So Routy, if you've put 8 5/16th holes you have enlarged the exhaust exit thingy by 2 1/2 inches, or is my maths wrong?

JC Stokes,  It doesn't add that way.  Each hole's square area is less than it's diameter.  Find the area of the drilled hole and multiply by how many holes.  ( I think ).

justin_o_guy2 wrote on 11/01/12 at 00:04:52:


I tried, every shop near me wanted 50 bucks plus.(maybe they smelled my newbishness) One guy quoted me 180. So I just went and bought the ryca version. that didnt pan out either. Still had leaks because there were two clamps now. I said no thanks to all that mess and jumped on a supertrapp in the marketplace for 170. For that price and ease of install it was worth it. I fired it up and it had a little more beef to the idle and a whole lot better of a sound and it made me  . the kicker was when I pulled away from my house and there was no more VW bug sound coming from my morterbike. very pleased.

To recap for the original posterman. The dyna is the best bet granted you can get one for under $25, and get the adapter for about the same(might get lucky and not need it). You can probably get away with bending the brake arm in a wee bit in a vice.

Honestly, you or anyone can have mine for the price of shipping, it aint pretty but it will work. Its just sitting on my back seat collecting dust. its got a pretty bad blueing/rubber stain in the middle of it.

Well, after reading through this thread, I went out and looked at the end of my muffler, which I've always assumed was completly stock. It's got two extra holes in it, each about 1/2 inch; one at 12:00 and one at 6:00. Even with the extra holes the exhaust on this bike is pretty quiet.

I can't wait to discover what other mods the three previous owners might have done as I get more familiar with this thing.

"How big Is the ID on the stocker, anyway?"

Mine is about 5/8". Not a lot of area even with the extra holes.

Let's say you have two exhaust pipes each with an ID of 2".  Inside each of those exhaust pipes, you weld a 2" diameter baffle.

One baffle has 20 holes
-- each hole about a 1/4 inch in diameter, ~.126” radius (.126 because the guy doing the drilling used a battery-powered handheld drill with dull Big Lots bits not even made in China)
-- each hole  ~.05 square inches in area
-- 20 x .126 x .126 x 3.142 = .998  
-- total area for exhaust flow approx 1 square inch.  


The other baffle has one 1 1/8" hole, punched at a machine shop.
-- 9/16” radius
-- total area for exhaust flow about 1 square inch.  (9/16 = .5625, so.....  .563 x .563 x 3.142 = .996)

For the sake of discussion, let's say the total area available to flow exhaust is the same for each baffle.  

Which exhaust pipe baffle will have less back pressure?

Which will make more noise?  

Routy wrote on 11/10/12 at 07:24:23:



Correct.  Every hole has a bit of turbulence around the edge that adds drag and therefore restricts flow a tiny bit.  More holes=more turbulence=more drag for a given area of holes available for flowing the exhaust gases.  There are a large number of variables (heat, pulsing (lack of smoothness), rpm, back pressure, etc.), but in this application, the total area of the holes in the 20-hole plate should probably be 10%-15% greater than the single hole for the same flow.  As you might expect, this negates, somewhat, the noise reduction from the 20 holes.