Ford Rat Rod with custom turbo piping

Hot vs Cold: Understanding Turbocharger Piping

Can you tell a BOV from a wastegate? Here’s what everything does, besides just make that “whoosh” noise…

Way back when dinosaurs roamed the earth in 1984, McDonald’s introduced a new concept in hamburgers. Well, at least it was new to fast food – you see, at that time when you went to the Golden Arches and ordered up a cheeseburger, a Big Mac, or a Quarter Pounder, the friendly person behind the counter would turn around and take a premade sandwich from the heated storage rack to fulfill your order. The diligent staff back in the kitchen would be busy making and wrapping up burgers to keep the rack full, and depending on the time of day and how well that particular store managed their production line, you might get one relatively fresh off the grill, or one that had languished for quite some time. This meant that there was a good chance that everything from bun to beef to lettuce and tomato would have all met at the same lukewarm temperature by the time you got it.

As you might imagine, this led to a suboptimal hamburger experience, and in a flash of marketing brilliance, McDonald’s came out with the McDLT, which was a fairly ordinary burger with unusual packaging and an inspired tagline – a two-chambered styrofoam box that “kept the hot side hot and the cold side cold”. Diners would take the top bun, accompanied by the lettuce, tomato (hence the “LT” in McDLT”), cheese, pickles, and condiments on the cold side, and add them to the lower bun and meat patty on the hot side just before eating.

McDonalds McDLT hot and cold side AD

Of course, the real solution to a better hamburger was “don’t make them 45 minutes ahead of time,” not a special box to let it hang out in until somebody bought it, and the fact that there was a lot of non-biodegradable packaging involved didn’t help, with the McDLT going the way of the triceratops by 1991. “So what does all this have to do with turbochargers?” you may quite reasonably ask. Well, you’ll often hear about these forced induction systems having a hot side and a cold side, and there’s sometimes a bit of confusion about what the reason for specific components on each side is. Today, we’re going to straighten that out so that you know your terminology and understand the purpose of all the additional hardware that makes the turbo work the way it should.

Hot rod with twin turbochargers

Recycled Power

The idea behind turbocharging is to take energy in the form of hot exhaust gas, which is normally wasted, and use it to compress intake air in order to allow more fuel to be burned for each power cycle. A turbine (that’s the ‘turbo’ part of the name) is powered by the engine’s exhaust, cooling and slowing it down in the process, and through a shaft it connects to a compressor (or ‘supercharger,’ which donates its back half to the name) which forces air into the cylinders at higher-than-atmospheric pressure.

Unsurprisingly, the part of the system that powers the turbine is called the “hot” side, while the part that supplies fresh air under pressure is called the “cold” side (even though an inevitable consequence of compressing air is heating it up to temperatures none of us would consider to be cold if applied to our persons – more on that in a bit). Though it’s possible for the plumbing to be as simple as a turbocharger connected to the intake and the exhaust, in practice some other pieces are necessary in order to make the engine run properly at best, and to keep it from spontaneously disassembling itself at worst.

Blue Bisimoto EF Civic shuttle turbocharged

Feeling The Heat

The one component that every turbocharged engine relies upon (with very rare exceptions) is a device called a wastegate. As a forced-induction engine makes more power and RPM climbs, more exhaust gas is produced, and more energy is available to power the turbine. Without some method of regulating this power, at full throttle the engine and turbocharger would be locked in a feedback loop, with more exhaust flow providing more power to compress intake air, until one of several possible bad things happened. If the turbocharger is small, it might simply overspeed and spin past its mechanical limits, destroying itself. If it’s large, it might raise intake pressure beyond what the engine’s compression ratio and the fuel’s resistance to pre-ignition could tolerate, turning the pistons into a fine mist of aluminum coming out the tailpipe or lifting the head gasket. If the turbocharger is medium-sized, the compressor may be forced into an operating region where instead of making more pressure, all it does is heat up the intake air because it’s no longer operating at a reasonable efficiency.

Ford Rat Rod with custom turbo piping

All of these things are highly undesirable, and while it’s possible to design a turbocharged engine to operate in steady-state conditions without any way to regulate how much exhaust energy is driving the turbocharger, it’s not really practical for automotive engines. This is the purpose of a wastegate – by opening and closing, it can deliver more or less exhaust gas to the turbine, and by doing so, regulate how much pressure is being created in the intake tract by the compressor.

At its simplest, a wastegate is typically a flapper valve built into the turbine housing inlet of the turbocharger itself. An actuator using spring pressure holds the wastegate closed, and a line connected to the intake manifold signals the actuator to open the wastegate when the desired boost pressure is reached, sending exhaust flow around the turbine instead of through it. In a purely mechanical system, as boost pressure rises the wastegate will begin to open, affecting how quickly the turbo ‘spools up’ in response to the throttle opening. Most factory forced induction setups using EFI will have an electronic solenoid valve as part of the control line from the manifold pressure signal to the wastegate; this allows the computer to keep the wastegate closed to quickly build boost by venting off the pressure that would otherwise cause the valve to open, providing better throttle response.

double external wastegates on the hot side turbo piping

While integrated wastegates are inexpensive to build, compact, and reliable, they aren’t optimized for efficiency either closed or open. This is why it’s common for high-performance and racing turbocharger systems to use a separate wastegate, or even multiple wastegates. The advantages are that more precise control is possible, often using a separate regulated supply of CO2 instead of pressurized intake air to both open and close the wastegate valve in response to commands from the ECU. External wastegates are also a way to avoid overwhelming the ability of an integrated wastegate to release exhaust gas and lose boost control because of its limited size.

Cold Is A Relative Term

On the intake side, there are a couple of components almost every properly-engineered turbocharger system will utilize. As was mentioned earlier, one of the unavoidable consequences of pressurizing air is heating it up in the process, and turbocharger efficiency is a measure of how well the compressor packs more air molecules into a given space instead of just heating them up. Much like shoppers at a Black Friday sale, the tighter the molecules are pushed together, the more they jostle against each other, and that motion is measured as heat energy.

Once a turbocharger starts producing intake manifold pressure in the double-digit PSI range above the ambient air, no matter how efficiently it’s working, the actual intake air temperature will be quite a bit higher than the cool, dense air the engine would like to work with. Sometimes this can be several hundred degrees higher, in fact – this is why we say that the “cold side” is only cold relative to the exhaust. To remedy this problem, the intake plumbing can include some form of heat exchanger, often referred to as an intercooler.

Porsche twin turbo setup

The name ‘intercooler’ in strict engineering terms refers to a heat exchanger placed in between two separate compressor stages, and the correct term for one that sits between the turbo and the intake manifold is actually ‘aftercooler.’ This never really caught on, though, so we’ll continue to use the word intercooler irregardless of it not being technically correct. An air-to-air intercooler passes pressurized air through a heat exchanger that looks very much like a radiator (because it is one) that uses flowing ambient air to carry away some of that excess heat. The output from an air-to-air intercooler will always be higher than the outside air temperature, but it will be far lower than what came directly from the turbocharger’s compressor section.

Some forced induction designs (primarily positive-displacement superchargers, which are a subject for a different article) use an air-to-water intercooler, which takes heat out of the compressed intake air and transfers it to liquid coolant, which then circulates via a pump to another cooler to transfer it to the outside world. Because of the extra steps, air-to-water intercoolers (which are really air-to-water-to-air if you think about it too much) are less efficient than air-to-air, but they offer packaging advantages and can be put into places air-to-air coolers won’t fit or be practical.

drag race between a gen3 camaro and a buick

Air-to-water charge coolers can also do one neat trick that’s frequently used in drag racing. Instead of having a second heat exchanger exposed to outside air, the system can be connected to a tank filled with ice water. In an air-to-ice setup, it’s possible (given big enough components) to actually have intake air that’s both highly pressurized and cooler than the ambient air temperature, with obvious advantages for power production. The down-side, of course, is that once the ice is melted, you’re back to square one, but for brief dragstrip runs it’s a very practical setup.

The Thing That Goes “Whoosh”

The one component we haven’t talked about so far is the one that usually draws the most attention – the blow-off valve, or if you want to use the technical term, the compressor relief valve. This is a device plumbed into the pressurized part of the intake tract that responds to a difference between air pressure ahead of the throttle body and past it – when the pressure ahead of the throttle blades is more than a preset amount higher than what’s in the intake manifold headed to the cylinders, the valve opens and releases compressed air to the outside world (a “vented” BOV) or back into the intake ahead of the turbocharger inlet (a “recirculating” BOV).

This accomplishes a couple of important tasks, besides sounding cool when you shift. When the throttle is suddenly shut, like between gears or when transitioning from acceleration to braking, a wave of pressure is reflected back to the turbocharger’s compressor. At best, this slows down the turbo’s rotation, which isn’t what you want if you are in the middle of a run up through the gears. At worst, it can damage the compressor or even cause the shaft that connects it to the turbine to shear off from the sudden shock load.

Blow off valve on an RX7 rotary engine

Cars with factory-turbocharged engines typically use recirculation style compressor relief valves because they’re quieter, and to prevent a momentary fuel-rich condition when it operates. Electronic fuel injection systems that directly measure incoming air usually have their mass air sensor at the very front end of the intake, so when a vented BOV releases pressure, some of the air that was already accounted for never reaches the engine, and too much fuel is added as a result. A recirculating BOV just loops the air around past the point where it has already been measured, eliminating that problem.

Even with mass airflow ECUs, clever programming can mitigate this issue for aftermarket turbo systems with vented blow-off valves by adjusting the fuel map to run lean in circumstances when the throttle is suddenly closed, and speed-density fuel injection systems that calculate airflow rather than measure it directly don’t have that problem in the first place. There’s sometimes confusion among those new to forced induction about the role of the blow-off valve versus the wastegate – just remember that the latter is what controls boost on the hot side, while the former handles pressure surges on the cold side and you’ll be on solid ground.

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