Muffler sizing

[Syscrush]

Hey all.

I'm planning to order a replacement muffler for my bike today - it's a stock '79 with a Gary Wolf 6-2-1 header.

My plan is to go with a 16" oval Stealth muffler from Hindle with the dB killer insert. What I'm trying to decide is whether to go with a 2" or 2.5" dia.

My concern with going with the 2" is that it may be too restrictive for a 100hp bike, and my concern with the 2.5" is that it may be too loud, especially given the shortish 16" length.

Assuming restriction is proportional to length and inversely proportional to cross-sectional area, the 16x2 would be 56% more restrictive than a 16x2.5, and 25% more restrictive than a 20x2.5.


I'm inclined to go with the 16x2, but I'm open to suggestions.

Thanks!

[Syscrush]

Screw it. I just decided to go with the 2". 16x2.5 is a little too close to an open pipe IMO. If I have any concerns that the bike is down on power or too quiet, I can always remove the dB killer insert - but I really don't see that happening.

[Rick Pope]

I seem to remember reading that a muffler needs to be sized to quiet only the displacement of one cylinder, as that's all that's firing at a time. So, the 16 x 2 might well be enough, given the 6 even firing pulses. Perhaps even a 16 x 1.75 might do it.

Personally, the older I get, the more I appreciate quiet exhausts. I don't care to draw the attention of the locals if I'm exceeding the speed limit through their territory.

[Syscrush]

I seem to remember reading that a muffler needs to be sized to quiet only the displacement of one cylinder, as that's all that's firing at a time. So, the 16 x 2 might well be enough, given the 6 even firing pulses. Perhaps even a 16 x 1.75 might do it.

I am no expert, but I always go on the assumption that the muffler should be sized based on hp. My rationale is that 1/3 of the energy of the gasoline ignited in the combustion chamber is going out that muffler regardless of the state of tune, and that energy is going out in the form of heat and noise.

It's hard for me to imagine that the muffler from a 150cc 20hp single-cylinder bike wouldn't choke down a 600cc 115hp I4.

With all that said, a 16" muffler is not long, and I agree that 16x1.75 would probably be OK, but the smallest ID Hindle makes is 2", and I do want a Hindle for sentimental reasons - which is why I'm going with the dB killer insert too. I had that setup on my SV650 a few years ago and I thought it sounded great.

Personally, the older I get, the more I appreciate quiet exhausts. I don't care to draw the attention of the locals if I'm exceeding the speed limit through their territory.

Same here. I'd rather have 95hp and be able to go WFO without setting off alarms all around me than have 110hp and know I can never fly below the radar...

Even so, with an exhaust note as special as the CBX's, I'm willing to bark a bit louder than I would for other bikes. ;)

[Rick Pope]

Phil,

I understand your logic on muffler sizing, but let me clarify my point. The muffler's first job is to muffle, so it has to handle each individual "bang" or power pulse. That's why I implied we can use a muffler sized for 175cc (1047/6).

However, you are correct in thinking that you need to have enough flow. A 150cc single won't rev like an i-4 600, thus the 600's need for more flow capacity.

But, I'd say have fun with it. If you are getting the dB killer insert, you might modify it to suit your taste.

[Syscrush]

It's an interesting idea. At 6,000 RPM, a 6-cylinder bike would have 150 exhaust pulses per second. If the pressure waves are travelling at about the speed of sound, then each individual pulse wavelength would still be longer than your whole exhaust system. I don't know anything about how this stuff actually all works together, but the notion that there's only one pulse in that muffler at any given time seems to have some merit.

On the other hand, if you plumbed all 6 pipes into one muffler from a 6-into-6 system, it's hard for me to believe that it wouldn't get a lot quieter. Where are the mythbusters when you need 'em? :)

[Rick Pope]

Perhaps rather than the length of the whole wave, it's the pressure front of that individual wave? Thus the "pop"? If the muffler breaks up that pressure front, it softens the noise, spreading it out to blend with the next wave?

I don't know for sure. Maybe it's just the jolt of this first cup of coffee.

I do know that the SuperTrapp uses difuser discs to break up the sound. My system has, I believe, a 1 1/2" hole through the muffler and about 7-9 discs, and is very quiet. I could be wrong on the hole size though.

[Syscrush]

Perhaps rather than the length of the whole wave, it's the pressure front of that individual wave? Thus the "pop"? If the muffler breaks up that pressure front, it softens the noise, spreading it out to blend with the next wave?

Probably.

I do know that the SuperTrapp uses difuser discs to break up the sound. My system has, I believe, a 1 1/2" hole through the muffler and about 7-9 discs, and is very quiet. I could be wrong on the hole size though.

Well, soon (in a week or two) I'll know what a 16x2 with dB killer sounds like. :)

[Syscrush]

I'm happy with how the muffler looks & works on the bike. I am going to have a bracket fabricated so I can move that mount strap back to the middle of the muffler, and replace the current cut & weld midpipe with something mandrel-bent, but for now I'm very happy with this setup:



I was a bit concerned about how the modern part would look on the vintage bike, but I think it turned out great.

[daves79x]

Don't fret too much about the muffler. Looks like you picked what you liked and the sound level you can tolerate. It will be LOUD with modest baffling. But none of your choices will affect carb tuning much as long as you keep the stock intake system.

Dave

[steve murdoch icoa #5322]

Nice you got to keep the center stand with that header.
Looks good and i bet it sounds great.

[Syscrush]

Nice you got to keep the center stand with that header.
Looks good and i bet it sounds great.

The header was built by Gary Wolf for the previous owner, and he specified that he wanted to keep the c-stand. Oil filter access is also easy with the exhaust in place.

As for how it sounds - it sounds pretty tame and smooth - a bit throaty with no real bark when idling or blipping the throttle in the parking lot. At WFO from 6-9k RPM it's another story. However, right now there's a lot of intake noise due to a missing airbox lid and a K&N filter - and it's a bit tough for me to tell the difference at speed between the intake and exhaust noise at this point. I'll have a better idea once the intake is sorted out with a new airbox. Also, the current midpipe is protruding farther than I'd like into the muffler, thereby negating some of its muffling ability, so there are a few variables to sort out still.

When we started it up at the shop, a bunch of guys kinda crowded around to hear it. The conversation basically went like:

Some dude: It sounds like a modern superbike.
Some other dude: BETTER!