Start with the turbo itself
A turbocharger is two fans on one shaft. On the exhaust side, a turbine wheel is driven by the hot gas leaving the engine. On the intake side, a compressor wheel spins with it, drawing air in through the filter and forcing it into the engine. Faster exhaust flow spins the turbine faster, which spins the compressor faster, which packs more air into the intake. The pressure that builds up in the intake manifold is what we call boost.
Why more boost means more power
Compressed air is denser. Every intake stroke then carries more oxygen, which means you can burn more fuel and make more power from the same displacement. That's the whole appeal of a turbo.
It's also the danger. Left unchecked, a turbo keeps accelerating as exhaust flow rises, and boost keeps climbing until something gives, whether that's the engine or the turbo. Turbo shafts already spin well beyond 100,000 RPM, and over-speeding them can destroy the unit. So boost has to be limited, and the way to limit it is to control how much exhaust reaches the turbine.
The wastegate: letting exhaust skip the turbine
A wastegate is a valve that lets some exhaust bypass the turbine and go straight into the exhaust system. Energy that bypasses the turbine doesn't spin it, so opening the wastegate slows the turbo and caps boost. Throwing away that energy sounds wasteful, which is where the name comes from, but it's what keeps the system safe and controllable.
Most wastegates are opened by a diaphragm fed with boost pressure and held shut by a spring. When the pressure on the diaphragm beats the spring, the valve starts to open. On its own, this setup holds boost near the spring's rating. That's mechanical boost control: simple, but it only gives you one boost level, whatever the conditions.
Electronic boost control: the ECU takes over
To run different boost in different situations, the ECU needs two things:
- A way to measure boost. That's the manifold pressure (MAP) sensor, reading pressure in the intake manifold between the throttle and the intake valves. Many Haltech ECUs, such as the Elite 950, have one built in. Otherwise an external sensor like the 5 bar TI MAP sensor does the job.
- A way to influence the wastegate. That's a boost control solenoid, like the 3-port 33 Hz solenoid, placed in the line feeding the wastegate.
The solenoid bleeds away part of the pressure headed for the wastegate. The less pressure the wastegate sees, the longer it stays shut, the more exhaust goes through the turbine, and the more boost you get. By pulsing the solenoid rapidly, the ECU can dial in exactly how much signal is hidden from the wastegate.
A worked example
Say your boost target table is set up against engine RPM and ethanol content from a flex fuel sensor, and you want about 20 psi (roughly 140 kPa) at 6,000 RPM on E85. As you accelerate, the ECU reads the MAP sensor, compares it with the target for the current RPM and fuel, and adjusts the solenoid pulses to steer pressure either toward the wastegate or out to atmosphere. On pump gas the target can be lower, and the ECU simply aims for that number instead.
Because exhaust flow changes constantly with RPM and load, the solenoid never gets to sit still. It's constantly balancing, which is why a good base duty table and sensible closed loop settings matter so much.
What electronic control opens up
- Boost by gear, so first and second don't just spin the tires.
- Boost by fuel quality, with more on ethanol than on pump gas. See how flex fuel control works.
- A dash switch for low and high boost modes.
- Overboost protection that cuts power if something fails.
Electronic control lets you run more than spring pressure, but the spring is still your fallback if a hose pops or the solenoid fails. Choose a spring you're comfortable running on, and always set an overboost cut.
Where to go next
For setting it up on your ECU, start with the boost control user's guide, then our Nexus boost control or Elite boost control articles. Electronic wastegates work differently again and are covered in electronic wastegate.
Boost control FAQ
Can a wastegate alone control boost?
Yes, at roughly the spring pressure. You just can't change it without swapping springs.
Does the ECU control the wastegate directly?
With a pneumatic wastegate, no. It controls the pressure the wastegate sees through the solenoid, and the wastegate responds to that.
