I recently installed the Woods TW-222 cams in my 2012 Road Glide. The "knocking" that I am hearing I've identified as coming from directly behind the air filter, in front of the throttle body....if you will.

Let me say that everything seems perfect. I probably have 500 miles on these cams, and they're pretty incredible. I am pretty sure that the "knocking", which goes away as there's an increase in RPM, isn't a sign of parts flying through my motor!

I've read that others have had the same sound after installing a more performance cam than the stock cams, and that it's called "reversion", which comes from the overlap in said cam. Why? What is reversion other than a sound? What about the overlap in a more "performance" cam causes reversion?

Curious if there's a more technical explanation to this than what I have found thus far.

 

Reversion is the backflow of exhaust gases. Reversion can be created by low exhaust velocity or pulses arriving at the combustion chamber at the wrong time. A major deterrent of drag pipes. Anti reversion cones can diminish/eliminate reversion.
http://www.ebay.com/itm/ANTI-REVERSI...-/271137096909

 

It's simply that exhaust and intake gases move backward for an instant while the overlap last. It happens because at lower rpm's, without exhaust flow momentum, the piston forces some of the exhaust gas into the intake passage. Makes for rough spots at low speeds and idle.

 

The reason the gases are mixing is that the intake and exhaust valves are open at the same time=Overlap. The more overlap the worse the reversion can be at low rpm.
Another issue is when you use a cam like the 255 which has almost no overlap the reversion hits the closed exhaust valve and sounds like the engine is pinging at low RPM.

 

What you are hearing is the reversion pressure wave bouncing off the throttle body in the manifold. The noise may vary in intensity or go away depending on cam, valve timing, throttle body, and intake etc. The noise is amplified with an ope A/C like a heavy breather; the SE A/C with the football cover will muffle the noise. Very common with fast ramp cams with generous overlap. See the graphic below for an illustration of reversion.

 

Thank you all for the replies. I do have an open breather system, a Big Sucker as a matter of fact, so the sound is amplified.

Is this something that would be problematic as far as you're all concerned? Do I need to buy cones or something else to eliminate it, or is it something that I'll get used to such that I won't even hear it any more? I am almost at that point already, but I just want to be confident that it's not slowly damaging something.

Thanks again for the responses. Very helpful.

 

Cones might help but could also have an effect on performance. The noise is not hurting anything; a characteristic of the cams and, as you point out, the noise is amplified by the open breather. If you are happy with the performance, ride and forget about it if you can.

 

"ride and forget about it if you can."

I can. Thank you!

 

What you are experiencing is good! At least in performance terms. This confirms the adverse effect of EPA noise regulations, in reducing vehicle noise on the one hand, while also reducing performance on the other, certainly where bikes are concerned. Once upon a time all vehicles made similar noises, except the most mildly tuned ones.

Ride on and enjoy. Also wear earplugs on longer trips, but you already knew that.....

 

I might suggest that the reversion characteristics can be altered by changing the exhaust tract

either the length, diameter, baffling etc.

this may not eliminate reversion, but can shift it to another rpm range.

( oh look, another advantage of tunable pipes such as supertrapps)

reversion can be interesting esp. on carbed bikes as the air/fuel vapour can sometimes be seen exiting the throat of the intake.
if an air cleaner is in place, the fuel may wash out oil from the air filter and make a mess- sometimes the rider thinks "blow-by"

mike