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weird member
1997 Softail Custom (FXSTC)
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4,340 Posts
Back on my wrecked bike when it had drag pipes, I used those inserts in the end to bring back the low end (bike had straight pipes when I got it and I couldn't afford a new set of pipes at the time). The ones that go in the end of the pipe. When you say torque cones, for a second I thought you were talking about the snake oil inserts that go up by the heads (those don't do anything).

The lollipop baffles in the end of the pipe work, depending on the pipe. There's two things at play, one is the harmonic reversion you mentioned (which could be remedied by as little as a screw or bolt in the end). The big thing these particular inserts do, is increase exit velocity. As soon as exhaust hits the air outside of the end of the pipe, slows way down, which causes a build up in the pipe that has to be overcome (the engine has to push it out, which is why it's more a problem at lower RPM). It's a problem with larger diameter pipes because although more free flowing (good at high rpm), they have a lower velocity (which is why the exit slow down is a bigger problem). The lollipop increases velocity just before where it would slow down as well as increases scavenging in the end of the pipe, helping to prevent that pressure build up.

The result is a smoother bottom end and eliminating some of the low-end sag (assuming it's caused by an exhaust that's too big for the engine and rpm range).
 

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weird member
1997 Softail Custom (FXSTC)
Joined
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4,340 Posts
Think I’ll pass on reading anything from tab as they are one of the worst performance exhaust you can buy
Never mind that they call it "back pressure".

To oversimplify it, you get better performance in various RPM ranges based on diameter and length. For street riding, if you put a too high output exhaust, that's where you lose power through the loss of scavenging and velocity, not back pressure.

And to be clear, these inserts don't give you more torque, you can't just slap them in any exhaust for more torque, they just give you back some of the loss when you have the wrong exhaust pipe (usually too big diameter) for the engine/cam setup for your application, which is really only a bandaid on the problem. But they aren't really creating back pressure, they're sort of correcting oversizing (for the intended RPM range and cam) and helping with some scavenging. It's more like what stepped headers do: not add back pressure, but add scavenging. That's why they go at the end of the pipe, where the everything has lost the most velocity. And it goes without saying, the proper exhaust for your setup would produce more power than a modified wrong exhaust.

Exhaust isn't that complicated to understand the basics. Yes, they can get complicated and there can be a lot to them, but...

"The reason why some old timers have said things like 'you need some back pressure to make power and torque' in the past was... well if you have like a really huge, way oversized exhaust, you lose your gas column inertia..." (key part here being "oversized exhaust"). Also "the trick is to get as much velocity through the pipe with as little back pressure as possible, a lot of that is in the correct size tubing for your engine":


 
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