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Message started by DragBikeMike on 11/28/20 at 10:49:19

Title: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:49:19

While working on my muffler shootout, I found that the Mac muffler flowed well but it’s too loud. I decided to see if I could reduce the noise level without restricting flow. There are at least two versions of the Mac muffler. A chrome reverse-cone and a black reverse-cone. I have one of each version.

I measured the outlet of each muffler and they both seemed to have a 1.4” diameter core. I assumed they were identical except for the exterior coating, one chrome the other flame sprayed black.

Looking into the core of the chrome muffler, I could see a straight-through design with punched louvers that protrude into the flow path. There was no wool packing surrounding the core. No wonder these things are loud.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:50:34

Since the inside diameter or the core is only about 1.4” (actually only about 1” in way of the louvers), inserting some sort of baffle inside the core will dramatically restrict flow. My goal is to retain good flow while reducing noise. The diameter of the muffler body is also small, and it’s tapered. There’s not a lot of room to incorporate a free-flowing baffle. The only option seems to be surrounding the core with wool packing to absorb the acoustic energy.

The mufflers are welded together, and they are manufactured from heavy-gauge steel. Any mods will require cutting the muffler open. Cutting requires touching up the exterior coating, so I chose the Black Mac as the most likely candidate for surgery. I can touch up the cuts with VHT Flame Proof Paint.

I had never run the motorcycle with the Black Mac installed, nor had I flow tested the Black Mac, so I needed to do some baseline tests. On the test bench, I immediately saw that the chrome Mac and the Black Mac are different animals. While the chrome Mac pulls 32-1/2” H2O on the bench, the Black Mac only pulls 26” H2O. I repeated the tests over & over. Same results. Why is the Black Mac less restrictive? I thought they were identical.

I set them side by side. The Black Mac is about one-inch shorter. Hmmmm? I would find out why the Black Mac flowed better when I cut it open.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:51:39

On the motorcycle, the Black Mac sounded different than the chrome Mac. The Black Mac made less high-frequency noise. It was very loud, but not as grating as the chrome Mac. In terms of volume, it’s totally unreasonable. As delivered, you really can’t run this thing on public roads. Here’s a comparison of the noise levels as measured on my iPad with the NIOSH app. Idle/3000 rpm/4000 rpm.

Chrome Mac: 70/83/86 dB

Black Mac: 73/82/86 db

BTW, I don’t think the NIOSH app is a true indication of exactly how loud these things are. It’s not calibrated, and the readings weren’t taken in accordance with (IAW) any sort of test specification. While both mufflers were measured under identical conditions, they weren’t tested IAW a state or federal procedure. In the real world of my 72-year-old ears, these things are both painfully loud and obnoxious. I fail to see how Mac can market them as “mufflers”.

So now I had a target. Reduce the noise levels below 73/82/86 dB without increasing restriction above 26” H2O.

Let’s get started.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:52:16

Cutting up the muffler requires a lot of crude work. I didn’t want to damage the flame-spray coating, so I needed a way to hold the work piece without marring the finish. A piece of aluminum scrap served the purpose. Once bolted to the muffler, you simply grab the aluminum in a vice. Solid as a rock.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:52:54

After a good look at the muffler, I decided to use a hole saw to cut through the end cap. The cavity around the reverse cone is deep, too deep to accommodate a hole saw, so the first thing I had to do was turn the reverse cone into a megaphone. I marked my saw cut with tape.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:53:30

And then lobbed off the reverse cone with a hacksaw.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:54:08

Now there is good access for a hole saw.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:54:51

We’re talkin a BIG hole saw, like 2-1/2”. It’s gonna want to bite and walk. A metal plug with a drive-fit was in order. Set it up with a ¼” pilot hole in the center and drive it into the exit pipe. It must be tight.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:55:29

You can see it’s a big saw.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:56:13

These Ryobi drills are powerful. When that saw bites, it’s gonna try and rip your hands off. Make sure the muffler is secure in the vice. You don’t wanna see it flying across the garage.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:57:11

It took about ten minutes to cut completely through the end cap. It’s very thick metal. Mac didn’t cheap-out on the steel, they just cheaped-out on the packing.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:57:48

You will need to make an end cap to hold everything in place when you put it back together. I cut one out of a piece of 3/16” steel plate. The OD is 3” and ID is 1.62”. There are four notches to accommodate screws. I used 10-32 stainless steel screws on mine.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:58:25

I was hoping that once I cut through end cap the core would pop out. No such luck. The core is welded on the inlet-end too. This weld must be ground away. Be careful not to grind through the wall of the muffler.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:58:59

I used this drill extension with a carbide burr and a grinding stone. The burr to plunge through the weld, and the stone to smooth off the remnant after the core was out of the way. I turned them with a drill. A high-speed grinder runs too fast; you have no control.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 10:59:38

The core came out with a little help from a hammer and pipe-drift.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:00:10

You can see that I damaged the end when I cut through the weld. This will have to be repaired.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:00:50

This shows why the Black Mac flows better than the chrome Mac. The chrome Mac uses a core made from one long continuous tube with an inside diameter that’s about 1” (in way of the louvers). The core for the Black Mac is two-piece. The major portion of the core is about 1.75” OD and the inside diameter in way of the louvers is about 1.4”. There’s a lot more room for the hot gas to flow through.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:01:41

I fixed the damage on the core with a short piece of ¼” mild steel rod and some JB Weld Extreme Heat putty. The putty is supposed to be good to like 2300°F. I bent the rod and filed it to provide a nice fit.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:02:16

Then positioned the rod and tapped it in place. Gave it a nice glob of the Extreme Heat putty and smoothed it off.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:02:58

Applied more putty on the other side. The hose clamp keeps things round as the putty sets up.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:03:39

To provide clearance for back-filling with additional wool packing, dress down the original end cap remnant on the core. File or grind it down until only about 1/8” remains. Just enough for the new end cap to bear against and hold the core in place. Red marks the area in question. I haven’t started filing on this one yet. Once done, it should only stick out from the core about 1/16” to 1/8” on the radius.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:04:24

To expose as much of the packing as possible to the acoustic energy, add perforations to the louvers. Don’t skip this step, it was good for an additional 1 dB reduction in noise and it didn’t affect the flow at all.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:05:01

To keep the wool packing from migrating through the louvers, wrap the core with a layer of stainless-steel screen.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:05:35

This is the secret packing material. It’s ceramic. One package should be good for about two packing jobs.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:06:14

Wrap the core with the wool packing and tie the packing in place with baling wire. One layer is all you can wrap at this point. If you try to wrap more than one-layer, you can’t get the core back in.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:06:55

Cut another section of stainless-steel screen and prepare it by folding over one edge. The fold is necessary to keep the screen from snagging as you insert the core.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:07:46

Wrap the wool packing with the stainless-steel screen. This outer layer of screen protects the packing when you slide the core back into the muffler body.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:08:23

If you don’t encapsulate the first layer of packing with the screen, you end up with this.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:09:12

Tape off the edge of the screen on the exit side of the core. Leave a dog-ear on the end of the tape so you can remove it later. Insert the core into the muffler. The tape allows you to back-fill more wool packing without snagging on the screen.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:10:01

Now continue filling the area between the core and the muffler shell. Just tear off small pieces of packing material and push them through the opening. Be sure to evenly distribute the wool packing around the core so that the core remains centralized in the muffler. You end up with this.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:10:44

Then peel the tape away. Note the four holes that I previously drilled & tapped for the 10-32 screws that hold the new end cap in place. There’s not a lot of thread engagement, but as long as you don’t get carried away with the screwdriver it should be sufficient to hold things together. If you strip it, then sheet metal screws will be in order.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:11:24

Use a mallet to drive the core home, then install the end cap and you’re ready to rock-n-roll. I put a dab of Permatex Optimum Black on each screw to keep them from backing off.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:11:59

It doesn’t look too bad. I like the megaphone style and you can’t even tell it’s been cut open.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:12:32

It looks fine from the back too.

Title: Re: Taming the Mac Muffler
Post by DragBikeMike on 11/28/20 at 11:15:11

How does it work? Not as well as I would like. It definitely is quieter, but not enough for my taste. It reduced the sound level about 3 dB. Here’s the new numbers for idle/3000 rpm/4000 rpm.

70/80/83 dB

The sound quality is much nicer than the chrome Mac, and a bit nicer sound quality than the as-delivered Black Mac. Almost no high-frequency rat-tat-tat, just a low frequency thud-thud-thud. Lot’s of cackle-crackle on deceleration, but no Ka-Booms. At idle, it is acceptably quiet. At small throttle openings, it’s a bit loud but acceptable. You can easily cruise around the neighborhood without making any enemies. But open the throttle and all bets are off. It’s very loud when you start getting on the throttle. You will most certainly attract attention. I could live with it, but prefer not to. I want to be able to wick it up without alerting the local constable.

As far as performance goes, it’s killer-good. It pulls 3rd gear 4K to 7K in 6.72 seconds. In comparison, the Dyna muffler does it in 7.83 seconds and the chrome Mac does it in 7.21. This is a fast muffler. Too bad it’s so loud.

I hope you find this report interesting and useful. It’s not too hard to install packing material in one of these mufflers, and the packing brings the noise level down a notch or two. The same mods will work on the chrome Mac.

As always, I welcome your questions and comments.

Stay positive, test negative,

Mike

Title: Re: Taming the Mac Muffler
Post by Armen on 11/29/20 at 06:05:35

Hey hey!
Thanks again for a well thought out and well documented project!
i have the chrome MAC, so cutting it up will mean dealing with finishing the cut edges. Not a huge deal, but unlikely it'll look as nice as it was.
I remember the Kerker baffles. They had a Competition baffle that you could put your fist through. Then they had one they called the 'DBAE' for Decibles And Emissions. It had a skinny little hole thru it. IIRC, the DBAE worked pretty well at low revs, but the Competition baffle was the ticket for high revs.
All that makes me wonder if it'll work if I reduce the ID a bit on the MAC a bit. I'd still want to do some packing to knock the sharp edges off the sound.
Lots of times a bike comes in with a painful sounding exhaust. Sharp edged sounds. Pulling the baffle, cleaning it off, and covering with fiberglass packing always made the sound more tolerable.
Once I'm settled in and can start serious puttering, dealing with the exhaust is on the list.
Thanks again!

Title: Re: Taming the Mac Muffler
Post by Edub on 12/16/20 at 15:36:36

Very cool step by step breakdown, DragBikeMike! I've really appreciated everything you do to share your knowledge!