Agostini Silencer Kit

[GuzziMoto]

47 minutes ago, Gmc28 said:

agreed, and thats the main reason i let this very minor  “unsolved mystery” fester in my mind.  exhaust flow dynamics can yield all kinds of results, but it does seem very unusual to have a non-2-stroke motor with an expanding outlet/exhaust, except in oddball cases, like the “power bomb” chambers on some dirt bikes, etc.  The double wall thing certainly would explain things, though i’m guessing that mid pipe isn’t double wall.  it is worth noting that while the V85 mid pipe pictured above is illustrative of my V11, it isn’t quite the same and is definitely more extreme.  the V11 mid has a much milder expansion where the header and mid join.

i’m sure its all fine, though it’s still mildly of interest as to why that mid pipe slightly flairs to a larger diameter (on top of the fact that the header slides into and not over the mid pipe, making it a double step change in diameter as it flows downstream)

Funny related bit of info. I have a Jeep Wrangler. It has a V6 engine with two sets of exhaust pipes, one per bank. They merge together back behind the engine and trans on the left side of the vehicle. Unless you did something to account for length, the length of the exhaust before where they merge would be shorter on the left side since the right side exhaust pipe has to go further to cross over to the left side of the vehicle. What the factory did to account for the difference in length was to bend the left side pipe into a loop. That loop makes the two pipes the same length from the head to the merge point. But that loop is prone to hitting rocks if you offroad. We in fact did that, putting a dent in the looped part of the pipe. The solution is to replace the loop with a fairly straight length of pipe that does away with the loop. But if the straight pipe was the same diameter you would be introducing an uneven-ness to the exhaust system, with the left side exhaust pipe being shorter then the right side exhaust before they merge. To overcome that the people who make the replacement pipe make the straight non-looped pipe using larger diameter pipe. Because the shorter section of pipe is larger in diameter the exhaust in it slows down so that it takes a similar length of time for the exhaust to travel through that section of pipe as it would have if the exhaust was going through the longer looped section of pipe it replaces. Thus, the two sides still reach the merge when they are supposed to and the two sides still have what acts like the same length exhaust system. Even though one side is actually shorter in length.

You never know but given the distance this change in diameter is from the exhaust valve I doubt it really makes a difference in power. But it may make a difference in sound. 

Edited 13 hours ago by GuzziMoto

[Lucky Phil]

On 1/14/2025 at 11:52 AM, audiomick said:

Well, I'm prone to that myself...

Maybe it would help to look at it from the other end, i.e. "going back".

It doesn't seem illogical to me at all that the pipe increases in diameter from front to back.

There are factors that I really can't judge relating to reflected pressure waves coupled with negative pressure at the head, resonance helping to suck out the gases, and so on, but the bottom line is "get that gas out of there".

It seems logical to me that the pipe is getting bigger towards the exit.

The governing factor in the case  of a 90 degree V twin with regards to the exhaust scavenging is the point of cross section dia of the secondary pipe. So the first major cross sectional change is what generates the negative pressure pulse. In multi cylinder engines they combine this with the collector shape so the designated cylinders "talk to each other" as well.

This is a Ducati Corse 888 "spaghetti" exhaust. The small short link pipes between the front and rear cylinders primary role is to create the large cross sectional change in pipe dia to generate the negative pressure wave to help with cylinder evacuation and subsequent filling. It's not there specifically for splitting the exhaust flow to both mufflers. Thats a secondary benefit.  There are two link pipes and the second one is obscured but you can see it's retaining spring underneath.

Phil

Edited 12 minutes ago by Lucky Phil