muffler mod engineering?

[bikesandcars]

I've searched google and this site... several different searches produce a variety of results regarding muffler mods.

I'm starting this thread as a hopeful home for some actual engineering discussion regarding stock-size muffler modifications.

There is plenty on the web dating back to the "2-stroke era" of the 60's and 70's and through today on tuning 2-strokes and optimizing saws for competition, but unfortunately not much in those articles I've seen applies to a working chain saw as we don't have the room or desire to fit a tuned pipe.

In my reviews of all the posts here, there is a lot of good practical advice.. ie.. "I drilled a hole 1 x my port opening and it worked" or " size the muffler hole about .8x the exhaust port".

What I've never actually seen is an engineer or scientist take the problem (keeping the muffler the same size) and trying to explain what is the optimum modification(s) due to some kind of math or science.

I'm the first guy to support seat of the pants type mods... but if it works better there must be a scientific reason and maybe a way to make it work better yet...

I'm just curious if someone more knowledgable wants to share some engineering info on the details of why a muffler mod makes more power (outside of just improving basic flow) , and is there a particular formula / modification that should be followed.

Interested in some engineering discussion on things like:

how big should the box style muffler be (is there an optimum size)?
What about an inlet tube off the exhaust port? (muffler inlet tube)?
Anyone try adding different length / area stingers? Results?

I've Seen some talk on adding Baffles, stinger sizing, etc.. but no discussion on relationship to size of saw, specifics of mods, etc... all is very generic and somewhat differing.

this topic has been beaten to death but not very scientifically.. or maybe I missed it

 

[rullywowr]

The problem is that as any good engineer would say, "It Depends." It really depends on a lot of factors. The most important (IMO) is that the carb is fully adjustable with L/H settings. When you mod the muffler you need to richen up the H jet or you risk running it lean.

As far as "It Depends": For example a saw could come with a too-small port in the first place so a "2x" size mod hole would be best. Other more pro saws may have a decent size hole so a "1x" would work better. I would start small and work your way up. My 550xp that i modded this weekend i just put in a 3/8" hole and it seems to be perfect. Can't imagine getting much more gains than what that has provided so far.

Just go for it, start small, and don't forget to adjust your carb.

 

[dl5205]

There has been Reams of Volumes, tens of thousands of Words written about Muffler Mods on this site. There are two stroke tuners handbooks out there. There are HillJack, cracker, Redneck and cowboy muffmods. Bring an "Engineer" to the genre, and there may be a heated discourse.

 

[bikesandcars]

Thanks

I guess to talk about it like an engineer we would need to define the problem and the inputs:

Problem: Design an optimum stock-size muffler

Easy Inputs (most folks know this off the top of their head):
Engine size (CC)
RPM: (max no-load and in the cut)

"measured" Inputs (usually not published but easy to check)
Exhaust Port Area
Inlet Port Area

"Hardest" Inputs (usually not published and hard for the average person to provide)
Port Timing (how long the intake and exhaust port are open in deg)
Ignition Timing
Carburetor details and Jetting (make, model, mnfr, size)

If some folks had done some "blueprinting" work on their saws and provided that info with some test results it would benefit this discussion.

 

[bikesandcars]

There has been Reams of Volumes, tens of thousands of Words written about Muffler Mods on this site. There are two stroke tuners handbooks out there. There are HillJack, cracker, Redneck and cowboy muffmods. Bring an "Engineer" to the genre, and there may be a heated discourse.

Awesome, if you could please provide more specifics on a chainsaw tuners handbook that explains stock muffler science

In the meantime, maybe we can pull together some of the lessons learned / rules of thumb.

 

[dl5205]

If you have not spent at least three weeks researching this site on muffler mods, you haven't scratched the surface of the information and disinformation. As far as chainsaw tuner's handbooks, I'm not immediately aware of a proper one, and a quick Google search on your part will provide you with the same books that are commonly available to everyone else. What I'm trying to say is, chainsaw performance is not always either subjective or objective, and to find what Really Works on your particular model, with your parameters, you will either have to experiment for yourself, like a true scientist, or use other's results as a Derivative. I have no empirical hard data to impart to you, maybe someone else does. Enjoy your experiment! Cheers!

 

[tommyd]

This is what has prevented me from hacking my muffler. There doesn't seem to be a whole lot of rhyme or reason other than "I made the port bigger...it works good" I am an engineer, maybe I can help, chances are I'll just get in the way either way the proof is in the math, it always is and I haven't seen any relating to muffler mods. If your looking for an equation there's a good chance I've got the right one in my stack of books here. However I may not be that useful as most of my experience is in the 4 stroke world. I can tune a two stroke but never tried to juice one before.

 

[blsnelling]

The most scientific information you're likely to find was done by Timberwolf. When it all boils down though, it's not worth the math or science involved. Simply open up that muffler and let it breathe. If we were talking about a tuned pipe, it would be entirely different.

 

[3000 FPS]

Stock mufflers are not designed for maximum fuel charge for the cylinder. The only exhaust I know of to do that is to use a pipe that has been tuned to that particular saw so that the harmonic waves create the maximum charge for the cylinder when on compression.

So what that leaves us with is the stock muffler and the only thing you can do with it is to increase flow by modifying it. You can not get the harmonics that you get from a pipe. So I really do not know what scientific information that can be had for a stock muffler other than make it flow better.

There are pro engine builders here that make these engine run faster and stronger by just increasing the flow, ignition timing, and increasing the compression. Other than a pipe I do not know what else there is.

It is all about getting the most fuel mixture in the cylinder in a small amount of time and with a stock muffler you do that by making it flow better.

 

[tommyd]

I tend to agree, the math may not be needed but there still has to be some ideal ratio. Would going past 1:1 port/muffler opening ratio be pointless? If it was truly just open it up and let it breathe then why not rem;very the muffler...just a rhetorical question but I think you see what I'm getting at.

 

[blsnelling]

Find timberwolf's thread. It will answer that very question.

 

[bikesandcars]

I tend to agree, the math may not be needed but there still has to be some ideal ratio. Would going past 1:1 port/muffler opening ratio be pointless? If it was truly just open it up and let it breathe then why not rem;very the muffler...just a rhetorical question but I think you see what I'm getting at.

good stuff.

I searched on "timberwolf muffler mod" and found what appears to be some promising posts. Don't have time now to dig in deep though.

There should at least be some solid rules of thumb...which hopefully have some science behind them, that we can use as a guideline moving forward.

I agree somewhat on the post regarding "this isn't a tuned muffler so what's the point"... I mean you can't make a 3" square box flow like a 24" long motorcross exhaust... but I'd like to elevate this process past "hog it out and hope for the best" approach.

please don't forget that I started the post because I couldn't find the answer in a couple hours of thread searching in the last week, maybe we need to pull it from a hidden spot or summarize it.. or I just need to find that post...

 

[old-cat]

I think the information you are looking for is here - http://www.arboristsite.com/chainsaw/44816.htm Look for the chart that he made.

 

[spindrift7mm]

It's simple !

bikesandcars the limiting factor of work saw mufflers is so simple it's overlooked. Muffler length is limited from ex port to case front ,height is limited from case top to cyl cover/brake handle. All modern saws being of similar design, then factor in weight (oz.s matter)on a hand carried tool. Lastly the spark arrester system which is just about a must for everyone but swamp loggers. These are the simple limits then comes stuff like USFS/Cal Fire tickets for modified mufflers,EPA Regs !!! Hope this gives you some help and don't take it as a neg. Engineer something small, legal,doubles HP,chrome and CHEAP I'll buy it Ken

 

[greg409]

I tend to agree, the math may not be needed but there still has to be some ideal ratio. Would going past 1:1 port/muffler opening ratio be pointless? If it was truly just open it up and let it breathe then why not rem;very the muffler...just a rhetorical question but I think you see what I'm getting at.

Tom, you might think of it this way:

An "ideal ratio" for exhaust tuning is elusive to say the least. As the vapor (exh) velocities change, so does the effective opening size of the vessel, (ports, muff tip, etc.) due to turbulance. (for lack of a better term)

Looking at it from that viewpoint, I believe that the better manufacturers design their exhaust for best all-around performance (i.e. idle, torque, speed, size, cost)

So, if you feel the need to modify, first identify the point that you want a power increase, then do the math to achieve that design.

If you want to get a little more "seat of the pants" performance, attempt to duplicate some things others have done here.

I'm confident most builders here don't run a simulation based on customer's power range requests.

Hence, no unanimous agreement on every detail.

luck,greg

 

[borat]

A two cycle engine isn't much more than an air pump. The easier the pump can work, the less resistance, the more available power. Sound simplistic but, when we're dealing with a little square box that's primarily designed to reduce noise and inhibit sparks, we cannot expect it to contribute much to engine performance other than providing sufficient back pressure for reasonable engine operation.

Considering how fast these engines spin, it's not hard to imagine that a stock muffler designed primarily to muffle sound, would likely provide excessive flow resistance and plenty back pressure. Trying to mathematically assess muffler modifications on such a small piece, would probably require specialized equipment capable of measuring very small changes in flow efficiency and engine performance.

I've read a fair amount on tuning expansion chambers for old two cycle Yamaha engines. The combination of variables of pipe configuration, cone segment/shape/diameter/length and stinger diameter/lengths etc. on a cylinder with a specific port configuration is mind boggling. Throw in some port changes, crank case modification along with intake configurations and the numbers grow exponentially.

The extreme efforts to study, experiment, measure, record and accumulate data for pipe tuning is a result of rewards from competing in and winning races. Motorcycle manufacturers also invested a great deal in the study of expansion chamber design. I can't see there being such a "carrot" to do the same to accumulate data for muffler mods by amateur chainsaw enthusiasts.

Accordingly, home experimenting, bench flow readings, seat of the pants dyno estimates and exchanging information will likely the best we will get. A lot of guys here have done plenty of muffler work, which has contributed to a respectable imperical knowledge base, despite no real scientific data to verify their modification results. Participants interested in a muffler mod, must trust their recommendations. You can rest assured that a bad modification will echo bad reviews.

 

[eiklj]

I'll chime in since I do have a Bachelor's in Mechanical Engineering. I've taken all the Thermodynamics, Fluid Dynamics, Materials Science and Machine Design classes that were required. But all I can add to this discussion is "it depends".

It depends on so many variables that we cannot control from day to day such as air temperature, barometric pressure, etc.

Guys that run race saws attempt to control as many variables as they can such as bar length, chain sharpness, tuned pipe length, etc. so the outcome is more predictable.

For the engineers that actually design chainsaws (or any 2-cycle engine) they go for the 80% rule. Build it good enough to satisfy 80% of the people.

I guess what I'm saying is there is really no magic formula for muffler size, volume, outlet size, etc. that I know of.

Auto manufacturers may use fancy CFD (Computational Flow Dynamics) software to predict what exhaust gasses do after they leave the engine, but I doubt many chainsaw manufacturers do.

Maybe they do, I'd like to learn their secrets.

Anyway, I don't suppose I added anything to the discussion other than my opinion.

Hopefully you find the answers you are looking for.

Seriously Borat, your avatar is giving me nightmares.

 

[borat]

"Seriously Borat, your avatar is giving me nightmares."

Hey man...... That's my girl!!!

A lesser man would be offended.

 

[parrisw]

Here is my advice. Don't over think it. Just open it up and let it breath.

 

[timmcat]

Good thread so far, My 2c is that without actually building a pipe from scratch using any of the formulae from any of the 2 stroke theory books or articles we are all simply left with the open it up a little and test technique. Having built a few pipes for sleds and other industrial 2 strokes that I wanted a little more out of I can say that the basic box shape is a huge limiting factor. If you look at most tuner pipes you'll no doubt see that the outlet stingers are always smaller that the exhaust port cross section. However due to the limitations of the box muffler that most saws are stuck with making the cross section of the outlet as large or larger than the port seems to work. A popular mod seems to be dual outlets, I've done this to all of my 60cc + Stihls and it works good. I too would like to see a clear cut formula but Hawking works in space not power equipment.