Power Adder Tech: Properly Sizing Blowoff Valves And Wastegates
There was a time many moons ago when turbocharging and centrifugal supercharging were forms of power-producing technology reserved largely for industrial equipment, big rigs, and high-end sports car and open wheel racing machines. But over time, the trickle down effect has delivered that very same technology to the masses. Now, armed with some fabrication skills and a little know-how, an enthusiast can affordably install a turbo or blower kit in their garage.
Drag racers who once would’ve scoffed at the idea of using these then-obscure power adders are hard-pressed to run anything else these days. With this influx of these power adders to the high performance world, the need for education on the intricate systems and their components has gone beyond the knowledge base of diesel technicians and motorsports engineers to the average consumer, and that’s what we’re here to accomplish with this tech piece as we dive into blowoff valves and wastegates, how they work, and more importantly, how you properly size them for your engine and horsepower needs and goals.
We’ve connected with Marty Staggs of Turbosmart USA, one of the industry’s leading manufacturers of turbocharger and supercharger components and accessories, to help us guide you, our readers, to a better understanding of how these two vital components work, and how they’re properly sized for a given engine combination and horsepower output. Staggs has also worked with us to provide a pseudo guide and examples to sizing, based on these same two factors.
Blowoff Valves: How They Work
Picture how a turbocharger system works here, as our first example. There’s a turbine side and a compressor side. The turbine side is driven by the exhaust gases from the engine, spinning the compressor side, sending a charge of compressed air to the engine. The distance this air discharge travels can vary. In alcohol-injected combinations, the pipes may run directly to the intake manifold, or through an intercooler and back to the intake on a gas-burning car. Regardless of that distance, however, there is highly compressed and fast-moving air charge being forced through the pipes between the turbocharger and the throttle blades, which are open since the engine is under power.
Now imagine what happens when you lift off the throttle and the blades slam shut?
All of that incoming air is suddenly stopped dead in its tracks, and the only place for that pressure to go is back to the turbocharger. This phenomenon is known as compressor surge, and it occurs when the incoming air is trapped by the closed throttle, sending a pressure wave back down the line. When this wave hits the turbo compressor, it will rapidly slow the wheel (or even stop it or turn it backwards), and over time, damage or destroy the turbo. If you’ve ever heard that long-lasting fluttering sound from a turbo car after a burnout, you’ve heard compressor surge.
When you lift, that turbo is spinning at over 100,000 rpm and that air has nowhere to go. By putting the blowoff valve in-line, it vents off that pressure spike to let the turbo freewheel. Marty Staggs
A centrifugal supercharger operates in a similar manner, but rather than a turbine and exhaust pressure, these use a belt or chain off the crank. So when the driver lifts off the throttle, the blower keeps spinning with the engine rpm, but there’s nowhere for the air to go. This puts an enormous beating on the compressor wheel, bearings, and shaft.
So how do you remedy this anomaly? With a blowoff valve.
“The blowoff valve basically protects the turbo or blower from surging and stopping when you lift of the throttle,” explains Staggs. “When you lift, that turbo is spinning it’s head off at over 100,000 RPM and that air has nowhere to go. By putting the blowoff valve in-line between the turbo and the throttle body, it vents off that pressure spike to let the turbo freewheel.”
Blowoff valves are of importance on any turbocharged vehicle, but they’re particularly useful on a stick-shift street car, because when you lift off the gas in between shifts, the turbo is still up to speed and the boost response is still there.
On a race car, they’re vital for when the driver lifts off of the throttle after the burnout and at the finish line to keep from beating the turbo to pieces and even breaking the shaft. Race teams that don’t use them are generally those than can afford to rebuild or swap turbos on a regular basis as a tradeoff to running the extra components that could cause a failure and add weight.
A blowoff valve essentially works through the use of a small diameter vacuum hose that’s plumbed into the intake manifold. When the pressure inside the intake is positive, meaning it’s under boost, the pressure in that hose keeps the blowoff valve closed. But if you lift off the throttle, the engine gulps up the last of the air, creating a negative pressure, or a vacuum. The blowoff valve now knows that the air supply has been cut off, and it opens the blowoff valve to release that pressure between the turbo or blower and the intake.
Sizing A Blowoff Valve
The primary question involved in sizing a blowoff valve is how much air do you need to bypass? Such a determination can be made by factoring in the cubic inches of the engine, how large the turbochargers are, and how much boost you’ll be producing.
“Typically, there is no such thing as too big of a blowoff valve,” explains Staggs. “For a drag racing vehicle or a powerful V8 automobile on the street, you’ll generally always want to put the biggest blowoff valve that you can on the car.”
In the Turbosmart lineup, the biggest blowoff valve is the Race Port, a universal BOV measuring a 52mm internal valve size that’s designed for drag racing, road racing, and big trucks. Designed for great performance via a faster spool-up from idle and reduced turbo lag between gears, this is the very same piece used by Mike Murillo, Andy Frost, and others on their 3,000+ horsepower race cars.
According to Staggs, V8-powered street cars will generally use the 52mm Race Port or a a pair of 38mm units. Turbosmart USA does produce blowoff valves in smaller sizes for performance vehicles — and imports — more in the factory horsepower range, including universal BOV’s and a full line of direct factory-replacement units.
If your combination is set up with a pair of turbos, you’ll want to run at least a pair, both of equal size to properly vent the air coming from each compressor. Racers who utilize an intercooler, be it on the nose of the car or in the cockpit, will often opt for another blowoff valve just fore of the intercooler inlet, to relieve the tank of that massive pressure spike when the air backs up in the system. In such cases, Staggs points out that a third BOV of equal size can be a great safety device.
Said Staggs: “Fortunately, picking the right blowoff valve or bypass valve is quite simple, as you can rarely have too big or too much valve. A general rule of thumb is for single turbo street cars — a single 52mm Race Port will generally do the job up to approxiamtely 750 horsepower. For full drag cars making more than 750 horsepower, a pair of Race Ports is needed, and sometimes three.”
Wastegates: How They Work
A wastegate is an incredibly important part of a turbocharger system, because it is the very component that controls the boost levels going to the engine. It is, to put it simply, an exhaust gas regulator, that works by limiting the flow of exhaust gases from the engine to the turbocharger turbine, thus controlling how much boost pressure can be produced.
The wastegate bypasses the exhaust gas around the turbo, so it doesn’t all go to the turbine. By regulating that exhaust gas, we thereby control the speed the shaft speed of the turbo. – Marty Staggs
Explains Staggs, The wastegate bypasses the exhaust gas around the turbo, so it doesn’t all go to the turbine. By regulating that exhaust gas, we thereby control the shaft speed of the turbo – the byproduct of modulating that turbine speed is boost. We don’t just say run that boost level and that’s it. No, we modulate the exhaust gas with the wastegate and it does boost control as a byproduct of controlling the turbo shaft speed.”
A typical wastegate is composed of an inlet and outlet port, a valve, and a pressure actuator. Because a wastegate is typically mounted on the exhaust manifold or the collector (or plumbed into the turbine itself in some cases), it has to stand up to some pretty intense temperatures. Whereas a blowoff valve is often billet aluminum or similar material, a wastegate demands a construction of stainless steel or other heat-friendly material.
Housed at the top of an external wastegate is a diaphragm and a spring, which is dialed in for a specific boost pressure. When the exhaust gas pressure coming from the engine exceeds the spring pressure, the valve is pushed open, venting some of that boost pressure out to the atmosphere. By doing so, less exhaust pressure is fed to the turbine side of the turbocharger, thereby delivering less energy to the compressor and ultimately to the engine. As we’ll depict in a simple formula below, the more boost that you need to divert, the larger the wastegate needs to be.
Sizing A Wastegate
Some of you with factory-turbocharged street cars may have tinkered with an internal wastegate, which is designed for the lower boost levels found on a stock, road-going vehicle. Turbosmart and most tuners consider 400 horsepower the threshold for moving to an external wastegate to better control boost and the power output of the engine, because higher amounts of boost with an internal piece can overpower the actuator spring and limit your maximum boost level. Larger frame turbos aren’t even manufactured with internal wastegates, so you won’t find them on race cars.
There are two primary factors that affect the size of a wastegate, according to Staggs. Number one is how much exhaust gas you need to bleed off or control. Contrary to most things automotive where more means bigger, a wastegate is quite the opposite — a fact dictated by how much air you need to divert.
“If you have a 400 cubic inch motor, as an example, and you tell us that your minimum boost is going to be 20 psi, we know that we don’t need to divert a lot of exhaust pressure because we really need that pressure spinning the turbo and creating boost.” What this means is you can run a smaller wastegate, explains Staggs, with an emphasis on the “smaller”.
Staggs went on to explain, for the purpose of comparison, that if you were to take that same motor and chose to run a minimum boost of just five psi, you’d need to divert far more exhaust gas, thus requiring a much larger wastegate.
“It’s just like an exhaust header — to free up a motor, you have to run headers with larger diameter pipes so you can bleed off the air out of the combustion chamber. It’s the same deal here,” said Staggs
- LS motors with a single turbo up to 88mm can get away with a single 50mm wastegate with proper mounting
- That same motor with twin-turbos will use 45mm and larger depending on the application and turbo size
- Twin-turbo Coyote motors with up to 67mm turbos are using a pair of 40mm wastegates
- Single turbo Coyote motors are using single HyperGate 45’s with turbos up to 80mm
The formula for a wastegate is simple:
Big turbo/low boost = bigger wastegate
Big turbo/high Boost = smaller wastegate
Small turbo/high boost = smaller wastegate
Small turbo/low boost = bigger wastegate
But to get more specific, exactly which size is best to fit your particular combo? Turbosmart USA offers their external wastegates in sizes ranging from 38mm (the Ultra-Gate 38) to the large 60mm Power-Gate 60. Between are units in 40mm, 45mm, and 50mm sizes. Wastegates are sized by the internal valve size, so a 40mm wastegate has a valve that measures 40mm.
“Street cars using turbo kits making up to 1,000 horsepower will typically use Turbosmarts’s 40mm wastegate. It’s a very small wastegate, but it’s an efficient system,” explains Staggs.
More hardcore applications, like Outlaw 10.5 racer Mike Murillo’s twin-turbo Mustang, will run a 50mm or 60mm unit, because they want to bleed off a lot of boost down low in the run, but crank it up on the top end of the track. “With massive turbos, big motors, and high compression, they need to bleed off a lot more exhaust energy so they can get the car moving, and then increase the boost as they go down the race track,” said Staggs.
“With ‘proper’ mounting, smaller gates will often get the job done at a lower price,” says Staggs. “Many times, the mounting of the wastegate is the last thing people tackle and space constraints then force the need for a larger gate due to the less than ideal mounting positions.”
Like blowoff valves, twin turbo applications will generally use two wastegates of equal size, because in essence, you’re operating two motors — one turbo is going to one side of the motor, and one to the other. There generally isn’t any exhaust crossover that would allow one to run a single wastegate, and by using two, you can certainly run a much shorter exhaust.
As you’ve read here, there really is no science or mathematical formula involved in the selection of a blowoff valve or a wastegate. The key is knowing your engine combination and how you plan to operate it — whether you want to keep the turbo in the higher boost range or whether you need bleed off a significant amount of exhaust gas pressure. And of course, before you do anything, it’s always a wise decision to educated yourself on how and where to install your blowoff valve and wastegate in the system to ensure the best performance possible.