FLOW CONTROL
UNDERSTANDING THE IMPORTANCE OF YOUR WASTEGATE
UNDERSTANDING THE IMPORTANCE OF YOUR WASTEGATE
The wise Keiichi Tsuchiya once said, “What is power without control?”; when it comes to boosted applications, the wastegate is front and centre in the fight on over-boosting. So, why is something so integral to the performance and reliability of the boost system often the last concern during the design process? Your turbo has an insatiable appetite for spent exhaust gases; left unchecked, it would consume everything, including your engine’s internals. The wastegate’s job is to keep your turbo in check. It does this by bleeding off exhaust gas pressure over and above a set psi to avoid over-boosting. This psi is set first and foremost by the spring located inside the gate, held in a sealed cavity between the flexible diaphragm and the top cap. The exhaust pressure inside the manifold must overcome the force exerted on the wastegate valve by the spring before the valve will open. A stiffer spring will raise the base boost, while a softer spring will lower it. These spent gases often put on a great show as they are vented to the atmosphere.
The next level in boost control is to add a boost controller to the mix. This will allow you to run boost over and above the spring rate, by leaking off the exhaust pressure pre diaphragm to fool your wastegate spring into thinking that it hasn’t yet reached its psi rating. Even with a controller, you still need a spring, as this will set your base pressure. You’ll find two types of controller available: pneumatic and electronic. Both achieve the same result, although the biggest difference is that an electronic controller is wired to the ECU and allows for adjustable pressures to be prescribed; for example, boost by gear. A pneumatic controller is simply a mechanical tap offering a single level of control dialled in the tap, hence the common name ‘boost tap’.
The wastegate’s performance is directly related to its installation/orientation. There are three very common issues that we see regularly: poor location, incorrect controller plumbing, and incorrect sizing of the gate itself.
To solve the issue of incorrect sizing, we recommend that you speak with your tuner before purchasing the gate. The size is dictated by the engine configuration, turbo sizing, and power goals. As a rough rule of thumb, running low boost will require a bigger gate than running high boost on the same turbo. This has to do with the flow: the lower the boost you run, the more pressure will need bleeding off. If the gate is too small, the resulting backlog of pressure will cause a spike in the exhaust pressure your turbo is experiencing, causing a boost spike. As we said, consult your tuner on this before purchasing any gate.
Equally important to the size of the gate is its location/ orientation on the manifold. Sadly, this is often a second thought in manifold design and most often dictated by the available space in the engine bay. The most efficient location is to have all primary runners merge and then divide equally to the turbo and wastegate at as shallow an angle as possible. However, as many manifolds merge directly at the turbo flange, again due to space restrictions, this is a common location to feed from. The only circumstances in which it is OK to feed the gate off a single runner is when more than one gate is employed; for example, in a 13B rotary with dual gates.