EXTERNAL WASTEGATES
An external wastegate does the same job as an internal wastegate, but as its name suggests, is a separate component to the turbocharger. Inside, you’ll find a spring and a diaphragm (as you would if looking inside an actuator), but rather than the parts moving an arm which opens a valve, they control the valve directly.
The main benefit of an external wastegate is that the part allows us to use a much larger wastegate hole through which exhaust gases can bypass the turbocharger. For example, many modern aftermarket wastegates can be bought with an exhaust gas channel measuring between 38mm and 60mm in diameter. To put this into perspective, the standard internal wastegate on a Garrett T4 turbocharger is just 22mm. A larger hole allows us to control boost far more accurately, primarily because more exhaust gases can be allowed to bypass the turbo.
A secondary benefit of using an external wastegate is that the turbocharger can use a more refined design of exhaust housing thanks to the absence of an internal wastegate mechanism.
On a conventional turbocharged engine, an external wastegate needs to be located in the exhaust manifold between the cylinder head and the turbocharger. Many aftermarket performance exhaust manifolds will have bosses already fitted to enable the use of an external wastegate. That said, be careful if using a twin scroll manifold (as described in last month’s issue of 911 & Porsche World)
— because of the divide within the pipework, you’ll need to ensure you’re using two external wastegates, one per scroll. In truth, with careful preparation of the manifold, you can use a single wastegate in this instance, however, there is plenty of evidence to support claims this is a compromise that’ll cost you many hundreds of rpm in boost response.
Most external wastegates offer multiple boost ports, meaning you can provide boost above and below the diaphragm for even greater boost control. Incidentally, when talking about wastegates, you may have heard owners of older turbocharged Porsches mention boost creep. This is where a turbocharger produces more boost than desired due to the wastegate not being able to ditch exhaust gases quick enough. An external wastegate will eliminate the problem thanks to a larger channel for gas to escape.
Some recently developed external wastegates, such as the Turbosmart E-WG45 and E-WG60, give you extreme control, the likes Porsche engineers working on the original 911 Turbo can only have dreamed about. Independent of any reference signal, without the need for compressed gas and fully adjustable from your tuning interface, there’s no need to change springs for different tunes or applications. In other words, boost control is now completely adjustable from a laptop. Put simply, as boost control strategies became more complex, a solution was required for maintaining performance and reliability, but bolstering it with truly infinite control. The resulting products operate on a current draw whilst providing fast and efficient boost control, in some cases against up 120psi. There’s provision for water cooling, too.