The problem it solves
With CO2 boost control, a popular approach is to have the Nexus control wastegate pressure, the pressure on the wastegate's top port, instead of manifold pressure. It's easy to tune and very repeatable. The catch is that the target numbers don't match boost. You might need to target a wastegate pressure a few psi above the manifold pressure you want, and that gap changes with the setup. That's fine for the tuner, but confusing for the driver changing targets between rounds at the track.
The Closed Loop Manifold Pressure Expected table is a helper table that stores the relationship between the two. Once it's filled in, you can set boost targets in manifold pressure and let the ECU work out the wastegate pressure it needs.
How to set it up
- Get wastegate pressure control working first. Tune the boost control system with wastegate pressure as the controlled pressure, until it hits different wastegate targets reliably.
- Collect data. Make runs at a range of wastegate pressure targets and log manifold pressure on each. You're building a map of what boost each wastegate pressure actually produces on this car.
- Fill in the table. Enter that relationship in the Closed Loop Manifold Pressure Expected table.
- Change the target table's axis. Set the Closed Loop Target Pressure table to use Boost Control Manifold Pressure Expected as its axis.
Now the target is set in boost. In Haltech's example, asking for 20 psi of manifold pressure leads the ECU to target 23.3 psi at the wastegate, because that's what the correlation data says it takes on that car.
Mapping it against other things
The expected table doesn't have to be a single line. You can add axes to it, so the relationship between wastegate and manifold pressure changes with other conditions. Haltech's example uses vehicle speed and a rotary trim knob, but many other ECU channels can be used. That's useful when the same wastegate pressure gives different boost at different speeds, or when you want a driver-adjustable boost setting that still reads in real boost.
Tips from our tuning
- Collect the correlation data in consistent conditions, and redo it after any change to the turbo, wastegate, exhaust or cam.
- Check that your logs agree with the table after every few passes. If boost is consistently above or below what the table predicts, update the table rather than chasing it in the targets.
- Keep an overboost cut set on real manifold pressure. The expected table is a translation, not a safety net.
Haltech's CO2 boost control dual solenoid kit includes the pressure sensor you need for wastegate pressure control. See also the closed loop base duty table.
