Boost control (i.e., regulating the pressure of the air charge coming from the compressor discharge side of the turbo) is accomplished by controlling the speed of the turbo under WOT conditions by means of a wastegate and then via blow-off valve after the throttle blades are closed.
Many street turbos come with an internal wastegate, which saves a lot in terms of space, installation time and money. The downside, however, is that internal wastegates have a very limited range of adjustability on both the high and low ends. External wastegates, on the other hand, can be set to regulate your own specific needs with replacement springs.
Some racers actually pre-load the wastegate or use more than one wastegate for the purpose of getting off the line and better traction control, but that’s going beyond the scope of this article. Just be sure to follow your supplier’s recommendations with regard to the size and placement of your wastegate so you can optimize your turbocharger’s operation.
Blow-off valves typically come into play once a driver is off the gas when he crosses the finish line. Even when the exhaust gases driving the turbine wheel are eliminated, the ram air effect of a car at high speed will still try to spin the compressor wheel―all of this while the throttle blades are closed. A properly positioned blow-off valve will vent excess pressure from the air charge into the atmosphere and help save your engine from unnecessary damage. Once again, check with your supplier for correct sizing and placement.
While all of this may sound daunting or intimidating, it’s stuff that’s overcome every single day by the best turbo manufacturers, component suppliers and professional installers. It just takes some thought and planning. When it’s all said and done, selecting a turbo is just the first step in getting a complete, functional system on your street/strip or full-time race vehicle. Once you get the right supporting cast, you’ll be on the quick path to success!
Text and Photos by Rod Short