why a muffler mod lowers internal engine temps

[drmiller100]

The sparky plug goes bang.
cylinder pressures go way up, combustion gas temps go way up, pushing the piston down.

eventually the exhaust port opens, and some of the exhaust gasses go running out into the muffler.

How much of the exhaust gas goes out? Well it depends on the pressure inside of the muffler. The lower the pressure, the more exhaust gasses run out.
If we open the muffler outlet, the muffler pressure will be lower. If we increase the muffler size, the muffler pressure will be lower.
The lower the muffler pressure, the lower the pressure inside of the combustion chamber.
Because of PV=nRT, the lower the pressure inside the combustion chamber, the lower the temperature.

Also, the more exhaust gas you can get out of the muffler, the more fresh, unadultrated mix can get in, and the more horsepower you can make.

 

[drmiller100]

why don't they do this from the factory?

I think it is because if a 50 cc saw made as much power from teh factory as a current 80cc saw, they wouldn't sell very many 80cc saws.

 

[COLD_IRON]

why don't they do this from the factory?

I think it is because if a 50 cc saw made as much power from teh factory as a current 80cc saw, they wouldn't sell very many 80cc saws.

Probably this little thing called the EPA

 

[scotclayshooter]

I thought the small exhaust exit was to control emmisions by stopping the saw from dumping so much unburt carbons.

 

[goatchin]

why don't they do this from the factory?

I think it is because if a 50 cc saw made as much power from teh factory as a current 80cc saw, they wouldn't sell very many 80cc saws.

job security

 

[Outlaw5.0]

EPA and noise regulations.

 

[chowdozer]

30 years ago almost all cars had Exhaust Gas Recirculation valves on them. The purpose of the EGR valve was to reduce NOX emissions.

The EGR valve mixed a portion of exhaust gas with the fresh incoming air/fuel charge lowering the flame temperature of the burn process. Adding exhaust gases to the incoming mix also increases the heat capacity of the combustible mixture, lowering flame temperature even more. NOX formation happens much faster at higher flame temperatures. That was how and why an EGR worked. The auto manufacurers spent much $ on this to meet emission requirements.

If you had a vehicle in that era, you may remember that when you bypassed the EGR you also had to knock the timing back, install a lazy cam or run higher grade fuel to avoid a detonation problem.

A restrictive muffler acts like an EGR, limiting the amount of burnt gases that exit the cylinder.

 

[Danger Dan]

The sparky plug goes bang.
cylinder pressures go way up, combustion gas temps go way up, pushing the piston down.

eventually the exhaust port opens, and some of the exhaust gasses go running out into the muffler.

How much of the exhaust gas goes out? Well it depends on the pressure inside of the muffler. The lower the pressure, the more exhaust gasses run out.
If we open the muffler outlet, the muffler pressure will be lower. If we increase the muffler size, the muffler pressure will be lower.
The lower the muffler pressure, the lower the pressure inside of the combustion chamber.
Because of PV=nRT, the lower the pressure inside the combustion chamber, the lower the temperature.

Also, the more exhaust gas you can get out of the muffler, the more fresh, unadultrated mix can get in, and the more horsepower you can make.

PV=nRT or the Ideal Gas Law?
Don't stop there...let's figure it out man
P = pressure
V = volume
n = number of moles of gas
T = temperature in Kelvin
R = a universal gas constant

PV/n=RT
PV/n/R=T or T=PV/n/R

LETS KEEP IT SIMPLE and SAY n AND R=1
T=PV/1/1 or T=PV

Therefore if you decrease pressure and/or volume, the temp will decrease.

So the assumption that increasing the muffler volume will lower the pressure/temp does not ring true. This may explain why expansion chambers run hotter than mufflers...VOLUME

 

[Lakeside53]

The temperature difference, caused by "back pressure " is tiny....

It's easy to calculate if you know the exact "back pressure"...

 

[Lakeside53]

A restrictive muffler acts like an EGR, limiting the amount of burnt gases that exit the cylinder.

I bet all the "burnt gases" exit the cyliner... the issue is the volume of "unburnt" gases...

 

[Danger Dan]

I bet all the "burnt gases" exit the cyliner... the issue is the volume of "unburnt" gases...

And the volume of burnt gases, (heat) contained in the muffler.
And I agree that unburnt gases are an issue...Ideally we want them all burnt. BOOM! :smoking:

 

[Haywire Haywood]

My dear ChainsawSam, Lakeside53 has forgotten more about chainsaws than you will likely ever know.

Ian

 

[ChainsawSam]

My dear ChainsawSam, Lakeside53 has forgotten more about chainsaws than you will likely ever know.

Ian

Yeah
Ian, problem is old "Jurassic Park" forgot all the correct information. HA HA HA HAAA HAAAA

 

[COLD_IRON]

Yeah
Ian, problem is old "Jurassic Park" forgot all the correct information. HA HA HA HAAA HAAAA

Wow. You are really stirring that pot.

opcorn: opcorn: opcorn:

 

[Scandy14]

opcorn:

 

[Haywire Haywood]

I smell a Cahoon.

 

[Danger Dan]

Cahoon? LOL...That's funny stuff!

 

[Lakeside53]

I smell a Cahoon.

My aging brain is having trouble recalling.. but I seem to remember that not ending well..

 

[Four Paws]

The lower the muffler pressure, the lower the pressure inside of the combustion chamber.
Because of PV=nRT, the lower the pressure inside the combustion chamber, the lower the temperature.

Your statements may be true at BDC only.

If the muffler mod helps scavenging, you will have the ability to run more fuel in the mixture. You will have more unburnt fuel escape into the muffler during transfer. You will also have a more dense fuel/air charge in the combustion chamber when the exhaust port closes. More fuel and air will make more power. More power = more heat.

The higher the exhaust port, the hotter the saw will run.

Who cares if the temp goes down or comes up, as long as it runs better and their is no adverse impact on reliability????

 

[DDM]

I smell a Cahoon.

Your sniffer is off Its WOJO.