Exhaust Burble Causesby John Albers
Exhaust Burble on Acceleration
Some cars' exhaust systems will produce an odd burble or bubbling noise. For the most part this is deliberate when it occurs in vehicles upon acceleration, but not always. Many "street racers" and similar cars have undergone a few modifications to give them better fuel efficiency at higher RPMs. In order for this to occur, the lifters and timing mechanism for the sequence of combustion in the engine's cylinders is altered. This means that, while the engine timing will work well at much higher RPM, the combustion process at lower speeds is slightly off. The burble is in actual fact a series of lean misfirings caused by this timing which rumble out through the exhaust system. It causes no serious damage to the engine, and soon disappears once the vehicle has reached high speeds.
Exhaust Burble on Deceleration
An exhaust burble, or throaty popping noise which could be likened to a babbling brook, is has a different cause if it only occurs when suddenly backing off on a vehicle's throttle. Essentially, the sudden easing off the throttle causes an immediate decrease in exhaust emissions to the degree that atmospheric pressure surrounding the exhaust pipe pushes cool air up into the exhaust system. This air collides with the warm exhaust gasses and creates a rumbling not unlike thunder. Normally this will only happen if the tailpipe has too wide a diameter to maintain equalized air pressure, or there is a seam in the exhaust pipes which is not air tight.
Some people like having the throaty noise of an exhaust burble; some do not. For those who do not want this sound, there are ways of fixing it. For an exhaust burble on acceleration, the engine must undergo a professional tune-up including resetting the timing mechanism and lifters; keep in mind this will decrease high speed engine performance, but increase the quality of performance at low speeds. For exhaust burbles upon deceleration, the seams of the exhaust pipe must be sealed against unwanted air entry, or the pipe must be swapped out for one of a smaller diameter to maintain equalized pressure between exhaust and the surrounding atmosphere.