Just another junk chainsaw

[LD1]

Two-stroke exhaust is part science and part art, as far as I can tell. A properly set up system not only scavenges the burnt charge from the combustion chamber, it directs a "reverse" pressure pulse to keep the incoming charge in place.

I have read that a 2cycle has a "secondary" compression ratio, that of the fuel air mixture in the crankcase. When backpressure is too low the mixture can exit the cylinder w/o burning called short circuiting or "4 cycling". An expansion chamber takes advantage of sound waves which travel through the exhaust pipe faster than the spent gases and both pull the gases out and when they return back toward the exhaust port help hold the next fresh charge in the cylinder.

All of that theory is correct.

Look at the science and design of a dirt-bike muffler.

The quickly exiting exhaust gasses create a vacuum. The piston being driven downward also creates a vacuum. Thus the fuel-air charge is drawn into the intake.

With the tuning and science of a dirtbike muffler.....they can actually draw this fuel/air mix into the exhaust to some extent. The pressure wave due to the design of a dirtbike muffler actually bounces off the cone and forces this air/fuel BACK into the combustion chamber right before the exhaust port closes. Some actually refer to this as kinda a "supercharger" effect.

Which is alot why dirtbikes have a "power-band". A range of certain RPM's where this effect is the greatest. And the engine will make significantly more power. And by simply changing mufflers can even change the power band higher or lower in the RPM range.

But none of this is to say that a 2-stroke "needs backpressure". An engine.....be it 2-stroke or 4-stroke is nothing more than an air pump. The more air that can be moved through the engine....the more power it can make. Hence Turbos or superchargers to FORCE more air in. OR the science of headers/H-pipes/x-pipes to help air exit faster.

A 2-stroke is no different in that aspect. The design of a dirt bike muffler is all about getting MORE air/fuel into the engine.

However, None of this applys to small 2-stroke engines like a chainsaw or weedeater. Because the muffler is NOT designed to aid in scavenging or airflow. Its simply there to reduce the noise and a place to bolt a spark arrester. And the easiest way to reduce the noise is just a big expansion chamber (a muffler on a chainsaw is roughly 2x-3x the volume of the cylinder), coupled by restricting the exiting exhaust gasses. By restricting/slowing down the exiting exhaust gasses makes it quieter....But it can still effeciently release all the exhaust gasses. Because this dont need to happen instantaneously. The volume of combustion gasses can be released slowly over the course of the next intake and compression cycle of the engine.

This back-pressure does indeed reduce noise....but at a cost of performance. The intake air/fuel mix doesnt enter the cylinder as freely......as it is also acting to push against the pillow of exiting combustion gasses.

This is why chainsaws respond so well to a simple muffler mod to make it breath easier.

Compare a saw to a hotrod. A Muffler mod is like doing long-tube headers. And piping it like a dirtbike (like some hotrod saws) would be like putting a small supercharger on.

 

[Snobdds]

All of that theory is correct.

Look at the science and design of a dirt-bike muffler.

The quickly exiting exhaust gasses create a vacuum. The piston being driven downward also creates a vacuum. Thus the fuel-air charge is drawn into the intake.

With the tuning and science of a dirtbike muffler.....they can actually draw this fuel/air mix into the exhaust to some extent. The pressure wave due to the design of a dirtbike muffler actually bounces off the cone and forces this air/fuel BACK into the combustion chamber right before the exhaust port closes. Some actually refer to this as kinda a "supercharger" effect.

Which is alot why dirtbikes have a "power-band". A range of certain RPM's where this effect is the greatest. And the engine will make significantly more power. And by simply changing mufflers can even change the power band higher or lower in the RPM range.

But none of this is to say that a 2-stroke "needs backpressure". An engine.....be it 2-stroke or 4-stroke is nothing more than an air pump. The more air that can be moved through the engine....the more power it can make. Hence Turbos or superchargers to FORCE more air in. OR the science of headers/H-pipes/x-pipes to help air exit faster.

A 2-stroke is no different in that aspect. The design of a dirt bike muffler is all about getting MORE air/fuel into the engine.

However, None of this applys to small 2-stroke engines like a chainsaw or weedeater. Because the muffler is NOT designed to aid in scavenging or airflow. Its simply there to reduce the noise and a place to bolt a spark arrester. And the easiest way to reduce the noise is just a big expansion chamber (a muffler on a chainsaw is roughly 2x-3x the volume of the cylinder), coupled by restricting the exiting exhaust gasses. By restricting/slowing down the exiting exhaust gasses makes it quieter....But it can still effeciently release all the exhaust gasses. Because this dont need to happen instantaneously. The volume of combustion gasses can be released slowly over the course of the next intake and compression cycle of the engine.

This back-pressure does indeed reduce noise....but at a cost of performance. The intake air/fuel mix doesnt enter the cylinder as freely......as it is also acting to push against the pillow of exiting combustion gasses.

This is why chainsaws respond so well to a simple muffler mod to make it breath easier.

Compare a saw to a hotrod. A Muffler mod is like doing long-tube headers. And piping it like a dirtbike (like some hotrod saws) would be like putting a small supercharger on.

You run a straight pipe and I will run a muffler modded chainsaw and guess who will win?

 

[LD1]

You run a straight pipe and I will run a muffler modded chainsaw and guess who will win?

Are you speaking as if I dont know?

The purpose of a straight pipe isnt backpressure if thats what you are insinuating. Its scavenging. Again.....its an air pump....whatever you can do to pull MORE air in and EXAUST air more efficiently the better.

There is a reason people dont run a straight pipe on a firewood saw and choose a simple muffler mod instead.

 

[Snobdds]

Are you speaking as if I dont know?

The purpose of a straight pipe isnt backpressure if thats what you are insinuating. Its scavenging. Again.....its an air pump....whatever you can do to pull MORE air in and EXAUST air more efficiently the better.

There is a reason people dont run a straight pipe on a firewood saw and choose a simple muffler mod instead.

I don't think the goal is just to move air. The goal is to have the most amount of fuel/air at the precise moment of detonation at the top of the compression/power stoke to throw that piston down with as much force as possible to create power.

Having backpressure optimizes that precise moment of detonation.

 

[cqaigy2]

I disagree

I don't. From riding dirt bikes and messing with exhaust. Pull the exhaust pipe off and it runs like crap, nice a loud, and the fire coming out is pretty cool looking but not near the power.

 

[cqaigy2]

All of that theory is correct.

Look at the science and design of a dirt-bike muffler.

The quickly exiting exhaust gasses create a vacuum. The piston being driven downward also creates a vacuum. Thus the fuel-air charge is drawn into the intake.

With the tuning and science of a dirtbike muffler.....they can actually draw this fuel/air mix into the exhaust to some extent. The pressure wave due to the design of a dirtbike muffler actually bounces off the cone and forces this air/fuel BACK into the combustion chamber right before the exhaust port closes. Some actually refer to this as kinda a "supercharger" effect.

Which is alot why dirtbikes have a "power-band". A range of certain RPM's where this effect is the greatest. And the engine will make significantly more power. And by simply changing mufflers can even change the power band higher or lower in the RPM range.

But none of this is to say that a 2-stroke "needs backpressure". An engine.....be it 2-stroke or 4-stroke is nothing more than an air pump. The more air that can be moved through the engine....the more power it can make. Hence Turbos or superchargers to FORCE more air in. OR the science of headers/H-pipes/x-pipes to help air exit faster.

A 2-stroke is no different in that aspect. The design of a dirt bike muffler is all about getting MORE air/fuel into the engine.

However, None of this applys to small 2-stroke engines like a chainsaw or weedeater. Because the muffler is NOT designed to aid in scavenging or airflow. Its simply there to reduce the noise and a place to bolt a spark arrester. And the easiest way to reduce the noise is just a big expansion chamber (a muffler on a chainsaw is roughly 2x-3x the volume of the cylinder), coupled by restricting the exiting exhaust gasses. By restricting/slowing down the exiting exhaust gasses makes it quieter....But it can still effeciently release all the exhaust gasses. Because this dont need to happen instantaneously. The volume of combustion gasses can be released slowly over the course of the next intake and compression cycle of the engine.

This back-pressure does indeed reduce noise....but at a cost of performance. The intake air/fuel mix doesnt enter the cylinder as freely......as it is also acting to push against the pillow of exiting combustion gasses.

This is why chainsaws respond so well to a simple muffler mod to make it breath easier.

Compare a saw to a hotrod. A Muffler mod is like doing long-tube headers. And piping it like a dirtbike (like some hotrod saws) would be like putting a small supercharger on.

Hmm. I have to say, this sounds interesting and does make sense.

 

[LD1]

I don't. From riding dirt bikes and messing with exhaust. Pull the exhaust pipe off and it runs like crap, nice a loud, and the fire coming out is pretty cool looking but not near the power.

A dirtbike muffler and a chainsaw muffler are worlds apart. Apples to oranges comparison.

Competiton saws do use tuned mufflers for quite a power increase. That isnt the goal of a chainsaw muffler.

This all started because of the notion that "muffler modding" a chainsaw is bad because it needs "backpressure". Which is simply an incorrect statement.

 

[Snobdds]

A dirtbike muffler and a chainsaw muffler are worlds apart. Apples to oranges comparison.

Competiton saws do use tuned mufflers for quite a power increase. That isnt the goal of a chainsaw muffler.

This all started because of the notion that "muffler modding" a chainsaw is bad because it needs "backpressure". Which is simply an incorrect statement.

They're not worlds apart though. The physics is exactly the same. Not a thing different.

The only difference is dirt bikes have room for a tuned exhaust where chainsaws don't. They both need backpressure to run right. A tuned muffler just synchronizes the pulse wave back a little better where a chainsaw is compact and a little more crude.

 

[cqaigy2]

They're not worlds apart though. The physics is exactly the same. Not a thing different.

The only difference is dirt bikes have room for a tuned exhaust where chainsaws don't. They both need backpressure to run right. A tuned muffler just synchronizes the pulse wave back a little better where a chainsaw is compact and a little more crude.

I'm kinda wondering if it's not specifically back pressure, but kind of like you mention in you last sentence, creating a pulse in the exhaust and intake so that the intake charge piles up against the back pressure, just before the port closes and goes into compression, and the opposite when the exhaust port open, it's being sucked out. I'm sure there is a vocabulary to describe this.

 

[LD1]

They're not worlds apart though. The physics is exactly the same. Not a thing different.

The only difference is dirt bikes have room for a tuned exhaust where chainsaws don't. They both need backpressure to run right. A tuned muffler just synchronizes the pulse wave back a little better where a chainsaw is compact and a little more crude.

We can agree to disagree. Cause you aint gonna change my mind or opinion, and I obviously aint gonna change yours