Exhaust reversion tech discussion.

[Excalibur]

Wasn't sure where exactly to put this thread but Harley Tech Talk seemed to be the right site.

Discussion theme: What ideas help reduce exhaust reversion on an older carbed Harley? Pan, Shovel, Evo etc.

Have been doing a bit of research and study on the topic so keen to hear comment from the knowledgeable and experienced.
There's talk of exhaust design affecting reversion and how in those cases, torque cones can improve it. Views are mixed. Others say a well designed exhaust sees no improvement from cones. But it poses the question, what constitutes a well designed system?

Straight pipes are another situation where lollipops can reduce the behavior.
Fishtails have attracted comment, something about going nuts trying to tune to suit.
Does a particular muffler style work better than others? Pipe size, length, design configuration?
What's worked best? What's shown to cure reversion or similar behavior.
What will disrupt the reversion phenomena?

Cams with lots of overlap have been pinged to be more of a problem because there's physically more time for exhaust shock waves to travel up the pipe, past the exhaust valve, into the cylinder, then through the intake valve, diluting the intake mixture with spent gases. If one is set on keeping their cam, what can be done to exhaust design to reduce or eliminate?

Often guys that change type of exhaust will say, bike runs different. So there must be a lot of info about what worked and what didn't

My personal interested is mostly to do with Pan and Shovel because I own both. The shovel project is soon to be running and I'd consider building into it, features that compliment engine running sweetness. Currently I have no exhaust.

Thanks for any thoughts, experience etc.

Couple of pages that define reversion. Some theory and opinion.
Intake reversion
Anti-reversion, scavenging

[Hossamania]

A little detail about your build will help with recommendations specific to your needs and wants.

[Excalibur]

Ok, good idea, thanks. Am adding details of the Shovel build and my regular rider. Either can be subject of discussion or experimentation.

Shovel bought in pieces after it suffered catastrophic big end meltdown. Both jugs, pistons, rods destroyed. It was originally made up out of parts. Bottom end is '78 with later flywheels. 80 cubes. FLH cam/ hydraulics. Rebuilding with single fire Dyna S. Carb SU. Exhaust to be decided.
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Pan, owned 6 1/2 years. 74cube. STD heads. FLH cam/hydraulics. 12v Single fire Dyna S. SU carb. 2-1 Fl style exhaust with fishtail. Premium gas, 50mpg average.

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Keen to learn. Always looking for an edge.

[76shuvlinoff]

I had a ton of reversion in my 76 with the Keihin butterfly carb. It was my first HD and this site didn't exist yet so I mistakenly assumed it was a carb problem.  I was running unbaffled fishtails. After changing to an E carb didn't really help, the fix was a bolt with a stack of nuts on it in each pipe.   

Running a 93" mill, 44mm CV carb and a Thunderheader now, no issues.

If you haven't seen this before it's in a sticky on the shovel page.
http://www.nightrider.com/biketech/exhaust.htm

[bobrk1]

I was  going  to  put  a set  in  my road king  the  guy  with  the  dyno  said  he  didn't  see much  of  a  difference  with  them, but  I  had  a  friend  who  had  straights  on a  low rider  with  a  S&S on it  that  ran great  but  would  sometime  stall at idle , he  called  S&S and  he  was  told  drill a hole  near  the  end  and  put  in a 3/8 bolt  for  back  pressure , never  stalled  after  that.
 

[Hossamania]

Lollipop, turned to about 3/4 open after this picture was taken.

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[Excalibur]

Thanks for the thoughts. Awesome.

76shuvlinoff,I expect the "bolt-with-stack-of-nuts fix" disrupted the reversion sound wave just enough to scatter it? Seem reasonable?

That's a very interesting dyno test on nightrider link. (Had seen before). A tiny change with the angle of the lollipop baffle has such a dramatic affect. The lollipop at 45° seems like a good compromise. Maybe the angle gets the wave tumbling?

My local Harley shop swears by gutting out Harley mufflers and sliding in perforated tube. Talk is bikes run well and sound is good. Apparently, exhaust gases rushing over a series of perforated holes, creates some back pressure?

Re a CV carb (incl SU), when reversion travels all the way back to the intake, vacuum will at least momentarily reduce. When this happens the carb-piston will drop, after all it floats on the exact amount of vacuum/pressure/flow. I can't decide if it makes CV more susceptible or less?

Comments are welcome. Thanks.

[HogMike]

1965 93" pan small bump in compression little larger valves, ported heads.
Single spark plug
S&S E carb
Stock single exhaust with stock (gutted) fishtail muffler.
Bike runs very nice, no issues except a pop now and then on decel due to loose exhaust system.

 

[turboprop]

Re a CV carb (incl SU), when reversion travels all the way back to the intake, vacuum will at least momentarily reduce. When this happens the carb-piston will drop, after all it floats on the exact amount of vacuum/pressure/flow. I can't decide if it makes CV more susceptible or less?

No it won't. Several reasons why it won't, but you guys can discuss in depth. You have obviously been next to a bike equipped with a CV while it was bing tuned on a dyno. Try it. Let us know how the slide in that CV carb drops momentarily.

[Excalibur]

Thanks for the comments. Keep them coming.

I have some Evo torque cones that I've been studying. Each is 2 opposing cones, like 2 funnels connected tail to tail. I can understand a cone for outgoing gases because it will speed it up as it goes through but why the cone facing the other way? I reason a single cone would be better because reversion gases will more likely be deterred, an air dam if you will.
Said to be effective, what about multiple cones perhaps at each of the exhaust joins or is it such that the further from the head, the less effective they become?
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Anyone using any type of cones? What's your experience?

Watched a video by Kevin Cameron on motorcycle exhaust. Amazing what has been worked out, including cause of the dreaded flat spot.

[Hossamania]

Those kind of work with oversized drag pipes, otherwise not needed, in my limited opinion. Spend the money on a good exhaust. If you just want sound and looks, performance is usually compromised.

[Tacocaster]

Excalibur, regarding your question, "....but why the cone facing the other way?"
I remember looking at TC's years ago when there was a lot of talk about them and found this as a possible answer to your question:

As the exhaust gasses leave the engine and travel down the pipe, at some point, some of the exhaust gasses reverse direction. When the gasses do that, they travel back against the pipe wall (in the shape of a do-nut). The reversion cone traps the wave between the cone and the pipe wall, not allowing it back into the engine.

Hope that helps explain.

[kd]

This article  explains it with a picture.  Fueling was heavily invested in a pipe design to do this also. 

[Excalibur]

Thanks for the ideas.

KD's article is interesting. It lead to another, and another, and another...

I suppose the real big question is, what constitutes a "good exhaust"? Is it mostly about baffle design?

30 years ago when I was circuit racing a vintage Triumph, I ran straight pipes. To quieten them, I cut two 1/8" slots about 5 inches long. The slots started 1" in from the end of each pipe. This took most of the crackle and bark out of the sound, making it much more pleasant. Amazing what a couple of slots can do. I mean who thought that up    


 

[Hossamania]

I might have to dig out my old drag pipes to see if I put a bolt in them, can't remember if I did, it's been 17 or more years (which doesn't seem possible). I really want to put them back on, but in all reality they should probably be just wall art. They do look pretty good all black, blue, yellow, chrome(ish).

[Excalibur]

Picked up this interesting comment....

'Megaphonitus' is caused by Exhaust Reversion due to a short (tuned?) exhaust system in combination with cam overlap. The reversion pulse of air up the exhaust pipe through the still open exhaust valve flows through the chamber and pushes the incoming fuel/air backwards through the carb, again loading it with more fuel. This charge of fuel/air will again be drawn into the carb on the next cycle again picking up more fuel for a triple richened mixture. This is why leaning out the carb seems to help.

It appears to suggest reversion effects can be modified or moved with exhaust design. On a bike we are obviously restricted with overall length of exhaust.

Still pondering how to define a good design. It seems like some baffling is desirable, not resembling a megaphone and not too short...

[Hossamania]

The reverse cone design of the Thunderheader has been proven to work well.
Ugly as sin, but puts out power.

[JSD]

I dont think you have to much to worry about using a stock cam (FLH) just try and keep the dual system length close

[Excalibur]

Thanks for the thought provoking ideas.

Equal length exhaust headers is something I hadn't given much thought to lately. Heard talk about it decades ago. So, would equal length be more important than actual length?

I picked up some drag pipes which I plan to test a variety of mufflers on. The object is to find something that runs well, sounds good and doesn't make obnoxious amounts of noise.

Has anyone used heat resistant ceramic coating for inside headers? An acquaintance has used such on his c1964 Bonnie and the chrome is holding up fine. A search around, I find KBS Xtreme Temperature Coating
Talk is ceramic coated headers have the same effect as heat wrap. I recall, gases move faster and engine temperatures are reduced (something like that?). As usual people don't agree so OK, what does anyone think? Experience? Theory? etc?

[JSD]

Exhaust length for your motor should be kept close to 40". S&S recommended 36" for hot rod motors . Keeping heat inside the pipe will help. Go for it i say.

[kd]

Are we discussing a hot rod engine here or a wide power band?  BTW, what is a hot rod engine?

[Hossamania]

BTW, what is a hot rod engine?

Every motor in every vehicle and motorcycle I've owned. Even if it wasn't meant to be.

[JSD]

KD the post starter has a mild build stock cam & Cubes so 40" is in his ball park. A hot rod shovel can be a stroker , big bore . Even a stock cube with high comp big cam etc . 

[kd]

 

[turboprop]

Wasn't sure where exactly to put this thread but Harley Tech Talk seemed to be the right site.

Discussion theme: What ideas help reduce exhaust reversion on an older carbed Harley? Pan, Shovel, Evo etc.

Have been doing a bit of research and study on the topic so keen to hear comment from the knowledgeable and experienced.
There's talk of exhaust design affecting reversion and how in those cases, torque cones can improve it. Views are mixed. Others say a well designed exhaust sees no improvement from cones. But it poses the question, what constitutes a well designed system?

Straight pipes are another situation where lollipops can reduce the behavior.
Fishtails have attracted comment, something about going nuts trying to tune to suit.
Does a particular muffler style work better than others? Pipe size, length, design configuration?
What's worked best? What's shown to cure reversion or similar behavior.
What will disrupt the reversion phenomena?

Cams with lots of overlap have been pinged to be more of a problem because there's physically more time for exhaust shock waves to travel up the pipe, past the exhaust valve, into the cylinder, then through the intake valve, diluting the intake mixture with spent gases. If one is set on keeping their cam, what can be done to exhaust design to reduce or eliminate?

Often guys that change type of exhaust will say, bike runs different. So there must be a lot of info about what worked and what didn't

My personal interested is mostly to do with Pan and Shovel because I own both. The shovel project is soon to be running and I'd consider building into it, features that compliment engine running sweetness. Currently I have no exhaust.

Thanks for any thoughts, experience etc.

Couple of pages that define reversion. Some theory and opinion.
Intake reversion
Anti-reversion, scavenging

Anti-Reversion for a trouble head?  Thunderheader. Done.

One of the Admins on this page has a 93" trouble head with a Thunderheader, CV44 carb and a good cam (we all watched as he tried different cams).