Hey all, I was poking around the forums and came across a post on these. They looked interesting so I did some searches and those who have them seem to all like them well enough but I noticed that nearly all the posts from those using them were for either Sporty's or those running straight/open pipes. Anybody used them on a bagger and if so what were your results? I just installed some cams and had mine tuned. While it pulls real nice..aren't we always looking for that little extra. It's a 2011 RG Custom running a SE Hi-flow A/C, decatted head pipe, and Klock Werks slip-ons. I like the sound of just wondering if there are any supportable results out there. I understand the concept of returning some of the back pressure lost to an open pipe and how that would also return some of the power. What I can't seem to wrap my head around is one of the claims made by DK Customs. They say they help to "increase exhaust gas velocity". Now, I am no engineer or exhaust expert by any means but how exactly does placing another decent sized obstacle in the end of your exhaust pipe increase the velocity of the exhaust gases?? So...what am I missing here?

 

like a carb venturi, narrowing a passage forces the gasses to go faster to get through the narrowed area.

if the volume of the gasses passing increases ( like more rpms...at 6000 rpms you miy have twice the gasses as you would at 3000 rpms*) at some point the narrowed passage will cause the gasses to back up and slow down upstream.

so "torque cones" will help below a certain rpm, and they are most useful in a straight pipe.

a straight pipe makes GREAT power at high rpms- so if you ride at 6000 rpms all the time, awesome.

but they give up power at low rpms, requiring the clutch to slip to leave a stop ( kinda like a top fuel dragster), especially on a heavyweight

and that explains why the 17 year old on the next block with the "performance exhaust" has to rev the crap out of his honda to leave the stop sign...

Mike

* and this will vary depending on intake, exhaust and cam exactly how much air/fuel or gasses pass at a particular rpm

 

Thanks! That makes a little more sense. While there would be a benefit, particularly on an open pipe, while the exhaust pulses were still moving at a rate in which most of it could escape before the next one, at the higher RPM's there could be an adverse effect due to the gases backing up. Sort of what I thought but I hadn't thought to adjust the rate based on RPM. I was stuck on increased velocity despite an obstructed exit path.

 

Depending on what size baffles your slip ons have, it is likely you lost some low end torque after decat and may not realize it. For $25 I would install the 258 TTI's and you will get the low end stronger. Kevin, one of the owners of DK Custom has a good write up in one of the threads of how restricting the exhaust makes more power.

 

I'm sure they are a well made product and like I said, those who have them all seem to like them. I was just hung up on the concept of increasing exhaust velocity by further restricting the exhaust. However, after MK's explanation realized I hadn't considered the "slower" rate at which the exhaust pulses were being generated through the lower-mid RPM range versus say 5k and above. Just added cams to my RGC and had it tuned and I'm over 100 ft/lbs by 2400 rpm and stay above 100 until about 46-4700 rpm with a peak of 108 ft/lbs. Now, after that 4.6k mark it starts dropping off pretty quickly but I knew from what I had read about the 255's that they characteristically "fell off the cliff" after that point. Which is fine since I don't ride up in the 5k range anymore. I ride 2-up 95+% of the time with the wife and we tour with the trailer a couple times a year so the power band is actually right where I need it.

 

An old shade tree mechanics trick to improve performance on bikes with straight pipes was to drill a hole through the pipe close to the end and insert a bolt.

I believe a couple years back American Iron mag did a dyno test on straight pipes with and without a bolt and found that it did improve output.

The thunder torque inserts are just a fancier version of the plain old bolt trick.

 

Exactly!

 

My road king has V&H mufflers with huge baffles - I put Big City Thunder quiet baffles in the pipes just ahead of the mufflers - low and behold all my low end torque came back! Another fancy version of the bolt and washer mod, which I tried but didn't work as well. It did quiet them down some too, which for me was a good thing.