So You’ve Decided to Go Turbo–Now What? (Part 3 – Boost Control)

The last segment of this tech special looks at Boost Control. If you missed Part 1 and Part 2, be sure to check them out first!

Boost Control 

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.

street turbo

Wastegates mounted on the exhaust side of the turbocharger are used to control boost pressure. When boost pushes open the large stainless steel valve contained within the body of the wastegate, exhaust gases are bled off, slowing down the speed of the turbine and compressor wheels. Replaceable springs contained within the top of the wastegate hold the valve head down.

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.

passenger side

The passenger side of Brad Personett’s Pro Mod shows the location of the turbo, wastegate and blow-off valves. Note that the turbocharger is mounted underneath the exhaust header up close to the firewall, not rigidly mounted to the tube frame. No intercooler is present since the car is running on alcohol.

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!

Air-to-air intercoolers

Air-to-air intercoolers are used most often on continuous-duty applications such as street cars or in round robin-type racing. To increase efficiency, applications should utilize as much surface area as possible (height x length) with minimal thickness (width) as space allows with a bar-and-plate design, which can safely handle higher boost pressures.


Text and Photos by Rod Short


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