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  • Porsche 911 silencers

    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.


    Early cars:

    All 911’s 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



    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



    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.



    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




    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.



    Silencing secrets:

    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.


    Copyright Joe Ellis 18 December 2005


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