table says what mixture you want.
What the VE number really means
The VE here is not a percentage of what would fit at atmospheric pressure. If it were, the numbers would climb every time boost went up. The ECU has already factored in manifold pressure, so the VE value only describes how effectively the engine pulls in and traps the air that's sitting in the manifold. On a well-behaved engine you'll see it follow the torque curve rather than the boost curve.
Get the foundations right before VE tuning
VE tuning only pays off when the ECU's model of the engine is accurate. Before you start, set up:
- Engine Volume
- Inj Flow Rate table
- Inj Dead Time table (see our injector dead time explainer)
Any error in these gets absorbed into the VE table, and the errors stack up. The payoff for getting them right is big. In a perfect model you could change Target AFR and watch the measured AFR follow exactly, or fit bigger injectors, enter their correct flow data and have the engine run the same mixtures without retuning. Real engines never quite reach that, so always recheck mixtures after a change, but the closer you are, the less you'll need to touch.
Our tuning order
- Fill in Target AFR (VE method only). Idle and light cruise usually sit near stoichiometric, which for gasoline is lambda 1.0 or 14.7:1. Heavy load needs richer targets for safety and torque. Start conservatively rich under load.
- Set up your wideband and confirm which base fuel map is active so you're editing the right one.
- Zero the corrections. Turn off or zero coolant, air temp and other corrections for now, so the base table is the only thing moving the mixture.
- Verify base timing with a timing light (see calibrating ignition timing), and keep the timing map conservative. A brief lean spot while tuning shouldn't be able to cause detonation.
- Warm the engine fully.
- Set injection angle for the smoothest idle. Around 420° BTDC is a typical place to be.
- Set idle speed with the idle control or the throttle stop.
- Tune idle. Bring the idle cells to your idle target, usually 14.7:1, while keeping a steady, misfire-free idle. More in tuning for idle.
- Tune the no-load range. In neutral, sweep through the RPM range you'll use and bring the unloaded cells to target. See tuning with no load.
- Tune under load. On a dyno or another way of applying load, step through the load sites at each RPM. In VE mode, adjust the table until measured AFR matches Target AFR. In injection time mode, add or remove time until you hit the mixture you want.
In VE mode, if you want a different AFR somewhere, change Target AFR, not the VE table. Using the VE table to richen things up breaks the separation that makes VE tuning worth it.
Troubleshooting
| Symptom | Usually |
|---|---|
| VE table looks jagged or odd | Wrong engine volume, flow rate or dead time being absorbed by the VE values |
| Mixture drifts after warm-up | Corrections re-enabled before the base table was finished |
| Changing Target AFR doesn't track | Injector data or dead time inaccurate |
| Rough idle at correct AFR | Injection angle not optimized |
FAQ
Which method should I use?
We use VE on almost everything. It makes later changes, like injector swaps or new mixture targets, far easier. Injection time is still fine for simple engines or if that's what you know.
Can the ECU help fill the table?
Yes. Once the base is close, the Quicktune feature can speed things up. If you'd rather have us do it, book a free consultation.
