michael whiteman
Well-Known Member
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- Mar 2, 2021
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Let’s Make a High Performance Muffler, Without Any Welding
Preface: This is the larger hot dog shaped muffler with ¾ pipe threads, not ½ inch.
This is used with the exhaust pipe for a 79cc 4 stroke Predator engine.
1. Start with one, or two, mufflers from Amazon. (pic.1)
2. Drill out the 3 spot welds which will allow you to separate the inner tube. (pic.2)
3. Cut around the center on the side nearest the outlet. Dig out that remaining piece under the lap joint in the bell. (pic.3)
4. To remove the center baffle, first remove the 2 spot welds through the center tube. When drilling at an angle start with a small diameter bit and increase the size until the baffle is free. I used a socket on an extension. Do not hammer this out with the outer housing being supported or you will definitely distort it. Instead, put the threaded pipe against your vise or bench. (pic.4)
5. Repeat this procedure if you are using a second muffler. You could also duplicate this tube with all the holes by using a piece of conduit or similar tubing. You’ll need to drill it full of holes on the area inside the muffler. Install this second tube over the first and secure it with two #6x3/8” self tapping screws. (pic.5)
6. Cut a piece of 2 ¼ inch exhaust pipe to the desired length. You will need a file or a grinder to remove some material from one end so it will slide easily into the bell on the original muffler.
7. Buy a Dorman freeze plug (part no. 555-085) at NAPA Auto Parts. This will become the removable end cap.
8. Chuck a cylindrical grinding stone in your hand drill and remove about half the thickness from the inside of the pipe on the outlet end. The freeze plug must not fit too tightly because you’ll want to gently tap it out from the inside. It’s OK to file some on the outer surface of the freeze plug if necessary. (pic.6)
9. Insert the plug flush with the end of the pipe and drill hole 1/8” from the edge in which to insert a #6x3/8” self tapping screw. This will prevent the freeze plug from falling out.
10. Lastly drill a hole in the center of the plug for the center tube to protrude through. Don’t make it a sloppy fit or the packing might blow out. (pic.7)
I highly recommend creating a muffler support bracket. The extra weight of this muffler acting on the studs in the head is not a good thing. This bracket can also hold the two pieces of your new muffler together like mine. A different design, or lack of, will require you to put a couple of self tapping screws through the exhaust pipe into the bell of the original muffler to hold it together. (pic.8-10)
Push the exhaust pipe into the bell of the original muffler and pack the cavity full of pink fiberglass insulation. Your local company that insulates houses will give you some scraps for free. You might want to try what I have done and use coarse steel wool. I think it might last longer. Now tap in the end plug, then insert the screw and you’re finished. It should sound better, and with the baffle removed eliminating any restriction there should be a feeling of increased power.
I have now riden my bike with this new muffler and can report the sound level with this straight through design is the same as the original "hot dog" muffler with the internal baffle. My big oversight on a performance evaluation is because I made two improvements at the same time. Porting and shaving the head along with this straight through, no backpressure, muffler made a tremendous difference. I can see where they claim over 7HP is obtainable with these 79cc engines. I feel these are the two most beneficial mods you can do for increased power.
Preface: This is the larger hot dog shaped muffler with ¾ pipe threads, not ½ inch.
This is used with the exhaust pipe for a 79cc 4 stroke Predator engine.
1. Start with one, or two, mufflers from Amazon. (pic.1)
2. Drill out the 3 spot welds which will allow you to separate the inner tube. (pic.2)
3. Cut around the center on the side nearest the outlet. Dig out that remaining piece under the lap joint in the bell. (pic.3)
4. To remove the center baffle, first remove the 2 spot welds through the center tube. When drilling at an angle start with a small diameter bit and increase the size until the baffle is free. I used a socket on an extension. Do not hammer this out with the outer housing being supported or you will definitely distort it. Instead, put the threaded pipe against your vise or bench. (pic.4)
5. Repeat this procedure if you are using a second muffler. You could also duplicate this tube with all the holes by using a piece of conduit or similar tubing. You’ll need to drill it full of holes on the area inside the muffler. Install this second tube over the first and secure it with two #6x3/8” self tapping screws. (pic.5)
6. Cut a piece of 2 ¼ inch exhaust pipe to the desired length. You will need a file or a grinder to remove some material from one end so it will slide easily into the bell on the original muffler.
7. Buy a Dorman freeze plug (part no. 555-085) at NAPA Auto Parts. This will become the removable end cap.
8. Chuck a cylindrical grinding stone in your hand drill and remove about half the thickness from the inside of the pipe on the outlet end. The freeze plug must not fit too tightly because you’ll want to gently tap it out from the inside. It’s OK to file some on the outer surface of the freeze plug if necessary. (pic.6)
9. Insert the plug flush with the end of the pipe and drill hole 1/8” from the edge in which to insert a #6x3/8” self tapping screw. This will prevent the freeze plug from falling out.
10. Lastly drill a hole in the center of the plug for the center tube to protrude through. Don’t make it a sloppy fit or the packing might blow out. (pic.7)
I highly recommend creating a muffler support bracket. The extra weight of this muffler acting on the studs in the head is not a good thing. This bracket can also hold the two pieces of your new muffler together like mine. A different design, or lack of, will require you to put a couple of self tapping screws through the exhaust pipe into the bell of the original muffler to hold it together. (pic.8-10)
Push the exhaust pipe into the bell of the original muffler and pack the cavity full of pink fiberglass insulation. Your local company that insulates houses will give you some scraps for free. You might want to try what I have done and use coarse steel wool. I think it might last longer. Now tap in the end plug, then insert the screw and you’re finished. It should sound better, and with the baffle removed eliminating any restriction there should be a feeling of increased power.
I have now riden my bike with this new muffler and can report the sound level with this straight through design is the same as the original "hot dog" muffler with the internal baffle. My big oversight on a performance evaluation is because I made two improvements at the same time. Porting and shaving the head along with this straight through, no backpressure, muffler made a tremendous difference. I can see where they claim over 7HP is obtainable with these 79cc engines. I feel these are the two most beneficial mods you can do for increased power.
