Start with the wastegate
A turbo is a feedback loop. Exhaust spins the turbine, the turbine spins the compressor, the compressor makes boost, and more boost means more exhaust energy to spin the turbine even harder. With nothing in the way, that loop keeps climbing until a part fails.
The wastegate breaks the loop. It's a valve in the exhaust ahead of the turbine that can open and send some exhaust around the turbine, either into the downpipe or out to atmosphere on some race setups. The less exhaust reaching the turbine, the less boost the compressor makes.
The valve is held shut by a spring behind a diaphragm. A hose from the compressor side feeds boost to the diaphragm. Once that boost can push harder than the spring, the valve cracks open and boost levels off. That's why people talk about "spring pressure" or "wastegate pressure": with no controller at all, the spring decides your boost. Change the spring preload and you change that base boost level.
Where the solenoid comes in
To run more boost than the spring allows, you have to stop the diaphragm seeing all of the pressure. An aftermarket ECU does this with a boost control solenoid placed in the hose to the wastegate. The ECU pulses the solenoid with a duty cycle between 0 and 100%. Each pulse bleeds off some of the pressure headed for the diaphragm.
- Low duty: little is bled off, the diaphragm sees nearly full boost, and the wastegate opens near spring pressure.
- High duty: more is bled off, the diaphragm sees less, and boost has to climb higher before the wastegate opens.
As a general rule, more duty means more boost. Haltech's 3-port and 4-port solenoids are the usual choices, and the plumbing differs between them.
Two ways the Elite can decide the duty
Because duty is just a number, the ECU can map it against whatever you like. The Elite gives you two strategies:
| Open loop | Closed loop | |
|---|---|---|
| How duty is chosen | Read straight from a table you tune | Base duty from a table, then adjusted by the ECU |
| What the ECU is aiming for | Nothing, it outputs your number | A target boost pressure |
| Reacts to conditions? | No | Yes, it adds or removes duty when boost is under or over target |
| Best for | First setup, diagnosing hardware | Consistent boost day to day |
Closed loop has a reputation for being mysterious, but it isn't magic. The controller follows fixed rules about when it's allowed to correct and by how much. Once you understand each setting's job, it behaves predictably. The same PID ideas apply across Haltech's closed loop functions, and our PID control mapping article is a good primer.
The first two choices in ESP
On the Boost Control tab, two settings shape everything else:
| Setting | Options | Our advice |
|---|---|---|
| Mode | Open Loop, Closed Loop | Get the car working in open loop, then move to closed loop for street and road use |
| Controlled Parameter | Manifold Pressure, Wastegate Pressure | Manifold Pressure for nearly every build |
Manifold Pressure means the controller chases boost measured by the same MAP sensor the ECU already uses for fuel and timing. There's nothing extra to fit, and you're controlling what the engine actually sees. Wastegate Pressure exists for CO2 systems that use two solenoids to pressurize the wastegate. That is a specialist setup, so check the wiring and sensor requirements for your system before choosing it.
A practical order of operations
- Set the wastegate spring to a safe, known boost level and confirm it with no solenoid duty.
- Wire the solenoid, set the output frequency to match it, and check it clicks when tested.
- In open loop, build a duty table that gets close to the boost you want across the rev range.
- Copy those duties into the closed loop base table, set your target, and switch the mode to closed loop.
- Log boost, target and duty together, and refine from the logs.
If the ECU's corrections are doing a lot of work, your base table is wrong. A good base means the controller only has to make small, smooth adjustments.
Set up an overboost cut before you raise boost above spring pressure. A split hose or stuck solenoid can send boost well past target in a second.
FAQ
Why does my car make spring pressure no matter what duty I command?
Usually the solenoid is plumbed wrong, isn't assigned to the output you think, or the wastegate is being fed from somewhere the solenoid can't bleed. Check the plumbing against the solenoid's instructions first.
Is closed loop worth it on a street car?
Yes. It holds a steady target through weather and temperature changes that would make open loop boost drift.
For the full ESP reference, see our Elite boost control page and the boost control user's guide. If you're running an electronic wastegate instead, start with electric wastegate control.
