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Hot Air 10/04/2005 (Reply) PDF Print E-mail

The highly polished stainless steel boxes you mentioned are also known as Jap-style boxes, sports cans and bullet boxes.  These silencer boxes have been, and still are advertised in all glossy sports car magazines and have become more and more popular due to their looks.


Let’s face it, they do look pretty.  These cans are available in different sizes with different tail pipes; some have removable tail pipes and tail pipe insets for more or less noise reduction; they are available in polished stainless, polished aluminum, titanium and I’ve even noticed carbon fibre cans, similar to what is available for super bikes.


Do cans add performance?


These boxes, generally speaking, employ the absorption method of noise reduction. I.e. just like a conventional free-flow or through-flow box used extensively in the local fabrication of free-flow systems.  If this can is used to replace a more restrictive black box, there will be a reduction in back pressure thus increasing power at a rate relative to back pressure reduction.


The absorption method however, is the least effective method of noise reduction.  Therefore, to obtain the same noise reduction as the original box, the volumetric size of the box must be increased substantially; i.e. the free-flow box has to be even larger than the original box!  Available space constraints make this impossible.  This is one reason a free-flow system is noisier than the original stock system.


Almost every sports can I’ve seen on various cars has been much, much smaller than the original.  In fact an O.D. of 91mm and length of 300mm being most common.  From a noise reduction point of view this is obviously mediocre.


A good performance exhaust system (not a cheapie) utilizes a specific silencer box combination which has been designed for minimum drone affect (the low frequency pressure wave inside the car) and installing such a volumetrically small box on a free-flow exhaust will mess up the combination, necessitating a change in the whole combination, which is in most cases not possible due to space constraints and other factors, e.g. different boxes attenuate different frequencies.  However, as mentioned, these sports boxes can be obtained in different sizes.


So, provided the correct volumetric size box (O.D, 154mm, length 350mm, I.D Perf 50,8mm) is installed on your Conquest (assuming your Conquest is either fitted with the 2E 12V motor or a 4AF motor) a perfect free-flow system can be fabricated with good results in terms of performance, consumption and sound, the only drawback being the price of the can, which will cost a bit more due to its larger size.


Just keep in mind that all a silencer does is reduce noise. It does not make power.  So the best silencer for maximum performance is NO SILENCER AT ALL!


There’s a thought!


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