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Boost Control: Wastegate Spring Selection

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Wastegate Spring Selection

03.58

00:00 - Most wastegate manufacturers offer a range of wastegate springs to fit inside their wastegates, that allow you to choose a base spring pressure for the wastegate operation.
00:11 The aim of this is to select a spring for the wastegate that will provide somewhere near to your desired minimum boost pressure, before any electronic control is applied.
00:22 This is important for two reasons.
00:25 Firstly, we want to make sure that we can control the minimum amount of power the engine is capable of producing.
00:32 Both from a reliability as well as from a usability perspective.
00:37 For example, with a two-wheel drive car it may be beneficial to have the ability to reduce the boost and hence power, low enough to reduce wheel spin on a wet road.
00:48 The flip side of this however, is that we can't achieve infinite control over the boost pressure using electronic control techniques alone.
00:58 What I mean by this, is that if our minimum boost pressure is perhaps 7 PSI, we may find that the maximum boost pressure we can achieve is 30 PSI, even if we remove the pressure source from the wastegate altogether.
01:13 The reason for this is that there are two forces acting to open the wastegate.
01:18 Understandably, we have the pressure source plummed to the waste gate actuator, providing boost pressure to open the valve.
01:26 But less obvious is the back pressure in the exhaust manifold.
01:30 I'll explain the situation in a little more detail.
01:35 The turbocharger actually presents a significant restriction to exhaust flow and in turn, this results in back pressure in the exhaust system between the exhaust valves and the turbine housing.
01:49 The amount of pressure varies depending on your turbocharger's size and engine capacity.
01:54 But in a factory turbo installation it's not uncommon to see the turbine inlet pressure reach double the boost pressure.
02:02 This means if you're seeing 15 PSI of positive pressure in the intake manifold, you could have 30 PSI of back pressure in the exhaust manifold.
02:13 This back pressure is acting on the wastegate valve and it has the effect of pushing it open.
02:20 This is why even with no boost pressure source on the wastegate actuator, at some point the back pressure will reach a point where it forces the wastegate open and the boost pressure stops climbing.
02:33 This is usually only an issue if you're tuning engines where you're wanting to achieve an extremely wide range and boost level.
02:42 An example of this might be a front-wheel drive drag car where only 10 PSI is used to launch the car but perhaps 45 to 50 PSI might be needed in top gear.
02:53 Options do exist to extend the range of boost control for these sort of applications, such as separate compressed air systems.
03:02 These are however, beyond the scope of this course.
03:05 Even in a street application, the effective turbine inlet pressure is still important to understand, as we'll see.
03:14 When it comes to choosing a spring for your wastegate, you'll find that the springs are rated to a certain pressure, a 10 PSI spring for example.
03:22 This seems simple enough, if you want your minimum boost pressure to be 10 PSI for example, just choose a 10 PSI wastegate spring.
03:32 The problem however, is that the spring rate will assume a certain level of back pressure in the exhaust manifold.
03:39 If your turbine inlet pressure is very high or very low, this will effect the actual boost level that you'll achieve.