Quiet Generator Muffler

For the very first Nerdy Thing I Do (NTID), I'll introduce you to the quiet(er) muffler I fabbed up for my portable generator.



I purchased the generator from my local Home Depot for use with my 5th wheel trailer. The soon-to-be-wife and I go camping a fair amount around Moab, UT, and it gets hot hot hot there in the spring and early fall. So having the A/C available while in the middle of the desert is a nice thing. To be honest, the generator was an impulse buy. I was looking for something else at HD (no doubt an odd sized drill bit that I just HAD to have for a project) and came across it at the end of the isle. It was marked down considerably since it was returned and "reconditioned." I don't remember what the price was, something around $300, but it was darn cheap for a 5kw generator (burstable to 6250w)! I knew the silly thing would most likely be louder than I really wished, but I figured, "WTH, might as well." (I would really really really like one of those fancy Honda super-quiet EU jobbies. That would rock. But I just can't swing the $2000 bucks for the appropriate sized unit.) So I came home with the Coleman/Briggs & Stratton option.

One trip to the desert and I wondered if I had made a mistake! The noise inside the trailer wasn't horrible...the roar of the A/C did a nice job of filtering out the racket of the generator...but the noise outside was considerable. I felt sorry for my buddies that were camping with me. (Although they all had there own generators running as well.)

So began my quest to try make this generator quieter. Lots of Google searches later and I came up with very little. I did find one option that I was tempted by. SuperTrapp makes a version of their tunable muffler for small engines. And people claimed that it worked really well! (I have a SuperTrapp on my Honda XR650R and really like it. It's loud...but only because I have it tuned that way. They can be quiet.) I found the info here. The info was encouraging, but the price was not. Over $100 for a muffler on a cheap-o generator just didn't make sense. I scrapped that idea.

I can't remember where I came up with this idea. It may have been while I was searching CraigsList for tools (which I do often). Someone was selling a generator and mentioned that they had become tired of the noise and stuck an old motorcycle muffler on it. Ka-Bing!! That should work, I figured.

So then my brain kicked in and I started pondering what motorcycle muffler would work. I finally settled on a sportbike muffler. My reasons were 1) from the factory, most crotch rockets are pretty darn quiet, 2) they are certainly rated at a high enough CFM so constricting the exhaust won't be an issue (calculate 600cc X a Gazillion rpm), and 3) any self-respected sportbike rider takes their cycle straight to the aftermarket shop from the dealer and has that shop install a free-flowing pipe for the added 5.9hp...which means there's got to be loads of this aluminum babies to unload! The market is literally flooded with stock take-offs from crotch rockets! Search ebay and see for yourself. Being an upstanding member of the "Ride Red" club, I, of course, could only consider Honda products. After a 2 minute search on ebay I came up with a take-off from a 90-something CBR 600 F3. For a cool $13.50! Of course no-one else wanted it, so in a week's time I had it in my Nerdy little paws.

Here it is, straight out of the box.



Here's a pic looking down of the stock muffler on my generator (with top fuel tank removed). Sort of boxy affair at best.


After some chin-scratching consideration I decided to mount the muffler in a vertical way. Make sort of a big-rig stack look for it. I originally thought I could fab up some sort of adapter that would attach directly to the outflow of the OEM muffler. Bad idea. 1) there's was no way to solidly attach a fairly hefty aluminum unit, and 2) I couldn't figure a good way to make a tight seal. I didn't want any hot exhaust gases (and sound) spraying out radially around the connection. So I decided on a whole new exhaust manifold/pipe arrangement.



I fabbed up a new flange to bolt directly to the engine head out of some 1/4" scrap I had laying around. I did buy a new gasket from the local small engine shop.

Since the space was somewhat limited I decided to make the exhaust piping out of 1" threaded black iron pipe. Cheap stuff and easy to come by. I did have to make one custom cut/weld near the flange to achieve a custom angle. The rest I fabbed from standard fittings.



I decided to use a standard pipe flange (for mounting pipes to flat surfaces) as the interface between the pipe and muffler. The diameters matched up pretty well and there was a good flat surface on each to make a good seal with. I tried to find some high-temp exhaust gasket material but failed. I ended up making the gasket/seal with some plain old fiberglass muffler packing. I just created a loose donut of material then clamp it flat and tight between the muffler and the flange. No leaks.





After one trip to the desert I'm quite pleased with this setup! It's MUCH quieter and I like the fact that the exhaust exits up (I run the generator in the bed of my pickup) instead of to the side. It effectively lowers the operating temperature of the unit (since the the old muffler cavity is just air now) and the majority of the sound is projected upwards instead of out. In doing a little research for this project I found that most people did say "You'll be surprised how much actual engine noise there is once the exhaust is toned down." It's true! These little engines aren't designed to be quiet. I don't know what's actually producing the noise, but there's a considerable amount. I guess it's the combo of bearings, valves, crankshaft, rod, lifters, and piston all added together that makes the noise. At any rate...I would highly recommend anyone make this mod to there small generator to help quiet it a bit. For around $50 it's well worth it!