Wastegate spring pressure theory

Hi guys,

This might seem like a simple thing but trying to get my head around the theory. When looking at wastegate manufacturers specs and the miriad of info online, everyone talks about XX psi springs in their wastegate.

Say we have a setup with proper sized wastegate area (i.e. no boost creep because of this) - hooked up to just the boost reference from the compressor housing and a "21psig" spring without any boost controller interfering. Won't this lead to a decreasing boost pressure as rpms goes up by default?

Since the exhaust flow increases as rpm rise and presumably EMAP does as well, does this not act as additional pressure forcing the wastegate open, therefore lowering the IMAP level that the "21psig" spring will enable?

Am I missing something here or is any "xx psi" advertised spring a simplification of things?

Followup question: does this also mean that achieving a flat boost pressure throughout the powerband is nearly impossible without electronic boost controllers with proper PID?

Thanks for any insights!

Remember that the wastegate is trying to maintain a "difference" between one side an the other of the wastegate, with one side being the force applied to the wastegate inside the turbo, and the other is the spring.

So lets say you have 21 PSI of pressure acting against the wastegate, and the spring is trying to hold it shut with a 21PSI spring. In this case the wastegate will remain closed as everything is in balance. However if you apply 22psi to it, it will start to open as its overcoming the spring by 1PSI, allowing some gas to bypass the turbine, and go through the wastegate opening, effectively lowering the amount of boost the turbo produces until it reaches the point the wastegate goes back into balance (21psi on both sides) and then closes again.

Now lets introduce boost reference. So now instead of their being atmospheric pressure being inside the wastegate. There is now the same pressure as what your manifold has.This extra pressure helps the spring hold the wastegate shut.

So keep in mind that RPMs, is about increasing the engine volume of air it is flowing, not pressure. So think Litres per minute, not KPA\PSI.

So an engine spinning at 2000rpm with 20PSI of boost, will have the same pressure as an engine at 6000rpm and 20PSI.

As a result the wastegate which is trying to keep the pressures in balance, will still work the same regardless of RPM.

Where you see differences based on rpm, is due to flow through the turbine and wastegate. These holes in the turbo have restrictions, as RPM's climb there is more gas per min to flow through the same area. As a result, this can change how much gas goes through the turbine vs the wastegate port and is different for every turbo design.

Its this difference in flow rate that requires "boost control" to help maintain. The spring in the wastegate, even boost referenced will only do so much, in some turbos\engine combos it might see the boost taper off as rpm climbs, in other cases it might climb. And this can be altered based on engine\exhaust temp as well as it changes the volume of gas (hot gas has more volume than cold gas)

This is where a boost controller comes in, it can monitor for these changes an then by changing how much "boost reference" hits the back of the wastegate using a solenoid therefore opening or closing the wastegate differently.

Paul put it very well.

I'm not sure what you mean about the boost pressure decreasing, unless you're thinking of the exhaust turbine having to spin the turbo' impellor faster to meet the air demand of the engine while holding the boost? While the airflow demand increases, so does the exhaust gas volume.

It might be easier to think of the turbo-charger as two parts, where the turbine is applying a torque to the common shaft, with the impellor receiving the torque to compress the intake air against the boost pressure.

In practice, the shaft assembly is rotating, and put simply, the exhaust gas volume and pressure is applying the power to the impellor that is pushing the air volume against the resistance in the manifold.

Obviously, power out depends on power in.

To limit the boost, power out, the easiest way is to reduce the power in.

Limiting the exhaust pressure limits the torque in, which limits the torque the turbine can apply against the intake pressure.

The easiest way to limit the exhaust pressure is to bleed some of the exhaust volume off before it passes through the turbine.

With a pure spring, it's a nominal value for lifting the bleed valve off it's seat, and because the spring increases it's force on the valve as it opens, the pressure will invariably rise slight (or significantly) as the balance between exhaust gas pressure, the spring force, and the valve opening find a balance. If the valve is too small for the flow, there may be a point where there's a noticeable climb in exhaust pressure, and hence boost.

It's also quite a crude system.

If one uses a controller of some type that aids the spring, it's possible to get better control over the valve as it opens, supplimenting the spring.

Ultimately, one could take the spring completely out of the valve assembly, using an external CO2 or nitrogen supply, but having one in place makes sure the valve is physically shut and can't leak.

Some campanies offer valves that are entirely controlled by electronics, and I think they aren't being fully utilised by many, as it is possible to tailor the torque curve of the engine to flatten it out and avoid, or at least reduce, the torque wheelie/wheelspin that can arise at maximum torque by reducing it, and/or help fill in the curve below and above that point, to make the most of the traction/chassis.

This varies massively depending on the wastegate you get Tom.

When it comes to external gates, a Tial wastegate will run at or near spring pressure unless exhaust pressure gets pretty extreme, but a Turbosmart wastegate will be very heavily impacted by exhaust pressure. As long as you're aware and plan for it, either can work well, but they do behave rather differently.