460 Too Much Exhaust?

[docbar85]

If the muffler is opened up too much during a mod. how would the saw run?
When considering the 80% - 85% rule (exhaust port to muffler) should you factor in the the area the screen takes?
I just finished up a 460 with about 95% ratio, got it taching around 13,300, seems to idle good. During the cut it seems like I can hear it, what sounds like skipping sometimes. Did not get to cut much so it could be my imagination.

 

[rbtree]

That's not open too much. Better rule of thumb is to have the exhaust ports at the muffler be 1.25 to 1.5 times the area of the exh. port at the cylinder bore, which is always less than at the cylinder/muffler interface.

 

[Lakeside53]

That's not open too much. Better rule of thumb is to have the exhaust ports at the muffler be 1.25 to 1.5 times the area of the exh. port at the cylinder bore, which is always less than at the cylinder/muffler interface.

I think by rule of thumb that's the same as 85% of the exhaust to muffer opening!

 

[Freakingstang]

what does a stock 046 or 460 turn? Mine is older and runs about 13,800 and it still four strokes. 13K seems a little low, maybe what your hearing is the four stroking? Just an idea. My muffler is opened up about 85-90% of the flange where the muffler bolts on. It runs pretty good imo.


-Steve

 

[Lawn Masters]

If you can hear it missing a bit once and awhile at full throttle, it is four stroking, which is good, you're not too lean possibly a tad rich, but thats still better than too lean.

 

[fishhuntcutwood]

what does a stock 046 or 460 turn? ...13K seems a little low...

13,500 stock if I recall. I think I've got mine turning just below 14k with an opened dual port, and it runs really well.

13K for an opened 046 sounds a bit low to me as well.

 

[docbar85]

Uh Oh!
I was using the area of the cylinder head where the muffler mounts. My mistake, I have always thought this was the area everyone was making reference to. This will definately put me over the limit. Can anyone tell me what the area of the cylinder exhaust is on a 460 and maybe also a 044.?

I plan to work on the carb. adjustment some more after I get this ratio correct. Will try to lean it out some more and see if it smooths out.

Thanks for the help.

 

[046]

I too use 85% of exhaust to flange area, works pretty well for me....

 

[Lakeside53]

Uh Oh!
I was using the area of the cylinder head where the muffler mounts. My mistake, I have always thought this was the area everyone was making reference to. This will definately put me over the limit. Can anyone tell me what the area of the cylinder exhaust is on a 460 and maybe also a 044.?

I plan to work on the carb. adjustment some more after I get this ratio correct. Will try to lean it out some more and see if it smooths out.

Thanks for the help.

no... that will put you right on where the so called "limit" is... If you're 80-85% of the muffler<> cylinder opening, just leave it at that. The screen is blocking up to 40% of this. Try it with and without the screen, and do timed cuts. You'll be surprised how little real world (cutting wood) difference it makes.

 

[docbar85]

Lakeside
I plan to do some more testing this weekend.
Are you saying if you have too much exhaust area the saw will just not perform as good as it could? It won't cause the saw to skip or idle rough etc.
By the way, I removed the factory screen and installed my own, the original appeared too restrictive.

 

[Mr.]

He's saying try with and without a factory style screen, throw yours into the mix as well. See which way it cuts the best.

Then you have your answer.

Don't tune your saw to RPM tune it to where it barely four strokes at WOT.

Fred

 

[Lakeside53]

He's saying try with and without a factory style screen, throw yours into the mix as well. See which way it cuts the best.

Then you have your answer.

Don't tune your saw to RPM tune it to where it barely four strokes at WOT.

Fred

I'll add the following:
...but as you have opened up the muffler, make sure you don't exceed the max allowable RPM [at that 4-stroking point] of 13,500 +/- 150rpm, with bar and chain. If you do, you'll need to restrict the muffler to bring it down.

 

[ShoerFast]

It won't cause the saw to skip or idle rough etc.
.

A muf-mod should have next to no change on the engine idle.

 

[046]

basic two stroke principle says you need back-pressure for max performance. otherwise fresh charged air would go out with exhaust.

that's why hotsaws and high performance two stroke bikes use expansion chambers.

so opening too far in not a good thing. you're better off going conservative. much easier to open up further, than to close up opening.

I've posted the EZ spagetti method of precision measuring odd size ports several times.

 

[docbar85]

Lakeside you posted 13,500 +/-150 rpm with bar and chain. Did you mean without bar and chain? If not, I definately missed the boat on this one.

 

[Lakeside53]

It's with bar and chain - as per the Stihl spec. And set it for the bar and chain you use the most... The engine needs some small loading to be tuned effectively. You will see a drop of 500->1000 if you you tune without the bar/chain then fit one.

Also, I should have said - "correctly tensioned chain".


All stihl specs are with the bar and chain. Madsen's says differently on their site, but follow the Stihl spec for a Stihl saw.

 

[Mr.]

I'll add the following:
...but as you have opened up the muffler, make sure you don't exceed the max allowable RPM [at that 4-stroking point] of 13,500 +/- 150rpm, with bar and chain. If you do, you'll need to restrict the muffler to bring it down.

I've never muffler modded without porting, so defer to Lakeside.

BTW I do not have 1 saw that I have modded that turns below the max RPM and I have 2 worksaws.

Fred

 

[docbar85]

Thought stihl spec. indicated no load rpm, which, I thought meant without bar and chain. Thanks for getting me on the right track. Plan to check it asap with the bar and see what it is turning.. Thanks again, I may keep the neighbors up tonight.

 

[rbtree]

basic two stroke principle says you need back-pressure for max performance. otherwise fresh charged air would go out with exhaust.

that's why hotsaws and high performance two stroke bikes use expansion chambers.

so opening too far in not a good thing. you're better off going conservative. much easier to open up further, than to close up opening.

I've posted the EZ spagetti method of precision measuring odd size ports several times.

True, but on my 346 and 5100 which I got from Ed a few months ago, he opened both saws's mufflers too much, imo....there's a total of three ports, two are unscreened....But the saws are incredibly fast.....Though I have to reduce the ports to lessen the noise, as it is too loud for sure.

 

[Monkeyhanger]

...but as you have opened up the muffler, make sure you don't exceed the max allowable RPM [at that 4-stroking point] of 13,500 +/- 150rpm, with bar and chain...

Hi,

just out of curiosity, do the USA 460's really have different max RPM's than the German ones? My 460 has a max RPM of 14000 according to the handbook (original German). However I often read 13500 here for the American 460's (Strangely enough my 346XPG has a lower max RPM than the 14700, often stated in the USA, according to the handbook.).

Is there a reason for the USA 460's having a lower max RPM?

Anway I'm only asking out of curiosity.

Thanks.