myth does not run Harley Davidson, costumer loyalty does, myth does not drive costumer loyalty, experience does long term experience, not just one or two years on the hottest rocket. And historical engines don’t augment any of that if it did Royal Enfield India would be the company to beat. So why are Harley customers so loyal? Because years of dedicated engineering to a very few engines has led to a very stable fuel efficient (per engine size mind you) power plant as well as a long list of racing performance parts. (read that last little bit carefully I wrote RACING parts, and I mean it) and most of all they rely on the consumer only know what they want to know, most don’t care for an 200 Hp machine but those that do it’s there and possible (no matter how difficult). Mix that with an easy to repair engine, that has as few failure points as possible (read as; moving components that are vital to said engine running) and top it off with real good low end torque, and decorative tins that look like something out of the 50’s (not really but most people don’t know the difference) and you got a winner.

And if that is not enough to convince those whom criticize without true experience (no listing to the graybeard complain about the AMF years does not count those are long over) think about this, how many of the naysayers of H-D would tattoo Honda, Suzuki or whatever; on your arm, back, or whatever? not many….

For any given size of engine a measure of its efficacy is expressed in its ability to pump air.
In a nutshell, get lots of the stuff into the cylinders – get the resultant combustion gasses out just as efficiently.

So called ‘back-pressure’ has no place in an efficient exhaust system. The performance of any exhaust system (and by analogy the engine) is degraded by the degree of energy required to expel the combustion gases.
Where performance gains are to be made is in the area of pulse-tuning. This being the art of using pulses in the hot exhaust gas to promote cylinder scavenging and prevent ‘charge’ loss.
This could be where this (HD) system appears to score, in that two ‘tuned lengths’ of header-pipe appear to be provided. However the header pipe(s) is just a component part along with the collector and tail pipe.
One just wonders, looking at the various diagrams how efficient the HD solution is with regard to header-pipes/collector and tailpipe.
My own experience with tuned exhausts (Road-race Sidecar outfits) demonstrated that significant gains could be made, BUT attention to the basic equations governing size and lengths was essential.
The other thing to remember that with regard to ‘pulse-tuning’ the exhaust system ‘stops’ at the first major section change, which can be viewed as the entry of the tail pipe into the expansion box (silencer).
The iron rule however is test the ‘packages’ and believe that the Dyno never lies!!
My own analysis of the Exup valve was that it provided differing ‘tuned’ header-pipe lengths. However I never attempted a close look at the system.

Regards John Davie

Absolutely not true! The EPA references a SAE standard for noise measurement, which requires a full throttle ride-by in 2nd or 3rd gear (which is used depends on a formula based on gearing and peak power rpm) at a specified speed. While the standard is gamed by manufacturers (for example, by shutting the EXUP snuff-or-not during this time of acceleration, or by having the ECU reduce the opening of secondary throttle butterflys as used on many Japanese four-cylinders at a very specific speed), it represents a fairly realistic test of motorcycle exhaust noise.