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Wastegates


As alluded to before, the wastegate is a system that can limit turbine RPM of a turbo. In a sense, a turbo is a positive feedback system. As the turbo creates more boost, it also creates more exhaust flow. If unchecked, the turbo would spin up to some ungodly RPM and something would eventually break. It is the wastegate’s job to limit flow through the turbine and thus control turbo RPM. In the simple case, the gate is controlled by manifold pressure. When the pressure is great enough (how great depends on the spring used in the wastegate) the gate opens and some of the exhaust gases bypass the turbine. Boost controllers generally manipulate the amount of manifold pressure the wastegate “sees” and fools it into thinking the manifold pressure is less than it actually is.

Blow off valves


The job of a blow off valve is to limit boost spikes and vent excess boost in the case that the wastegate is not doing its job. In a completely closed intake system, when the throttle plate closes during boost (as during a shift) a pressure wave would travel backwards to the compressor of the turbo (or supercharger theoretically) and cause a decelerative force on the compressor. Minimally, this would reduce compressor RPM and decrease performance after the shift, or in the worse case it could damage the turbo. A blow off valve, like a wastegate, samples manifold pressure. When the manifold pressure is less than the pressure in the intake piping, the valve opens and the pressure in the intake is reduced.

A bypass valve is like a blow off valve except that instead of venting the excess intake pressure to atmosphere, it pipes it back to before the compressor.

Remote Mounted Turbochargers


There seems to be an ever growing interest in placing remote turbo systems on F-bodies. The kits are generally seen as a practical solution to some of the difficulties in designing a turbo kit for the F-body (especially the V8s).

As stated before, the job of a turbocharger is to take the kinetic energy of the exhaust gases and pressurize the air in the intake. It is generally considered optimal to place the turbo as close to the exhaust ports as possible so as to take advantage of the high temperature and pressure exhaust gases before they have a chance to cool or leak. It should become obvious rather quickly that the location of the turbo is not ideal from a performance standpoint. The main limitation is that the energy available to drive the turbine has been greatly reduced on its journey from the front to the rear of the car. While the car is running, touch your exhaust tip and then touch the exhaust manifold (please don’t really do this). One is substantially hotter than the other. Generally to overcome this difficulty, smaller turbine housings are used. These housings are able to respond more rapidly, but at the sacrifice of greater backpressure. Additionally, if you are running a catalytic converter before the turbo, there is the risk that debris from a failed cat may destroy the turbine.

With regards to the cold side, the long intake piping must be pressurized with the intake manifold. This will further reduce response. It is also imagined that the long intake piping may act as a heat sink. While this is partially true, it has nowhere near the efficiency of an air to air intercooler. Many people have installed intercoolers with their remote kits.

The main strengths of a remote kit lie in the fact that it is easy to install and does not take up space in the engine bay. These kits have also been more economical than a traditional kit in some instances.

Variable Nozzle Turbine


The geometry of the turbine housing determines the boost threshold of a turbo on a particular application. There is however a compromise between responsiveness and restriction. Smaller housings allow faster spool but cause restriction at high RPMs. Large housings allow better flow potential at high RPMs but have higher boost thresholds and more lag, all things being equal. There are many engineering tricks that have been developed to combat this, one of the most advanced being the Variable Nozzle Turbines or Variable Area Turbine Nozzles. The turbine housing contains vanes that are manipulated in order to reduce lag, create a lower boost threshold, and permit greater flow at higher RPMs. The nozzle is smallest at low RPMs or when the turbo needs to be spoiled quickly, then expands as the compressor reaches the desired RPM. In OEM applications, the vanes of these turbos were typically controlled by the computer. In addition to creating a more efficient turbine, the design eliminated the need for a wastegate, the opening of the vanes is sufficient to maintain the compressor at the desired RPM.

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