Husqvarna 223L Trimmer Muffler Mod - Flowbench Data

[Team FAST]

3 hrs ·
My next door neighbor brought his trimmer over because it wasn't working properly.

His trimmer is a Husqvarna 223L which is a 25cc engine using a clamshell design bottom end. It has a Zama C1Q carb, a Cat muffler, and a foam air filter contained in a bolt on plastic housing/manifold attached to the inlet side of the carb.

I decided that rather than simply "repair" his trimmer, I would mod the muffler and hopefully return it in a much better state of tune that when he first purchased it!

I thought I would include some pictures and the flowbench data for all to see.


Pics to follow.

 

[Team FAST]

Here's the nasty beast as it was dropped off at my shop. Typical accumulation of gack.

this was bought new in 2008 from Lowe's and has run reliably until last weekend. Pretty amazing it did this well for that long given today's ethanol laced fuels....

I replaced all the primer lines with yellow tygon lines.

 

[Team FAST]

Typical Zama C1Q carb rebuild. Remove limiter caps, ultrasonically clean all the hardware, replace all the diaphragms and needle. Set pop off lever height.

 

[Team FAST]

Here's the muffler. It's a typical Catalytic converter with a peashooter exhaust. You can open it up by gently and patiently prying open the folded seam using flat screwdrivers. I have already removed the spark arresting screen in this photo.

 

[Team FAST]

Here is what the muffler looks like opened up. You can see the catalytic material shaped like a donut or a "biscuit" in the center. The pic shows the engine side of the cat baffle plate. The cat is fed via the 13 0.195" diameter baffle holes. The cat material resembles a brillo pad in its texture. No doubt very restrictive to exhaust gas flow!

The cat and baffles is on a thin plate which is sandwiched between the two muffler halfs. This plate is also important in that it locates the two sleeves which support the screws that attach the muffler to the cylinder. These two sleeves prevents the two halves from collapsing when you tighten down the bolts to the cylinder.

 

[Team FAST]

Here is the entire muffler assembly (note the backside of the CAT/Baffle plate is shown here) after I cleaned everything up using a Harbor Freight Soda blaster (love that tool!) Also shown is the spark arrester screen and the two internal support sleeves. The flange also resides inside to reinforce the exhaust port opening.

 

[Team FAST]

It's obvious to me that the simplest thing to do to improve exhaust gas flow through the muffler is to remove the cat. Using a cutoff wheel grinder, I carefully cut through the outer cage of the cat. After freeing and removing the cat material, I used a "dotco" style air disc sander to smooth the sharp edges. This leaves the 13 holes which are at .195" diameter each.

 

[Team FAST]

Prior to splitting the muffler open and removing the cat, I obtained an airflow baseline using my Superflow SF-110 flowbench. I machined an adapter plate that exactly matches the exhaust port dimensions.

 

[Team FAST]

Here's the Flow data summary. Stock, and then removing screen and deflector. Surprisingly, no change! This tells me that the cat material is the limiting factor.

After removing the cat material as shown above, you can see the flow benefit. The outlet hole is also extremely small, so I enlarged it to .375" The result of opening up the outlet and removing the cat baffle entirely DOUBLES the flow through the muffler. I flow everything at 10" of H2O on my bench.

By the way...the exhaust port flows 32 CFM at 10" H2O wide open. This would be the equivalent of running the engine without any muffler attached.

 

[Team FAST]

Here's the breakdown of the geometry of this muffler. All sizes shown are stock.

Please note that for the final configuration, I opened up the outlet hole to .375"

 

[Team FAST]

Finally, Here's the muffler reassembled. I opted to re-install the cat baffle plate (without the cat material) to provide a guide for the support sleeves. I think the gain in flow is plenty for this trimmer, given the very restrictive intake tract. The intake uses a very small sponge air filter, and the flow takes a tortuous path from the airbox down to the carb via a plastic manifold. This intake system is very restrictive, and therefore, opening up the muffler too much would be of little benefit and may even result in a loss of power or fuel efficiency.


To summarize my muffler mod:

After opening up the muffler assembly, I removed the catalyst material, and I opened up the outlet hole from .275" to .375" using a drill bit. I reassembled the muffler re-installing the cat baffle plate. I removed the spark arrestor screen and left the stock outlet baffle in place.

My oldest son, aged 9 helped me reassemble the trimmer to test it. I showed him how I tune the carb, and what happens when you adjust mixture screws, and how the engine sound changes (opened the "H" screw at full throttle to make it four stroke) as you adjust the mixture. What a great father-son bonding project!

It runs really strong now!

What a difference! Way more power and throttle response than stock.

At Team FAST, we don't "repair" two-strokes, we hotrod them!


Hope everyone finds this useful!

 

[GrassGuerilla]

Nice work! Very clean cat extraction.

 

[Tothemax]

Very cool. I like the data.

 

[computeruser]

Nice. I did a similar muffler job on a 326-series hedge trimmer. The difference was tremendous.

 

[ANewSawyer]

I did a very aggressive muffler mod to my 525LS and I think I over did it. It might be a little more powerful with a smaller outlet but is turning 9500 rpm happily. Four strokes not much below 9000.

 

[Chris-PA]

Very interesting. I've been looking at some cat mufflers lately, and your data confirmed some of what I was thinking. I notice that the biggest benefit by far was enlarging the outlet, which improved the flow from 3 to 5.5, and removing the cat only took it to 6. Given the size of the engine and other restrictions, I'm not sure how much flow it needs - I kinda guess the cat didn't matter much.

I also note that almost all the cat mufflers I've seen also have very restrictive outlets, and it seems to me that must be part of how they function for some reason as clearly you can make them flow better and still have a cat.

I'm considering playing with an opened up cat muffler for one of my ported saws, but leaving the cat material in. I noticed in one of Brad's threads that a Dolamr cat muffler already contained a bypass around the cat. Anyway, my objection to cat mufflers is the heat, which is especially an issue given the problem I have with vapor lock in the summer, but I like the idea of reducing the crap I breathe.

 

[Team FAST]

Very interesting. I've been looking at some cat mufflers lately, and your data confirmed some of what I was thinking. I notice that the biggest benefit by far was enlarging the outlet, which improved the flow from 3 to 5.5, and removing the cat only took it to 6. Given the size of the engine and other restrictions, I'm not sure how much flow it needs - I kinda guess the cat didn't matter much.

I also note that almost all the cat mufflers I've seen also have very restrictive outlets, and it seems to me that must be part of how they function for some reason as clearly you can make them flow better and still have a cat.

I'm considering playing with an opened up cat muffler for one of my ported saws, but leaving the cat material in. I noticed in one of Brad's threads that a Dolamr cat muffler already contained a bypass around the cat. Anyway, my objection to cat mufflers is the heat, which is especially an issue given the problem I have with vapor lock in the summer, but I like the idea of reducing the crap I breathe.

No the largest improvement was removing the cat material, leaving the baffle plate with the holes. The cat is very restrictive.

 

[Chris-PA]

No the largest improvement was removing the cat material, leaving the baffle plate with the holes. The cat is very restrictive.

Oh, I misunderstood the notation then - I was assuming the "Cat Baffle" still had the cat in, and "Baffle Removed" took it out, but I see now the cat was out of both.

 

[lightj12]

I did the same thing to my 335rx trimmer and it made a big difference; runs great now. Was running way too hot with a cat muffler.

 

[BGE541]

Awesome, Flowbench needed... maybe I'll try and build one.