Exhaust question: Helmholz resonators

[GentlemanRacer]

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I've been looking into commissioning a custom exhaust for a modern HD v-twin off ebay (sometime next year when there is money for it) and I wondered if anyone here has any experience tuning motorcycle exhaust notes with resonators. Is this done?

It's a big thing in the luxury car market but I don't know enough about it to use the google right. My grasp of computational fluid dynamics is that it is a term I read on the internet once.

There are a bunch of off the shelf resonators, but if you have made a custom exhaust, have you added a resonator and tuned it? Do custom shops do this kind of thing?

 

[CafeRay]

Automotive exhaust resonators are designed for audio characteristics only. That "growl" that used to sell some cars is just being replicated.
Some car tuners claim that the resonance waves, in the right phase, will improve exhaust flow at certain ranges. YMMV.

I seriously doubt any of that will work on a Harley motor or any Twin.

 

[GentlemanRacer]

I seriously doubt any of that will work on a Harley motor or any Twin.

There are race exhausts that use different manifolds for performance, and the prior art in F1 with their ceramic coatings shows that you can make a performance difference. I'm not really interested in extra performance out of a giant V-twin. The idea of changing the sound is that it changes the overall quality of the experience.

What is it about the twin that would make a resonator less effective? Math or it didn't happen.

 

[Brian P]

Normally on cars, the headers / front part of the exhaust system dumps into a chamber having significant volume (which may be the catalytic converter housing) and at that point, the tuning effect of the header lengths on the engine ends. From there to the back of the car (or to the main muffler) is a certain length of pipe which will have a certain resonant frequency, and if the muffler itself is not capable of dealing with that frequency, there may be another resonator that offsets the natural frequency of the back part of the exhaust system. The size, placement, and dimensions of the connection to the main exhaust pipe affect this.

To me, the design of noise-cancelling elements looks like a black art. There are a number of recent-model motorcycles that use a two-part muffler joined by a pipe between them. Normally there is one chamber underneath the bike somewhere near the area below the swingarm pivot and then a second muffler at the normal location. The length of the primary header pipes - from the engine to that first chamber - is critical for engine tuning.

I have no idea of the formulas used to size muffler elements but volumes of this sort of thing are usually somewhat in proportion to cylinder size, and the length of primary header pipes is certainly inversely related to the target engine RPM at peak power. I suspect that if you tried to do this for a low-revving V-twin with large cylinders, that the required length of the primary header pipes would put the first chamber in an inconvenient location (or quite likely, pretty close to where a "conventional" single muffler would have to be placed, leaving no room for a connecting pipe to the second muffler), and the size of the cylinders would demand a chamber size that is inconveniently large.

 

[CafeRay]

There are race exhausts that use different manifolds for performance, and the prior art in F1 with their ceramic coatings shows that you can make a performance difference. I'm not really interested in extra performance out of a giant V-twin. The idea of changing the sound is that it changes the overall quality of the experience.

What is it about the twin that would make a resonator less effective? Math or it didn't happen.

Applying this level of tech to a Harley engine is a bit ridiculous. The flow rates and rpm are not high enough to see an effect.

Ceramic coating in F1 like zircotec is not about engine power, it's about being able to route the exhaust better within the body to use exhaust gases to help under-body air flow for down force. On custom bikes, it allows more freedom of routing, as that stuff is pretty amazing at keeping the pipes cool.

You can change the sound significantly just by altering the baffle and acoustic dynamics with a larger/smaller can, different baffles. That's the aftermarket pipe industry is about, despite claims of HP increases, they typically just save weight and change the sound.

Go to any race weekend and watch the Harley 1200 class, these bikes sound pretty awesome.

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