Exhaust design stock manifolds

JoJo

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Jul 1, 2021
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2005 Rocket III
Hi, I am planning to fabricate a new exhaust for my 2005 Rocket.
I am planning on using the three cast manifolds and welding the popes on from there.
I noticed that on the face they are like two round openings joined in the center.
There are raised portions of cast steel both center top and bottom of port.
Is there a function for this or can they be removed and or smoothed out for better exhaust flow?
Thanks,
JoJo
 
The head is aluminum so answering your question is a little confusing right now.
But consider this if you add all new pipe and stay with the small exhaust port manifolds. Are you looking to improve the flow or just the typical loudness and looks? Just curious as to what you are after.
 
The head is aluminum so answering your question is a little confusing right now.
But consider this if you add all new pipe and stay with the small exhaust port manifolds. Are you looking to improve the flow or just the typical loudness and looks? Just curious as to what you are after.
Hey Warp, I guess that I am looking to consolidate the system to the front side of the engine in order to facilitate an open area for the rearsets that I am installing. I would like to maintain a decent exhaust note with no loss of performance and a possible loss of some weight.
 
The factory header is designed with small tubing to assist in getting the peak torque down low in the RPM band. It is very restrictive to higher total torque and horsepower out put. May I suggest looking at some other headers such as Viking and LMS.
 
If you want to experiment there's value in a stepped design. Have a look at modern sport bike headers near the flanges as an example, the change in diameter is used to create a harmonic and the sizes and length of the smaller section of the primary is very specific to cam size and RPM range.

There are only three R3 headers I would look at for inspiration these days, Carpenter Brutes, LMS Racing, and CES. They all perform exceptionally well, having distinct torque curves placing the peak values at different places with differing impacts before and after peak torque, and the peaks differ in their breadth between the three. They have different diameters, lengths, and merge collectors and are good examples of the influence each can produce.

There is a ton of info on this on the internet, but it takes a good amount of time to sift/sort through to make sense of it all. From what I recall off the top of my head:
  1. All values influence each other as a system so none work in isolation.
  2. It's better to be slightly under than slightly oversized on the primary diameter. Influences peak torque height. 2" ID or less on R3s, but more than 1.5" ID.
  3. It's better to be slightly too long rather than too short for the primary length. Influences RPM point peak torque is achieved. 20" or longer on R3s.
  4. It's almost universally best to use a long vs short merge collector. Influences the breadth of the torque peak and height.
  5. Merge Collector choke point, the narrowest part of the collector, heavily influences power output. This is the hardest to find info on, too big and too small is an overall loss everywhere but it can be smaller than is intuitive. Look at the Brutes, they choke way down and make great power.
  6. Post collector should be no less than 18" but the actual length is primarily influenced by packaging. Too short is a universal loss of power everywhere, the longer the better.
 
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