Muffler restriction

I replaced my mufflers last year - the ones that came with the bike were rusting. I got the mufflers without the welded seam, but found I could only go about 80mph. The aperture was too restrictive inside the muffler, especially when running 750-style pipes.

I used a Klein uni-bit on a Milwaukee extension shaft:




The original aperture was only 1/2". I drilled it out to 5/8, and got some improvement, then 3/4" and found no more bogging down at speed.

This particular bit fits snugly down the muffler, and the design lets one enlarge any hole in 1/8" increments. Other step-drill bits are available in 1/16" steps, and they make metric ones, too. These bits are primarily used by electricians to make or enlarge hole in panels and junction boxes to run conduit, and specifically designed to cut sheet metal in the 18-12 gage range.

Much neater than a hammer and long screwdriver or chisel, and you can ensure both mufflers are the same aperture.

With the original 1/2" aperture, cross section is .196 sq in, or .392 sq in with balanced pipes. With the 5/8 aperture, this grows to .307 sq in on each side. A 3/4" aperture yields a cross section of .44 sq in on each side, which is just a bit more area than running the original set-up through balanced pipes.

These calculations assume that the main exhaust pulse runs through the center aperture at speed, and disregard the annular slots at the rear of the baffle assembly.

That's useful and fun exhausting. I've been thru 4 peashooters sets so far, first 2 being factory issue the last 2 after market but they all were straight through tube about 1" ID. Only the bafffle holes varied by size and shape of the baffle louvers. Who sold you the restricted ones? I was pissed that my SV650 sounded more like a 4 wheeler than a cycle so looked in muffer to see flat baffle plate blocked through flow so shot two double taps of 4 .22LRs through it for two ~1/2" oblong holes that lowered the exhaust tone and opened up some low end power response. The bullets did not exit the muffler but don't rattle that I can hear. Do you note a lower louder exhaust note now?

I have not seen restricted peashooter such as those described - since around 1975 or thereabouts when they were "factory" replacements for noise requirements in the US. Instead of boring the center hole, we usually cut off the last couple inches of muffler so they looked in profile sort of like the dunstall type, roughed up the muffs with some 400 sandpaper, and painted them flat black. That got rid of the restriction and we thought the flat B mufflers looked WAAAY cool compared to stock! But all the peashooters I have seen from suppliers for many years now have been as Hobot described.


Hmm, I have an old set of Peashooters and some flat black paint...

One fella noticed none of the after market reverse cones had the twittering run over sounds of factory issue so did a survey of like half dozen baffle designed compared to his rusting out Norton insides to see that the factory had louvers of crescent shapes while the others had straight cuts or triangular shaped flaps. He was able to take photo's of insides but have lost them to show here.

My original peashooters had a blocking plate located about the widest part. The new ones I have, I can look through.

Dave
69S

As I said in another post, you can fire a ping pong ball straight through my pipes. No idea if they are original or a replacement. They have a Norton stamp on them. My riding buddy on a big Kaw Nomad complains about feeling my exhaust in his face when I leave him in the dust...

The restricted mufflers work fine if you have a crossover pipe. The restriction and the crossover make them very quiet. That is what they were designed for. jim

Many years ago Bob Schanz (DomiRacer) had a container full of peashooters that had a blanking plug in the center forcing the exhaust into the outer chamber and exit through narrow slits after the plug. They were real quiet, but you were lucky to get 35 mph going uphill. I believe these were Asian build. Bob wound up selling them at a huge discount. We used a 1" hole saw on a drill extension. The Norton snarl returned and it no longer bogged. Noise was no louder than a stock "S" (probably the loudest of the stock Commandos). Chrome was not top notch but pretty good at ten feet.

comnoz said:

The restricted mufflers work fine if you have a crossover pipe. The restriction and the crossover make them very quiet. That is what they were designed for. jim

Click to expand...

Yeah, Jim, that was my point. I can't remember exactly, but I think these were listed as 850 mufflers. If you mount them on 750-style pipes, they're too restrictive. I essentially did what Ron did years ago. By using a step bit, you can control how big the hole is, and keep the two sides the same size.