Thanks! You’ve used a stainless silencer and tail pipes. I came across a (2001) contribution from Bruce Cunningham regarding the “attractiveness” of mild steel twin pipe exhaust systems for the XK 120, which I would like to share with other interested readers, including Nick and other “non-believers”. I think the best title for this contribution would be “The sound of music”.
One of the characteristics that creates the unique twin-pipe XK sound is the fact that you have a split manifold which produces essentially two three-cylinder engines singing in harmony. With the pipes formed to create equal length paths for each set of three cylinders, the harmony is two identical frequencies that are 180 degrees apart in their wave forms. The two-in-two-out muffler with straight through pipes on each side allows some mixing of the sounds which enriches the sound before it exits the tail pipes. This music quality cannot be duplicated in a system that has both exhausts merging into one pipe prior to exiting the system.
The silencer has two straight pipes inside that are perforated. The perforations just allow the sound waves to be attenuated as the gasses pass unimpeded through the straight pipes. Some part of the sound waves pass through to the opposite perforated pipe where valleys in the primary sound wave are partially filled in by the peaks of the one from the other side.–
The sound waves produced by the exhaust valves aren’t sinusoidal: it’s the complexity of the wave forms that creates the richness and uniqueness of the sound quality. Air vibrating in a pure sine wave has a very bland sound. Example: piano strings are three to a key and are tuned slightly out of tune with their mates to give each key a richer quality.
The first resonance point is at 1600 rpm (where the peaks are at the end of the pipe as the wave`exits). The next is at 3200 (the next octave) for six cylinders and at this point there is a harmonic present as this is the resonance point for each of the 3 cylinder sets. There’s a really beautiful sound at 2400 (midpoint between 1600 and 3200) rpm where the sound swell slightly with a pleasant mixture of harmonics. This is about 53 mph where I tend to drive just for the sound - it’s almost like cruise control as my foot tends to adjust the speed for the sound almost automatically.
Listening to the various crescendos and diminuendos and the multiple harmonics throughout the range is a big part of the joy of XK driving.
Bruce Cunningham (XK Lovers Forum 2001)