Porsche 911 Exhaust Systems
One of the most persistent problems faced by any enthusiastic 911 owner who wishes to compete in their car is noise. Whatever variant they decide to use, the rear engine layout always determines that because of the extreme packaging constraints and the proximity of other noisy components the sound level measured from the exhaust tailpipe exceeds that of other sportscars. Add to this the desire that most competing drivers have to try and improve the performance of their car by increasing power and/or reducing weight and you have an idea of the scale of the difficulties faced by an exhaust designer.
All 911s up to the introduction of the 993 have essentially the same limited amount of space available for silencers. This led the original designers to come up with the banana shaped silencer that fits between the rear of the engine and the rear bumper.
The impact bumper cars introduced around 1974 still used this as the main silencing element, with the addition of a small pre silencer alongside the engine. If a fibreglass replica of the rear bumper is used as per the 911SCRS race and rally cars, there is a bit more space available behind the engine for larger silencers. At the time the SCRS was competing noise was not really an issue and the most efficient design of twin tapered megaphones that ran straight from the end of each 3 into 1 manifold to exit just clear of the bumper were only nominally muffled with tiny oval cans. Today if this independent configuration is desired for maximum power, it is necessary to fit twin transverse round silencers on top of each other, with the left hand bank of cylinders exiting through a tailpipe on the right and side and vice versa.
Linking both cylinder banks together via a 2 into 1 pipe can be desirable for torque on smaller capacity engined rally cars, but the complicated routing required of this pipe inevitably will compromise the space left for a silencer, which is why the two functions were combined within the OE silencer. The standard OE silencer remains a good compromise between power and noise, but its single tailpipe design will compromise the power of the highest output engines that can now be fitted to these early body cars. Pattern copies of this silencer exist but should be avoided because they often feature power sapping internal baffling arrangements that cannot be seen from outside and often are not as efficient at attenuating the unique flat six noise frequencies.
BTB have developed a unique "Figure 8" shaped silencer which incorporated a central X pipe, to equalise the secondary length of twin 3-1 manifolds and which gives an efficient 6>2>1>2 twin tailpipe system.
The 964 body affords a little more scope for more silencing with a combination of simpler shapes of oval and round boxes, but inevitably the power output potential, pipe sizes and hence noise volume increases in line with the arrival of 3.4 and 3.6 litre engines so the same challenges largely apply. It has become quite common to use 964s as the basis for backdated Retro Mod cars. In these cars more creative solutions are required to match the conflicting requirements of performance, packaging and quality of sound.
The more emphatically rounded rear end styling of this model freed up considerably more space in the rear arches just behind the rear wheels, which allows fitment of symmetrical oval reverse flow silencers each side.
Although road noise levels dictated the use of somewhat restrictive internals to the OE system, at last the scope exists for a free flowing system to be designed that uses the extra volume, and pipe length available to give a useful increase in power without producing an unacceptable increase in tailpipe noise. If this power is to be regularly exploited in competition or trackday, it is recommended that the silencers are only made from the highest grade stainless steels and preferably are of a repackable design to enable the noise attenuating material within the silencer to be replaced without the cost of replacing the entire system. This is because of the higher exhaust gas temperatures resulting from the more extreme ignition timing and leaner fuel mixtures made possible by much more precise electronic engine control systems.
Initially when the 993 was raced in the UK they ran with the boot lid/spoiler propped open, which meant that the noise from the air-cooled engine (and amplified by the fan) was directed in the same direction as the tailpipes, i.e. straight at the scrutineers noise meter. However, when the RS biplane spoilers were fitted the engine noise was directed upwards away from the exhausts, which meant that the efficient straight through systems were still viable.
The 996 and onwards are fully water-cooled of course, which means and end to the noisy engine mounted fan, and more sound absorbing water channels in the engine castings. 996's also have yet more room behind the rear bodywork for mounting silencers, which means that even fire-breathing race-tuned GT3RSR's can be fitted with enough silencers to comply with trackdays, without compromising their awesome performance.
For racing the emphasis is on ultimate performance and as such the smallest silencers are fitted that only just comply with circuit noise limits, so as to minimise the weight slung out behind the rear axle.
The essential element to a performance system is the straight-through or absorption type of silencer. This consists of a perforated duct surrounded by sound absorbing material, which converts sound energy (vibration) into heat. This type of silencer typically attenuates the high frequency noises from the engine leaving the characteristic low bassy burble to emanate from the tailpipe. There are two problems with this type of silencer when fitted to 911's, firstly with the earlier smaller bodied cars it is difficult to fit a silencer that contains enough volume of sound deadening around a big enough perforated tube within the confines of the bodywork.
Secondly, the short pipe run on any rear engined car between the engine and silencers, doesn't allow for much dissipation of heat before the gasses encounter the duct. This can lead to rapid deterioration of the sound deadening fibres which are then sucked into the exhaust stream and blown out of the tailpipe.
One of the tricks to reducing the noise further is to combine the two cylinder banks, to allow a degree of noise cancellation to take place. This is where the peak pressure wave from one cylinder is cancelled by its opposing wave form being present in the same section of silencer. A mild version of this can be effected by having a carefully positioned crossover or balance tube between the two independent pipe runs from each cylinder bank. This minimises the possibility of reversion waves being reflected into the cylinder head during valve overlap, which can have a detrimental effect on the incoming fuel air charge and efficient cylinder filling. If more sound reduction is necessary both cylinder bank exhaust flows can be arranged to directly face each other, preferably within a damped chamber in the silencer.
Turbocharged cars tend to produce lots of low frequency noise, because the high frequencies are lost in the exhaust turbine, and they need much larger ducts to cope with the hotter gas flows. However, they don't suffer as much from reflected waves as the incoming charge is pressurised above atmospheric pressure, and the turbine interrupts the reversion pulses anyway. The lower frequencies can only be attenuated in large well damped expansion chambers unless restrictive baffling is used which will sap power by increasing back pressure in the system preventing effective spool-up of the turbo.