For those with cat mufflers: point of reference

[themaddhatter]

Just thought you folks would be interested in my little "mini-experiment"

Finally got around to doing a little work on my Dolmar PS420 (only had the parts for a month or so).

Had put ~ 5 tanks through it, and took it back to the shop for a free tune (part of purchase), then decided to swap out the cat muffler for a factory non-cat (didn't even mod it, just out of the box).

Here what the results were (measured with tinytach Fast-Tach) using the same gas, same carb settings, same bar & chain & ~ 3 minute warm-up on a 64 degree day:

CAT MUFFLER: Idle - 2,600 RPM; WOT - 12,000 RPM
NO-CAT: Idle - 2,800 RPM; WOT - 12,500 RPM

If I am correct (and I am sure the pros here will tell me if I am not):

-the cat creates significant back pressure
-by getting rid of it, the engine can move more air (not as restricted)
-If you are moving more air, you will need to richen up your air:fuel screw
-If you are able to move more air and more fuel, that SHOULD translate to more power

What I am wondering is why the RPM change amount would be different between idle and WOT (i.e. not both 200 RPM or 500 RPM). Would that be indicative of something? Or is that just a function of piston velocity/venturi effect/Coriolis effect/lunar pull/cosmic rays?

 

[KenJax Tree]

DING DING DING!!! We have a winner

 

[Stihl Wielder]

Just bear in mind that two cycles rely on SOME back pressure to run optimally. Watch you don't over-rev your saw. When I port-mod a muffler I make sure that the total "area" of the outlet of the muffler is just a tad smaller than the total area of the exaust port on the cylinder. I know, it's not scientific by any means, but it does give you just a little backpressure, and yes I am a redneck motorhead but it works for me.

 

[themaddhatter]

Just bear in mind that two cycles rely on SOME back pressure to run optimally. Watch you don't over-rev your saw. When I port-mod a muffler I make sure that the total "area" of the outlet of the muffler is just a tad smaller than the total area of the exaust port on the cylinder. I know, it's not scientific by any means, but it does give you just a little backpressure, and yes I am a redneck motorhead but it works for me.

Yeah I know you need SOME in there, that's why I just used the factory non-cat muffler.

I was tempted to grind out the louvers behind the spark screen on the new non-cat muffler, but figured I would just keep it "factory" (well, factory outside the borders of the EPA oops I mean USA)

 

[kevin j]

Its a little more complicated. With less restriction, it moves more air, but the 'air' went through the carb and already has fuel in it. So you don't richen up to add fuel to make up for the extra air directly.

It was probably lean to begin with, that is one factor.

I don't understand fully why, but the carb mixing and assorted pressure wwaves bouncing around do change the tune, but it is not as 'more air I have to add more fuel'. It is usually true, just not for the reason that first makes sense.

 

[nmurph]

If you think a saw needs a muffler to create back pressure you obviously haven't looked at many of the vintage saws. Short, straight pipes; nothing else.