Muffler Mod Theory

[tdi-rick]

TW's zero back pressure muffler :greenchainsaw: (knew I had a pic somewhere)

 

[husq2100]

Jesus! dont get your fingers in there :jawdrop:

i would be interested to hear how it performed and if the saw was modded with this set up in mind(as in a whole package)

Serg

 

[tdi-rick]

Pretty sure there's youtube of it running.
I'll see if i can find it.
I remember the boys commenting it sounded like it was a proper pipe.

BTW, JC on AULRO had a 300Tdi come in with a porous/split bore last week.
I mentioned you'd seen something similar when you did you head a few years back.

 

[scotclayshooter]

I know your car probably isn't a 2 stroke, but pull the exhaust off at the cylinder head. See if it's got more, or less power. More than half the battle when building an exhaust on anything, is scavenging exhaust pulses to extract burnt fuel on the exhaust cyle.

Same on a car if you go too wide on the bore of the exhaust pipe, The gas goes slower and you lose the syphon effect at the pistons.
Yep that gas going up the pipe at 120 mph (guess) dont just stop.

But i remember a thread on using a straight pipe on a saw and i think it needed to be 2' in lenth to have any effect.

From what ive read full on motorbike type exhausts on a saw are a short term thing and are prone to cracking even just being used at GTGs and races.
Never mind as a work saw.

I will agree that on my FS250 strimmer if i temporarily fit a longer pipe it makes it quieter to the user.

 

[tdi-rick]

TW's stock appearing 359.
Removable head, bigger carby, ported.

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[edisto]

Don't know about that. Pretty sure if you wanted to get technical, you'd run an expansion chamber with a timed exhaust for the most power. I punched a 5/8 hole in my 385 husky muffler, then welded about a 2" long pipe to it. It was quieter, but and I'm fairly confident it made a bit more torque with the pipe than vs a straight hole

.

The pipe would create more backpressure for the same sized opening. If scavenging is efficient, some charge gets pulled through with the exhaust, so (short of having a tuned pipe) the right amount of back pressure will stuff some of the charge back in the cylinder.

I know your car probably isn't a 2 stroke, but pull the exhaust off at the cylinder head. See if it's got more, or less power. More than half the battle when building an exhaust on anything, is scavenging exhaust pulses to extract burnt fuel on the exhaust cyle.

You're comparing apples to scotch tape. Scavenging is important in 4-strokes, and usually is accomplished by having the right lengths in the header, so that a pulse from another cylinder helps scavenging BUT (and this is a great big J'Lo but) in a 4-stroke, the intake is closed when this happens.

So, in a 4-stroke, ALL of the battle is scavenging, and in a 2-stroke half the battle is scavenging, and the other half of the battle is not tossing the baby (charge) out with the bathwater (exhaust).

 

[edisto]

TW's zero back pressure muffler :greenchainsaw: (knew I had a pic somewhere)

Nice...I'll bet she's loud! Straight pipes will aid scavenging, but the problem is that they aid too much, and you toss some fuel out the exhaust. My understanding is that you just start long, and cut back until you reach an optimum length for your running rpm. It looks like this length was constrained to fit the muffler, i.e., to keep the stock dimensions.

 

[Evan]

TW did a quick test.
that would have taken much longer and been alot more work welding in short pipe everytime he opend the cover up on the 026.

not worth it for what he was doing

 

[tdi-rick]

<snip>
Straight pipes will aid scavenging, but the problem is that they aid too much, and you toss some fuel out the exhaust. My understanding is that you just start long, and cut back until you reach an optimum length for your running rpm.
<snip>

Which is all they did on GP bikes back in the day until Walter Kaaden worked out how to stuff the charge back in with a reverse cone.

 

[hh1341]

My mod

I put 4" of pipe in the muffler and then added a short megaphone.......made a big difference.

 

[edisto]

TW did a quick test.
that would have taken much longer and been alot more work welding in short pipe everytime he opend the cover up on the 026.

not worth it for what he was doing

That's exactly the problem I'm going to run into, so I'm going to go with my best guess, and try to optimize for what I have. No way I'm going to make up 5-10 mufflers to compare...I probably will compare what I wind up with to a simple, opened up stock muffler though.

 

[hh1341]

That's exactly the problem I'm going to run into, so I'm going to go with my best guess, and try to optimize for what I have. No way I'm going to make up 5-10 mufflers to compare...I probably will compare what I wind up with to a simple, opened up stock muffler though.

You could fit a large diameter pipe and then use inserts to neck it down.

Also you can slide the inserted pipe in and out, in an attempt to find a sweet spot.

Here is what I did, if that will help.
http://www.arboristsite.com/showthread.php?t=90616

 

[edisto]

It's what's going inside the muffler that would be too much of a pain to modify.

I had planned on just opening up the muffler to find the sweet spot once i had it together, but I like the insert idea...my only fear is that I might continue to get improvement as i cut back, and I'd have to start over!

 

[scotclayshooter]

.


You're comparing apples to scotch tape. Scavenging is important in 4-strokes, and usually is accomplished by having the right lengths in the header, so that a pulse from another cylinder helps scavenging BUT (and this is a great big J'Lo but) in a 4-stroke, the intake is closed when this happens.

Theres normally an overlap where the intake and exhaust are both open at the same time this allows the exhaust to syphon through some fresh charge to clear the cylinder, So more oxegen in as theres less burnt crap that wont burn.
Fuel injection makes this even more efficient.

Ford made a 2 stroke turbo direct fuel injected prototype a few years ago and they used the turbo with valves to completly clear the cylinder for the next charge. Never heard any more about it but they were planning it for the small cars.

 

[edisto]

Theres normally an overlap where the intake and exhaust are both open at the same time this allows the exhaust to syphon through some fresh charge to clear the cylinder, So more oxegen in as theres less burnt crap that wont burn.
Fuel injection makes this even more efficient.

Smokey Yunick, who was responsible for more rules being made up on the spot than any other race mechanic, once got busted for drilling small venturis in the headers for the same effect (more complete burning of the exhaust).

My favorite story is where he almost got away with running a 7/8 scale car!

 

[hh1341]

He He

He got greedy......he should have just tried 15/16's

Must have been allot of work to build.

 

[hh1341]

Dyno

Any one seen plans for a DIY dyno, so a guy could test his mods?

 

[Metals406]

In reference to port shape on the muffler... TW's flow bench tests have shown that a elongated rectangular shape flows better than a round (pipe) shape.

That's the way I made the one on my 670, and it sounds really good, with good improvement.

The saw wanted to run at 14,700 before it two stroked... To avoid a lean condition (I'm a puss), I tuned it to 13,500. Makes me wonder what it would do ported?

 

[edisto]

Any one seen plans for a DIY dyno, so a guy could test his mods?

 

[ShoerFast]

There is not a better one out there!