Exhaust Reversion.

I read a recent post reply awhile ago and "exhaust reversion" was mentioned. Can anyone here explain this term, it's cause, and corrective actions to take to eliminate it? I do not think it is the problem I have with my Sporty, but would like to know more about it.

I'll give it a try. Just as it implies it's the exhaust gases or pulse reversing towards the engine. From what I know the most common way a regular tinkerer causes this is by modifying his exhaust to flow more freely and going too far towards more freely. Or by putting larger pipes on the engine that what came with it. The diameter of the pipe is determined by the expected flow of the exhaust gases. If the gas pulse enters a larger pipe, the pulse looses momentum and can be sucked back towards the engine. Torque Cones are available to help eliminate this effect ( besides modifying the cams, valves etc ). The cones are installed into the pipe at the exhaust port. They speed up the exhaust pulse by forcing it thru the smaller diameter of the cone.
There are other brothers on this site that could correct me if I have it wrong.

Would the smaller diameter cone affect the exhaust note?

And this is why, (or so I've been told) that on a kick start motor with custom (or drag) pipes, one has to "clean out" any gases that may remain in the combustion chamber before attempting to start it. Makes it easier to start with less kicks.

The clean out process is simple. First, you hold the throttle all the way open with the ignition off and kick the motor over a few times, clearing out any gases. Then, choke the motor, give it some gas (a couple blips), turn the ignition on and kick until it starts. Should only take one, two, or three kicks at the most if motor is in tune. The clean out is usually only necessary when the motor is cold.

Any of you that have had a kick start bike know what I'm talking about LOL.

Here's a couple of links explaining cause and effect of exhaust reversion...
http://www.hotbikeweb.com/tech/0603_hbkp_custom_motorcycle_exhaust/viewall.html

http://www.circletrack.com/enginetech/ctrp_1011_race_engine_reversion/viewall.html

Good links, Lucifer, though the info isn't conclusive LOL. I suppose the debate could go on forever. Experimentation is the best way, at least for this garage mechanic, to get the most out of one's pipes. In one of the links you posted, it said that factory pipes were at best a compromise in performance, looks, etc.

Truth is, all pipe designs are a compromise. Depends what you want. I sacrificed a little performance in favor of looks and sound, but think I gained some of it back because my bike is so light. JMHO.

Exhaust reversion has little effect on Tattoo's or my bobber because they are 300 - 400 lbs. lighter than most big bikes. For those who need to extract every last HP from a heavy touring bike, it makes good sense to upgrade the exhaust system. Again, JMO.

Thanks for all the replies.

Every four stroke motor sees reversion to some degree; some worse than others depending on the motor configuration. FLHTBiker's explanation is a good one; same cycle in marine motors as well. Cam overlap and lobe separation angle are major contributors to reversion. Larger displacement motors are more susceptible to reversion; the bigger they are the more they suck. Some of the negative effects of reversion can be mititgated via the configuration of the exhaust system but if cam overlap is 260* and LSA is 120*, reversion will be a factor to contend with.

There is also a great video on the S&S website about it with drag pipes and they show you dyno charts after they stick the bolt in the end too. Its the second video in the instructional video series.

Saw the SnS vid on youtube, it was very cool
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