Boost control and wastegates

The spring sets minimum boost. The solenoid hides pressure from the gate to raise it. Spool, creep and altitude are the surprises.

A turbine driven by exhaust gas spins a compressor. More exhaust energy means more boost, which means more exhaust energy: positive feedback that keeps going until something limits it. The wastegate is that limit — a valve that bypasses exhaust around the turbine. A spring holds it shut; boost pressure on the actuator opens it.

The duty table

With 0 % duty the gate sees full boost and opens at the spring pressure: that is the minimum boost. The solenoid bleeds pressure away from the actuator in proportion to duty, so the gate sees less than real boost and opens later: more boost. A rough rule is boost ≈ spring ÷ (1 − gain × duty). In practice the gate also needs a little extra pressure to open far enough to bypass the surplus, so real boost sits above the simple formula — which is why the stock table is found empirically, and why you will adjust it.

Things that will surprise you

• Spool overshoot: the turbo comes on faster than the gate can respond; boost spikes above target right at the rpm where the rods like it least. Taper duty down through the spool.
• Creep: at high flow the gate cannot bypass enough even wide open, so boost rises with rpm regardless of duty. That is hardware, not a table.
• Altitude: the turbo must make a higher pressure ratio for the same gauge boost in thin air, so shaft speed rises. Think in pressure ratio, not psi.
• Boost cut: the fuel cut that saves you when a cell is wrong. Set it before the first pull.

Closed-loop boost

Real ECUs rarely run open-loop. In closed-loop (Scalars → Boost control) you fill in a Boost target table in boost pressure per rpm, the duty table becomes the base (the feed-forward starting point), and a PI controller trims the base toward the target using the MAP sensor, within a trim limit, whenever the throttle is above the lockout.

• Kp acts now: it shuts the gate harder the further boost is below target. Too much and the boost hunts fast.
• Ki acts on persistence: it removes the steady offset a wrong base table leaves. Too much and the boost hunts slowly.
• The trim limit is the honesty check. A controller sitting at its limit is telling you the base table is wrong by more than the limit, or the hardware cannot make the target. Fix the base table; do not widen the limit.
• The base table still matters. Tune it in open-loop first, then switch the mode on: a base that is nearly right needs tiny trims and small gains, and spools exactly like open-loop did.
• The target is rpm-only here. A real ECU also trims it against intake air temperature and gear, which is how a cold day is handled; in this app the cold day has to be in the table.