Turbo Talk: Downpipes and Blow-Off Valves

Source: Turbosmart
Source: Turbosmart

It sounds like teenage locker room language, but blow-off valves and downpipes really are crucial pieces to the turbocharged automotive puzzle. For all their naughty connotations, they are not nearly as filthy-sounding as other car related terms — the next time you are at a dinner party try saying  “dipstick,” “lube job,” or “drain cock” around a bunch of people who aren’t auto enthusiasts and note their reactions for us.

Naturally aspirated — or non-turbo or supercharged — cars are pretty simple pieces of machinery relative to force-fed applications. In order to keep a boosted vehicle from self destructing, a slew of unique components must be put in place. The overall layout of a turbocharger in an application is critical for ensuring that the system performs properly, a topic that we covered to some detail in an article last November, so if this all seems new to you, it’s worth suggesting that you look there first.

While exhaust manifolds are critical to the engine’s ability to remove exhaust fumes, it would all go up in smoke if it weren’t for a tiny compressor bypass valve on the intake side. Commonly referred to as a blow-off valve, it functions like a relief valve for your car’s pressure cooker. Regardless of whether it is improperly assembled, malfunctioning, or simply not there, without a fully functional blow-off valve, it’ll be beans on the ceiling in no time flat.

Simply explained, the blow-off valve (BOV) is a pressure relief device on the intake tract that prevents the turbo’s compressor from going into surge, a dangerous area of airflow instability that can wreak havoc on turbine thrust bearings. Typically installed somewhere between the compressor discharge and the throttle body, the BOV prefers to be downstream of the intercooler so that when the throttle is closed rapidly and airflow is suddenly reduced, it can help stabilize pressure fluctuations and vent any unnecessary back-pressure out into the open air.

All of these rapid pressure fluctuations can be audible. When a BOV detects that a throttle is closing, it uses a combination of manifold pressure signals and a tightly wound spring to vent any extra boost pressure trapped within the intake tract. This creates a noticeable “Pssssshhhhhh” noise that somewhat resembles the prolonged opening of a two-liter of soda pop.

Naturally, there are multitude of different blow-off valve styles. From full atmospheric race applications and dual port designs that are re-configurable to quiet plumb-back models and chassis specific set-ups, there is something out there for every application.

Micah Wright/Autos Cheat Sheet
Micah Wright/Autos Cheat Sheet

On the exhaust side, once spent fumes have hit the turbocharger’s compressor wheel and are rearing to get dumped out the rear of the car, it must first hit a series of pipes. The first of these pipes is called the downpipe, and it plays far more of a crucial roll than you may think.

For most turbocharged vehicles, this pipe isn’t really a pipe at all, but a high-temperature resistant catalytic converter designed to filter exhaust fumes and protect the environment.

Micah Wright/Autos Cheat Sheet
Stock downpipe beside a custom unit | Micah Wright/Autos Cheat Sheet

However, stock downpipes are not very good for performance. Unlike naturally aspirated engines, turbocharged motors hate back pressure, so not having a free-flowing exhaust can hurt fuel economy, power, and reliability all at once. Blasting out of a turbine and straight into a complex honeycomb of piping is the last thing turbo exhaust needs. Thankfully, there is a very simple solution to this issue.

By upgrading the restrictive factory downpipe with a high quality aftermarket unit, turbo enthusiasts can receive an increase in both performance and efficiency, all while adding a refined growl to the exhaust note. Expelling exhaust gasses at a faster velocity through a larger low-turbulence downpipe ensures that back pressure is never an issue. Simply put, the easier a turbocharged car can exhale, the better it can perform.

Naturally, there are some additional considerations that must be taken into account, like the proper placement of oxygen sensor bungs like the ones seen here. Also, since most aftermarket downpipes don’t have a catalytic converter in place to filter gasses, a defouler will sometimes be necessary in order to keep the O2 sensors from tripping an engine code. As for keeping the vehicle emissions legal, there are several options, and since most people who opt for a downpipe install an aftermarket exhaust anyways, placing a free-flowing catalytic converter from a company like Magnaflow further downstream is an easy choice that isn’t prohibitively expensive.

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