Car Mechanics (UK)
THE IMPORTANCE OF BACK PRESSURE
Klarius’s Doug Bentley explains why back pressure is so important to consider, especially as amateur tuners get it so wrong: “Aftermarket sports exhausts are, usually, the first place we look for an example of how to get things right and wrong. The quality varies so much, from being works of art to low-grade, generic big bore mufflers. Sports exhausts are usually tuned for loudness, which tends to be achieved by using fewer sound-deadening materials and baffles within the silencer boxes. Should the pipe be a wrong/ inappropriate diameter though the system, a freer-breathing lowend exhaust system can actually reduce engine output. This is because any engine is designed for a specific flow of gas through its piston chambers, valves and exhaust system. The pistons rely on a degree of back pressure to push against, in order to balance the gas pressure within the engine to maximise power and torque.
“As an analogy, if you blow through a small diameter drinks straw, the pressure in your cheeks rises and can become uncomfortable. When you choose a wide straw, and the pressure reduces, there is only atmospheric pressure remaining and you soon run out of puff. With an engine, should you remove the exhaust from an engine entirely, it will run poorly, misfire and produce less power. Conversely, if you impose too many restrictions into an exhaust system, the engine will labour unnecessarily. Getting the balance right is critical.
“Should an exhaust system start to blow, the back pressure drops, engine power will reduce and both fuel consumption and emissions will rise. In these cases the ECU might put the car into ‘limp-home’ mode or, in extreme cases, shut the engine down entirely. We have also witnessed some cheap aftermarket exhausts that have disastrous effects on engine performance, efficiency, emissions and wear, due to their incompatible back pressures. This is why Klarius places such an emphasis on only supplying Type Approved systems.”
A further consideration is noise. It is illegal to modify a car’s exhaust so that it is louder than it was when new. When measured subjectively by an MOT tester’s ear, it is a valid failure point. The police are also enforcing the law and the government has been reported as trialling ‘noise cameras’ that are designed to crack down on, what the former Transport Minister described to the press as, "nuisance drivers".