How to Choose a Wastegate Spring

The good news is that selecting the right wastegate spring is a relatively simple and cost-effective way to ensure the best possible spool up and boost stability, with minimal high-RPM boost drop-off, so let’s get to it!

First, a word of caution – when changing wastegate springs you MUST re-adjust your boost control settings afterwards. If you have a manual or electronic boost controller this is easy to do, but if boost is controlled by your ECU and you don’t have access to change it, you should consult your tuner before going any further.

In this article we’ll be concentrating on turbo systems with either an internal or external pneumatic wastegate, connected to a boost control system that uses boost pressure to actuate the wastegate. Other systems such as VNT, vacuum or electric actuators, and C02 boost control systems fall outside the scope of this discussion.

We’ll be using the term “gate pressure” to refer to the boost pressure your turbo system achieves with no input from the boost controller. Gate pressure is directly influenced by the wastegate spring.

Please note however that whilst wastegate springs typically have a boost pressure “rating”, this is a guideline only. It is impossible to predict the exact gate pressure a certain spring will achieve due to the vast number of variables involved in the turbo and wastegate system, but the spring should deliver a gate pressure that is pretty close to its advertised “rating”.

You may have come across this rule of thumb before, but understanding where it comes from will help greatly when it comes to wastegate spring selection.

A pneumatic wastegate is a self-regulating, closed-loop control system, which uses a boost reference signal as a feedback loop. As boost pressure rises, the boost reference to the wastegate diaphragm causes the valve to open more, which prevents further boost increase. If boost drops, the valve will open less to prevent boost dropping further. The spring in the wastegate determines the “equilibrium” point, where all the forces are balanced and boost is stable. This is how “gate pressure” is achieved and regulated by the wastegate.

A boost controller works by bleeding a controlled amount of pressure from the wastegate’s boost reference hose, thereby raising the boost pressure at which “equilibrium” is achieved. The more you bleed off, the higher the boost pressure goes.

But there’s a limit – once you’ve bled off most (or all) of the boost reference signal, there’s nothing left to control the wastegate – the feedback loop is gone, or at least seriously compromised.

This limit occurs sooner than you’d think. For example, it is not uncommon for a small factory turbo with a gate pressure of 7psi to make around 12-13psi stock, which fits within the “rule of thumb”. However, turn the boost up to 16psi and you’re at more than double the gate pressure, and the boost reference signal to the wastegate would be no more than 2-3psi.

Remember that the wastegate uses this reference signal to regulate boost pressure and compensate for variations. But since the reference signal has been slashed from 16psi to 2-3psi, the boost controller’s “authority” over the wastage is significantly diminished, and variations in boost pressure cannot be effectively corrected to maintain stable boost.

Push this same turbo to 18psi and the boost controller is now just a passenger!

Once the boost pressure is more than double the gate pressure, external variables that typically affect the turbo’s ability to make boost (i.e. atmospheric temperature/pressure, engine RPM and load, exhaust backpressure etc) cannot be effectively corrected for and boost control becomes less stable. Spool up will not be as aggressive (since the wastegate will be blown open by the exhaust pressure), peak boost may differ from one day to the next, and boost will likely drop off significantly at high RPM.

You can see from the examples above that the more your target boost and gate pressure differ, the more your performance suffers.

Whilst keeping the boost to no more than double the gate pressure offers acceptable boost control, better performance can be achieved by selecting a wastegate spring that gives a gate pressure 10-20% lower than your target boost. This ensures your wastegate and boost controller have a strong boost reference signal to work with to make corrections, which results in the best possible spool up and boost stability, with the least boost drop off at high RPM.

But what if you plan to use multiple or variable boost settings? You might map your ECU to deliver lower boost at part throttle or in low gears, or you might want to set a boost controller to deliver low or high boost settings depending on road/track conditions.

In this case it is best to select a wastegate spring that achieves your lowest boost setting (i.e. your low boost setting = gate pressure), and ensure your highest boost setting is no more than double the lowest setting.

If this is not possible, a compromise will need to be determined. Either the low boost setting remains where it is, and the high boost setting will not be optimised for performance, or the low boost setting is increased with a stiffer wastegate spring, and the high boost setting is given the best possible performance.

Selecting the right spring for your wastegate is a simple way to ensure you get the best possible performance from your turbo!