One hundred twenty six full throttle dyno pulls. We're pretty sure Jesse Rodriguez of Diamond Star Specialties had no idea what he was getting into when he volunteered his car for our boost controller test. Amazingly, the only problem we had was paint melting off his rear bumper.
For such a seemingly simple device, boost controllers vary wildly in cost and complexity. Electronic boost controllers offer in-cabin adjustability, supposedly better boost response and advanced mapping capabilities, but some claim mechanical systems work equally well at a fraction of the cost.
The small hole to the left...
The small hole to the left of the turbine is the internal wastegate valve. These internal wastegates are sometime called swing valves because they swing open.
In an effort to learn the real difference between $50, $250 and $750 boost controllers, we ran all the controllers we could find on Rodriguez's very angry Eclipse GS-T Spyder.
Boost Control Basics
Every modern turbocharger uses a wastegate to regulate boost. The wastegate is simply a valve that allows exhaust to bypass the turbine, limiting the energy available to drive it . When designed into the turbine housing, they're called internal wastegates, the kind found on nearly all production cars. External wastegates are located separately from the turbocharger, somewhere between the exhaust ports and turbine housing. The remote location allows the valve to be bigger and flow path smoother, resulting in better boost control and the ability to safely manage a more powerful engine.
The gold-colored can is the...
The gold-colored can is the wastegate actuator. Boost pressure is this can pushes on a diaphragm to open the wastegate.
The wastegate is held shut by a spring, and as boost builds, the wastegate actuator diaphragm pushes the wastegate open against this spring. The size of the diaphragm and strength of the spring determine how much boost it takes to open the wastegate.
All the boost controllers in this test act on the pressure signal that opens the wastegate. Reducing this pressure will keep the wastegate closed, allowing more boost to build. When the desired boost setting is reached, the controller lets pressure into the diaphragm, opening the wastegate.
Electronic controllers use either stepper motors or solenoids to do this. Stepper motor assemblies vary the opening of a precision valve, similar to a water faucet. This action is accurate, though somewhat slow. Solenoids, however, operate in either a fully open or closed position, but can change this state very quickly, feeding and releasing full pressure so quickly that the pressure in the diaphragm can be accurately controlled. Dual solenoids will give an even greater degree of precision. The HKS EVC EZ, EVC IV, and GReddy PRofec A all use an identical stepper motor assembly. The GReddy PRofec B and Blitz SBC i-D share a common dual solenoid design, though they drive them differently. A'pexi stands alone with its single solenoid design.
If a wastegate is set to 10 psi, it may start to open around 6 psi, slowing the turbo's acceleration and finally reaching equilibrium at 10 psi. Boost controllers can improve turbo response by keeping pressure off the wastegate as long as possible, making more energy available to drive the turbine. Just as boost reaches the set point, pressure is dumped on the diaphragm to rapidly open the wastegate, and hopefully keep from spiking the boost.
The Testing
Testing was performed on a 1997 Eclipse GS-T with a front-mount intercooler, big injectors, cams, a popular hybrid turbo known as a Frank Stage 3, and a 3-inch exhaust. Two key points to remember when comparing these results are that this car exhibits minor boost creep and it has an internal wastegate.
We intentionally tested a car with boost creep because it's a common problem. In a later test, we'll use these same controllers with an external wastegate.
A Dynojet was used to simulate the load of accelerating on the street in third gear. Our two basic tests tracked boost when the throttle was slammed open at 2000 rpm and at 4500 rpm. The 2000-rpm test would give the controllers a chance to highlight any improvement in response, and illustrate their stability through the rev range. Tipping in from 4500 rpm challenges the controllers ability to suppress boost spiking as the turbo is already up to speed.
Interpreting the Results
When interpreting the data, don't dwell on the final boost level. We were shooting for 12 and 17 psi, but didn't spend too much effort landing right on those values. More important is the rate at which boost builds, and the controller's ability to prevent spikes and hold a constant boost level. The tests at 2000 and 4500 rpm are shown on separate charts.
The transition from building boost to holding it is a demanding event for the boost controller. The longer boost can be kept from the diaphragm, the quicker the boost will build. Wait too long and it will overshoot, open it early and response is soft. Even if you get the timing right, there's the problem of how much to open. Again, not enough and the boost keeps going up, too much and your gauge will bounce like a basketball.
Finally, remember these tests were done on a car with an internal wastegate that does exhibit boost creep. Some controllers specifically warn against use on engines like this, and not surprisingly, they didn't work so well.
How We Did It
Testing is all in the numbers, and you need lots of numbers to really understand what's going on with seven different boost controllers. For this test we used an Edelbrock QwikData. Boasting a maximum sampling rate of 500 Hz, it had no problem keeping good records of our boost controllers' performance. We used two of the eight analog inputs for TPS and boost, one of the six digital inputs for rpm. The Windows-based software allowed us to monitor all the sensor readings in real-time on screen to make sure everything looked good. Watch for a complete review as soon as we get more creative with this great tuning tool.