Interesting exhaust comparison ..

[David Barnby]

Like everything on stock bikes Dave - built to a price and not a performance standard.

An immediate doubling of the cross sectional area as the exhaust pulse enters the pipe so the gas velocity immediately halves in speed and pressure as it leaves the port. You won't see a mess like that on any aftermarket system. Similarly again on the Tourers, some of them have the front pipe leaving the cylinder and running along the frame rail towards the rear muffler. Just before the muffler they then have a point where the rear cylinder header joins the same pipe. This is the opposite of the last situation as you have a 1.75" pipe discharging gas from one cylinder and then another 1.75" pipe discharging the same amount of gas at the same pressure is dumped into the existing flow path causing an instant doubling of pressure at that point. None of this kind of stuff is conducive to performance.

Others have a pipe that goes underneath from the right side to the left of the bike that attaches to a bracket behind the transmission. Here, the pipe goes from 1.75" round to nearly flat so that it fits in the bracket before opening up back to round to accept the left muffler. Exhaust design has a HUGE effect on the performance characteristics of a performance built engine. Not so much a standard engine as all of the components are so mild that the best pipe in the world can't improve on low lift, low duration cams, low compression and poor head design but once these factors are improved then the exhaust becomes more of a major player and can either make or break the build.

There are good and bad in aftermarket designs too though. The majority of V & H systems are all designed and tested on stock engines

thought pressure and speed of gas due to volume changes had an inverse relationship so doubling one reduced the other??

the under cross pipe on tourers is a snake in the grass. As Alex says, flattened to make space so flow characteristics all limited by this bit of pipe. Stock Harley touring silencers are not flow symmetrical. The left side has a much smaller baffle internally. The intention is not to have too much flow take this route, the journey via the left is also crazy long and way off desirable equal length header before the mufflers objective. I believe the whole thing is designed to block the flow taking that hard right turn and follow along to the right muffler and operate as a 2:1.

Aftermarket slipons are symmetrical flow (same size baffles) but often have the flattened cross pipe. Just swapping mufflers with stock headers will improve looks and make a “big boy pants” sound. Performance? Nah.

i also think one of the reasons the MOCO restricts flow to the left is to keep rear tire temp under control, this is likely to be an idiotic fantasy but seeing the temp and corresponding pressure rise since fitting a TPMS under all riding conditions started the worms wriggling in my head and I blame the non flattened cross pipe from the Power Duals which along with the less restrictive silencer on the left creating a BBQ effect on the tire. Hence the cupping challenges I have been facing. But could just be all in my head

will confirm after I fit my Bassani 2:1 over the winter

V&H have confirmed many times their design philosophy is not about performance but satisfying the large market made of of people that care about show, not go. They design great looking pipes (I still look at pictures of the V&H Propipe on my Softail thinking how much more pretty they were compared to the Supertrapp now fitted and providing far improved performance) and are extremely well put together. Perfect for those where that is all that counts.