How big a hole?

[woodyguy]

Been reading up and trying out muffler mods on my saws.

I'm a little confused though as people talk of drilling a couple of 6mm holes or one 13mm hole. Trouble is the 13mm is equivalent to 4.6x 6mm holes.

Is there any science behind how much extra flow you should allow?

I'm not into fancy welding but drilling holes is attractive as so simple.

Would having no muffler (not practical) be the best solution to performance.

I'd love to hear your experience.

[Stubby]

Been reading up and trying out muffler mods on my saws.

I'm a little confused though as people talk of drilling a couple of 6mm holes or one 13mm hole. Trouble is the 13mm is equivalent to 4.6x 6mm holes.

Is there any science behind how much extra flow you should allow?

I'm not into fancy welding but drilling holes is attractive as so simple.

Would having no muffler (not practical) be the best solution to performance.

I'd love to hear your experience.

 

You need some back pressure so no muffler would not be good . In an ideal world exhaust should be matched to suit inlet at working rpm . Hence exup type valves and moving inlet trumpets on bikes etc that continually adjust to suit what ever rpm your at at any one time . However on a relatively simple 2 stroke saw there is an optimum for working rpm ( flat out in the cut ) . There is a formula but I can't remember . Ask Spud !!!

[wyk]

The only 'science' I've seen has been borrowed from bike tuners. Even then, there's not a lot of consensus. I have seen 75-80% of the exhaust port size as a guide on non-ported saws. However, I have seen good results in high compression saws with a 100% or more opening. In my personal experience, in a non-ported saw, I have seen great results with just doubling the size of the stock muffler exhaust. On some Echo's, I have done even more than that with excellent results. It really depends on the CC's of the engine, the muffler size, the porting, the carb, etc etc.

 

Which saw is this for?

Edited August 9, 2013 by wyk

[spudulike]

The only 'science' I've seen has been borrowed from bike tuners. Even then, there's not a lot of consensus. I have seen 75-80% of the exhaust port size as a guide on non-ported saws. However, I have seen good results in high compression saws with a 100% or more opening. In my personal experience, in a non-ported saw, I have seen great results with just doubling the size of the stock muffler exhaust. On some Echo's, I have done even more than that with excellent results. It really depends on the CC's of the engine, the muffler size, the porting, the carb, etc etc.

 

Which saw is this for?

 

Pretty much agree with that, 80% is the figure banded around and that is the area of the internal exhaust port in the cylinder wall.

 

It depends a bit on the size of the saw and where you are going to use it, a MS660 with a free flowing tube and no baffle will be earth shattering and not please many - if you do a lot of residential town work then a real loud saw is a bit out of the question so a compromise is sought.

 

If you port a saw - you really need a decent muffler mod as hindering the extra flow causes a boggy saw that runs rich - as I found out on a 395XP where the owner wanted it a bit too quiet:001_rolleyes:

 

It came to life with a few large holes in the baffle plate!

 

Personally - don't get too hung up on the outlet hole size - it is easy to make a small hole larger but more difficult the other way round!

[wyk]

 

Personally - don't get too hung up on the outlet hole size - it is easy to make a small hole larger but more difficult the other way round!

 

Even on a 460, the dual port is freakin loud without the spark arrestor. Sometimes adding a spark arrestor to a large hole will work if you get a bit too eager with the port - assuming it's possible to do so.

 

I can't use my 385XP for built-up areas. It is insanely loud. When I was in England, it literally echoed up and down the hillsides.

[spudulike]

Even on a 460, the dual port is freakin loud without the spark arrestor. Sometimes adding a spark arrestor to a large hole will work if you get a bit too eager with the port - assuming it's possible to do so.

 

I can't use my 385XP for built-up areas. It is insanely loud. When I was in England, it literally echoed up and down the hillsides.

 

That sounds like Stubbys saws - my ears are still ringing:lol:

[wyk]

But I bet it puts wood on the ground in a hurry.

[woodyguy]

Thanks for feed back. I'll try it on several saws but initially trying it out on a little 211 that I use for hedging just for practice. Not bothered about noise as I only use it in my own wood where I can be as noisy as I wish! Drilled a couple of 6mm holes which helps. Might add a few more and see. As Spud says though, you can't take them back. Pity there isn't some way of measuring power easily rather than timing cuts which is a bit inexact.

[wyk]

Don't forget to richen up the carb to compensate for the breathing.

[Gerbutt]

Don't forget to richen up the carb to compensate for the breathing.

 

Some folk say you don't need to richen them up. I did my 200t and then took it to a shop for a re-tune and he said it was fine. It does rev like a mad thing now which does make me tempted to try and take that plastic restrictor thing off to drop the revs a little but should be fine the way it is!