Wastegate setting, "What's right for me?"

Posted by Robert on 11/6/2014 to ARD Blog Articles
Proper wastegate adjustment can have a considerable impact on how the Volvo turbocharger control system both develops power and maintains boost pressure across the rev range. A wastegate setting that is too low/loose can prevent full boost target from being reached and create lethargic power development. Consequently a wastegate setting that is too high/tight can cause the boost to over shoot target and, in some cases, can cause the ECU to initiate limp mode. So proper wastegate adjustment is critical for best performance and power. 

Let’s start by defining terms since there is some mixing of terminology in the industry. First off we have the wastegate actuator. Pictured below, this actuator is acted on by boost pressure and in turn pushes it’s threaded rod outward toward the wastegate valve. 

The wastegate valve is a part of the turbo exhaust housing and when opened, allows exhaust gasses to bypass the turbine wheel. Exhaust gasses are ultimately responsible for creating boost pressure. The more the wastegate valve is opened, the less exhaust gas acts on the turbine, and in turn develops less boost pressure. 

The last part of the equation is the Turbo Control Valve, commonly referred to at TCV. This valve is controlled by the ECU and when activated it blocks boost pressure to the wastegate actuator. This blocking keeps the wastegate valve closed allowing exhaust gas to act on the turbine which in turn increases boost pressure. So as you can see, if the TCV were to fail (not react to ECU command) it would ‘Fail safe’ and prevent boost pressure building any higher than the initial wastegate actuator setting. 


Finally, let’s define shortening the threaded adjustment on the wastegate actuator as higher or tighter and let’s define lengthening the threaded adjustment on the wastegate actuator as lower or looser. 

To fully understand the system we need to do a little math. We need to understand how much exhaust back pressure would be necessary to force open the wastegate valve when we might actually want it kept closed. If we measure the diameter of the wastegate valve, solve for area, then measure the mechanical leverage difference between the length of the wastegate valve arms we can determine how much wastegate pretension is necessary for a given boost pressure. 

 Recall from our previous wastegate articles that, in almost all cases, there is more pressure between the exhaust valve and the turbine wheel than there will be pressure in the intake manifold (boost pressure). For an intake manifold at 18psi as an example, exhaust back pressure at the turbine might be as much as 25 psi or more in certain cases. At 25 psi exhaust back pressure and a calculated effective leverage ratio of 7.2 we can see it would take 3.47 psi of pretension at the wastegate just to hold it closed by virtue of the exhaust back pressure. 

So with that said, the only true way to know what optimal wastegate setting is for your setup we need to measure exhaust back pressure. This is not viable for most folks so accepted averages for a given setup are typically fine to calculate from. However there is a more DIY friendly way to check for proper wastegate tension. For best results have a proper wastegate adjusting tool so you know exactly where you're at, however in a pinch here’s how we do a ‘seat of the pants’ method if we don’t have a tool handy, or find that the recommended setting isn't quite enough for a particular setup. 

1. Tighten the wastegate to the recommended setting and test drive, keeping an eye on boost pressure. Ideally a 3rd gear run at wide open throttle to redline if it can be done in a safe and proper environment. 
2. Monitor for boost overshoot. If there is none, increase the wastegate tension by a further 1.0 psi until slight boost overshoot is seen. 10% above target is our rule of thumb for overshoot allowance. So for an 18 psi tune max allowable overshoot would be 19.8 psi. 
 3. Once overshoot is reached then back off the wastegate setting until the overshoot is gone and target boost pressure is reached. 

Depending on the ECU upgrade provider, wastegate setting may vary. This can be a function of how the tuner adjusts TCV maps in the ECU software, so you should consult your tuner for what the best wastegate setting is for your setup both hardware and software. 

That’s it, it’s really that simple :) Got questions? Reply in the comments below!
 

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