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Turbo Exhaust Headers

Turbo-tuning exhaust options for force-fed engines

The beauty of a turbocharger is that it makes use of wasted energy thrown away by the engine, the pulses and pressures of the exhaust gasses. The exhaust flow spins a turbine wheel that shares an axleshaft with an air compressor. Once their rotation comes up to a certain speed, pressure or boost builds up in the manifold. This pressurized intake charge rushes into the cylinders and "overfills" them, producing more torque and horsepower. Sounds simple enough, but given a turbocharger's dependence on the exhaust system, it's obvious that the headers feeding it merit some close attention.

Flow

Every hot rodder knows the value of a good set of exhaust headers for cheap, bolt-on power. Actually, that's not entirely accurate. Aftermarket exhausts don't actually make horsepower, but instead choke off less power by reducing backpressure for better flow. Exhaust pulses are "scavenged" out of the cylinder heads and then progress through the system, cooling and contracting as they go. The challenge is to get exhaust out of the system as quickly and thoroughly as possible—greater heat dissipation allows the engine to run cooler, and less exhaust in the cylinders allows more fresh air in for better burn.

A set of performance headers, whether they feed a turbo or not, achieves this faster flow by several means. One of the simplest is by a cleaner bore with fewer obstructions. In contrast to the intake plumbing (which needs a bit of friction on the inner surfaces to create turbulence for better mixing of fuel and air) inside the exhaust system, smoother is better.

Unfortunately, mass-produced factory headers are usually fairly crude-looking, cast-iron pieces with lots of rough edges. Also, most production exhaust systems use press bends, which constrict the inner diameter at the radii.

One way to clear the exhaust path of texture is by extrude-honing, which forces a grit-removing putty through the tubing. Better yet are custom headers that are carefully fabricated inside and out to maximize airflow. Aftermarket systems typically use ball-mandrel bends, which maintain a constant inner diameter at the bends for less-restricted flow.

Balance

That doesn't mean, however, that a turbo engine's exhaust tubes should be made as large as possible. Large-diameter headers are fine for engines that operate in the high-rpm ranges, but they suffer at low- and mid-range engine speeds because of slower exhaust flow. The trick is to strike the right compromise between exhaust-gas velocity and a pipe diameter that's sufficient to extract the gases. A smaller-diameter header boosts exhaust-gas speed at low rpm (great for hill-climbing torque or spinning a turbo), but can become restrictive at high rpm. You have to balance the need for a quick spool-up of the turbo against a need for sufficient exhaust-flow volume. Otherwise, the extra intake flow supplied by the turbo will encounter too much backpressure on the exhaust side.

In fact, turbo cars are notorious for having exhaust go up the intake during cam overlap, because the backpressure may be two or even three times the boost pressure. Turbo engines like camshafts with lots of lobe separation, which minimizes the time when both valves are open and this problem occurs. That's why backpressure testing of the exhaust system is more important on turbo cars.

Convergence

Another important factor in an exhaust system for a turbo engine is the distance of the collector (the section where the individual tubes converge) from the exhaust manifold. Longer tubes mean more low-end torque; shorter ones move the powerband to a higher rpm range, so determining collector distance should include factors such as the weight and application of your vehicle.

The design of the collector itself and the individual header pipes that feed it are also critical for good performance. When an exhaust valve closes, the gas pressure inside a primary header tube is reduced, and the exhaust-gas flow (airflow) is accelerated. When this fast-moving gas hits the headers' large collector tube, it slows down, creating an obstruction to proper and rapid airflow. By keeping the length of each primary header tube the same (tuned-length headers) and carefully placing the tubes within the collector, exhaust flow to the turbo is enhanced.

Heat

In addition to exhaust flow velocity, much of the turbo's power comes from heat. Hot exhaust gasses expand rapidly, which helps to spin the turbo faster. The problem is that factory cast-iron exhaust manifolds are big heat sinks. They retain a lot of the exhaust heat right next to the cylinder heads, which isn't good for the rapid evacuation of exhaust gases. A custom or aftermarket stainless-steel system doesn't absorb as much thermal energy, so it can be directed to the turbo instead.

Insulating the exhaust tubing with a ceramic coating or a thermal wrap can also be of value on a turbocharged engine. The main benefit of insulation from a performance standpoint is that it holds heat in the pipes to improve exhaust scavenging for driving the compressor. Getting the gas out faster also means that the cylinder heads run cooler, which lowers the operating temperature of the engine. In addition, because the heat is better contained within the header tubes, less is radiated into the engine compartment, and underhood temperatures come down. Lower underhood and engine water temps mean the intake charge becomes denser, which makes power. Another way to lower the temperature of the intake charge is with an intercooler, but that's a subject for another article.

In summary, a turbo system relies on a high-quality exhaust system. The headers should be of equal length and consistent diameter, with smooth bends and surfaces. Also, the diameter of the tubing should be small enough for high-velocity flow, but large enough to prevent excessive backpressure. In addition, the collector's location affects the torque characteristics of the powerband. And don't forget that heat insulation can improve your turbo's performance.

Resource

GP One Turbo Performance, 310/891-3512