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Old 04-29-2008, 10:49 PM   #1
motocash74
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Default Square Exhaust Mod

Ok this is the advice I received from Cary @ schnitz. I sent him a drawing of what I was going to do, he replied with some changes and here is the result.

This is my first how to, so lets hope it doesn't suck.

First off this is for the Big Gun Q series pipe modification.

Remove the rear section of the Big Gun & drill out the rivets & unscrew the bolts that hold both the front & the rear end caps.



This is what you should be looking at. The perf plate inside is what you are trying to get out. It is sleeved into the front end cap and can be rather difficult to remove.

This is what you are after. The core. It is exactly 2" in diameter and slips into both the front and back end caps.



Here is what I replaced the stock core material with. It is 304 SS perforated plate.
I ordered it from smallparts.com. I'm sure I over paid for the stuff, but it was such a small order I didn't care.
0.117" Round Perforations on 5/32" Straight Centers

I then use a piece of 2x2 square tube I had laying around as a forming die. I used a ball peen hammer to form the perf plate around the square tube. I then used tin snips to cut the perf plate just a bit longer than needed so I would have 1/8" overlap. All told I used 8-1/2" x 12" of the plate. This means you only need order a 12" x 12" sheet of the stuff.



After speaking with Cary he told me to use 4 chevron perf plates as internal baffles.

For these I used 5/32" Round Perforations on 3/16" Straight Centers. This because it is lighter material and is 63% open vs. the 51% of the other plate. Cut 2"x5" strips of the stuff to make the chevron plate baffles.



This is what they are supposed to look like after you weld them into the interior of the square perf tube you just made.



This is what they look like welded in place. Cary recommended brazing them in place. I had already put my son to bed before I started this project, so I couldn't leave to get some brazing materials. So I instead chose to weld the chevrons in place. It took a long time to spot weld 100 or so places. The material is too thin, to weld normally, so a spot is all you can do.

For the next step you will then need some exhaust tube that is exactly 2" OD for both the front & the back of your 12" long square tube. This is the important part.

The entire fabricated piece needs to be exactly 14 1/2" long. You can be 1/8" shy, but not any longer. This is important, otherwise you will not have proper fit up. If you are too long, you will not be able to assemble the silencer again. If you are too short, you may have the rear of the silencer slip out of the rear end cap and begin blowing all your exhaust packing out the tail pipe. If you put the screen back in, you will clog the screen with exhaust packing. 14.5" is what you want.



You will have to form the square end to the round pipe. Luckily the material is thin, so light tapping with the ball hammer will get you where you need to be.



Ok, heres the finished product. Looks like crap eh? Well that's because the first one was 14" long and didn't seat properly inside the can. After that I cut the inlet pipe out & re-welded a longer one in its place. 14.5" is what it needs to be.


So here it is installed.



All I can say is that it sounds great! It is way deeper than the Q series & isn't incredibly loud. I still have the option of putting the Big Gun spark arrester screen in, if I need to quieten it down a bit. But for now, I'm happy with the way it is.

Cary if you read this, this is preparation for the rest of the job, coming your way soon.

Last edited by motocash74; 04-29-2008 at 10:53 PM.
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Old 05-07-2008, 05:57 PM   #2
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Default Re: Square Exhaust Mod

Thanks to Cary for taking the time to describe this modification. The bike pulls much harder down low & has decent gains up top.

Now the head & jug are off to Schnitz for the full monty. According to Cary there is a 9 week backlog on head work, but 7-10 days for the 705 treatment. Looks like I won't get to dyno the 705 until July.
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Old 05-08-2008, 10:03 PM   #3
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Default Re: Square Exhaust Mod

cool mod. I was wondering how this mod went together. I read about it in patman's dyno page. now I know how. tks
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Old 05-26-2008, 04:01 PM   #4
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Default Re: Square Exhaust Mod

is it quieter than the muffler originally was? What did you use for packing?
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Old 05-26-2008, 04:39 PM   #5
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Default Re: Square Exhaust Mod

Patman's version used an earlier style of baffle. This works a little better. You can add as many baffle plates as you want to get the sound level where you desire it. Thanks Rob, for documenting this.

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Old 05-26-2008, 09:18 PM   #6
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Default Re: Square Exhaust Mod

Quote:
Originally Posted by dragoontwo View Post
is it quieter than the muffler originally was? What did you use for packing?
Unfortunately the pipe is a little louder than the big gun "Q" was, but not much. I did leave the spark arrestor screen out so I can go a little quieter, however I sent most of the engine to Schnitz before I could do a test with the screen in place.

As for the packing, you'll laugh, but I've used this stuff for years in all sorts of bikes & it works great.

The fiberglass mat material used as body filler for rusty cars. I bought a roll of the stuff at advanced auto parts & crammed as much of the stuff as I could in all the pipes I've put it into. Works like a charm. Other than the square mod I used the same stuff in a new set of cans for the Duke. I took the Akroproviks off & rolled my own cans from diamond plate. They work great with the fiberglass mat.

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