HOT AIR: More About Silencer Systems

After forty years of working on exhaust systems, in particular performance exhaust systems on two stroke and four stroke bikes, road cars, rally cars and all sorts of hybrids and kit cars (we’ve even worked on micro light and paraglider exhausts!), I could write volumes of factual and anecdotal stories.  So little time…

There is a particularly enjoyable time in my life every time a decision is made to develop a new performance exhaust system.

This done, the next step was to introduce free-flow silencers to the resulting exhaust system piping. Both the Run-X and the 4AGE 20V run best on 63,5 mm tubing and this in itself is a problem.
Being four cylinder motors, the exhaust firing frequency is low (as opposed to 6 cylinder or 8 cylinder) This is a major factor, creating low frequency resonance inside the cab (droning).
In addition to that, to make the problem worse, the bigger the bore on the exhaust pipe, the worse the drone; and we have to deal with 63,5mm pipe, which is a very big bore for a 4 cylinder motor; we cannot compromise on the optimal pipe diameter, so we must now find silencers that not only must be free-flowing (absorption type), but must be of 63,5 mm perforation, must be the correct shape to fit the available cavity and must be of the correct front box-back box combination to obtain a great sporty-yet-refined sound which would not be obtrusive to the enthusiast and which is acceptable in terms of in-cab low frequency noise attenuation.

More and more cars are built with more inbuilt rigidity in their body structure.  Their harmonics (sound level at which the structure begins to vibrate) are very low.  The factory exhaust systems are carefully tuned not to have sound frequencies that would excite the body structure of the vehicle.  It is paramount to obtain and maintain super –high standards on stock vehicles, even on high-performance models.  However, we are interested in performance-oriented exhausts and most enthusiasts want a “performance” sound. Most enthusiasts are happy to compromise sound for performance.  However, most car enthusiasts “live” in their cars and are not too partial to “sonic disturbance” inside the vehicle.
When a free-flow system is installed on most 4 cylinder vehicles, the frequency of the exhaust pressure wave often matches the harmonic of the vehicle’s body shell at certain rpm, (2000 – 35000rpm) causing a loud and very annoying “drone”.  It can be so bad that  a palpable pressure wave can be created inside the cab, which can be quite intense and extremely annoying!

So, what mufflers can we wrap around this 63,5 mm exhaust pipe to adequately attenuate the noise inside the cab and outside the vehicle without losing power and obtaining the required tones?

I receive a bundle of queries about silencer boxes and in particular “ ….. What is the best silencer box ….”,   as a  general question.  We now know that each particular vehicle requires a specific exhaust pipe diameter and a specific silencer box combination to achieve the required sporty but unobtrusive sound.  So such a question cannot be answered in general terms.  Another question often posed, “… what silencer makes the most power?”  Here we have a very simple answer.
Silencers do not make power;  they rob power!
I have yet to see an engine producing more power with exhaust silencing than an engine with open branch and secondary pipes.  So, the best silencer box is no silencer box!
The question to ask, therefore should be, “What is the best flowing silencer box we can use?”

Under-car design often leaves exhaust consideration to the last stage of design, necessitating restrictive silencers to muffle the exhaust to acceptable noise levels instead of allowing ample space (more space) for silencers more attune to performance.
To install a custom-made performance free-flowing exhaust system that meets the average enthusiast’s needs, considering the mentioned drawbacks, can become more complex a job than just replacing restrictive silencer boxes with off the shelf universal free flow mufflers.

To be continued…
Abel dos Santos

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