Muffler Mod on 088, Increase by !!!!

[redoakneck]

I am making a new exhaust cover for my milling 088. The stock cover has a 14 mm diameter (Area=49pi=154sq.mm) opening in it. Behind that cover is a huge oval opening that is much larger than the opening at the exhaust at the cylinder. This gives lots of room to make a big exhaust outlet easily.

The muffler is a completely hollow aluminum can, no baffles at all. I bought the saw used but I do not think it was modded in any way before I got it.

The opening at the exhaust at the cylinder is 18mm X 36mm rectangle which is 648 square mm , so the gas gets squeezed down from 648 to 154.


Current idea is make that exhaust opening 85% of 648, so around 520 mm squared. 520 in circular terms is about a 26mm diameter hole!!! That means going from a 14mm diameter hole to a 26 mm diameter hole!!! I am using 1" (25mm) o.d. pipe so mine will not be that big, prolly closer to 20mm inside diameter.

I know to richen the h screw on the carb after all said and done, but is my thinking right on that 85% value or is that used if other mods like porting and larger carb are used???

I am mainly doing this to get the exhaust to flow to the back of the saw away from me and the wood. I am going to attach a section of flex pipe to the 1" pipe and direct it to the back. Some extra power from the bigger outlet would be a bonus, but 14 to 26 seems radical.

Comments/ advice???

 

[mdavlee]

That sounds about right. I usually open a 7/8" by 5/8" opening in a 70cc saw muffler plus the original opening. Those are large engines compared to what I usually mess.

 

[mtngun]

Current idea is make that exhaust opening 85% of 648, so around 520 mm squared. 520 in circular terms is about a 26mm diameter hole!!! That means going from a 14mm diameter hole to a 26 mm diameter hole!!! I am using 1" (25mm) o.d. pipe so mine will not be that big, prolly closer to 20mm inside diameter.

Your numbers sound good.

I assume there is no spark screen to restrict flow ?

 

[redoakneck]

The spark screen is very large and covers the entire area at the cover. It is recessed into the muffler so I do not think it will restrict much. Plus out here we have record rainfall and the sparks could not start a fire in the swamps where I have been milling.

Bob L. had a similar design with ring baffles but I do not know if he has had a chance to experiment as he has had some bad luck with an injury.

 

[blsnelling]

Just open it up as much as the deflector will cover. Forget the numbers.

 

[Stihlman441]

I am making a new exhaust cover for my milling 088. The stock cover has a 14 mm diameter (Area=49pi=154sq.mm) opening in it. Behind that cover is a huge oval opening that is much larger than the opening at the exhaust at the cylinder. This gives lots of room to make a big exhaust outlet easily.

The muffler is a completely hollow aluminum can, no baffles at all. I bought the saw used but I do not think it was modded in any way before I got it.

The opening at the exhaust at the cylinder is 18mm X 36mm rectangle which is 648 square mm , so the gas gets squeezed down from 648 to 154.


Current idea is make that exhaust opening 85% of 648, so around 520 mm squared. 520 in circular terms is about a 26mm diameter hole!!! That means going from a 14mm diameter hole to a 26 mm diameter hole!!! I am using 1" (25mm) o.d. pipe so mine will not be that big, prolly closer to 20mm inside diameter.

I know to richen the h screw on the carb after all said and done, but is my thinking right on that 85% value or is that used if other mods like porting and larger carb are used???

I am mainly doing this to get the exhaust to flow to the back of the saw away from me and the wood. I am going to attach a section of flex pipe to the 1" pipe and direct it to the back. Some extra power from the bigger outlet would be a bonus, but 14 to 26 seems radical.

Comments/ advice???

So with that larger exhaust port have you removed the H limiter cap,wilth the limitted coil i found it hard to tune mine i have made the size of mine port some and the H screw is up against the stop all the way rich.
Sorry just realised you have an 088 which would not have the limiter coil.

 

[Beer Gut]

Your numbers sound good.

I assume there is no spark screen to restrict flow ?

Not to cut in but it was mentioned no spark arrest screen, I too cut in wet woods or bucking in the gravel shop yard no worry of fire, does removing the spark arrester improve performance and does carb need to be adjusted?

 

[redoakneck]

Thanks for the input!!!

Brad, you say go as big as possible so I will do the open pipe and not worry about any baffles.

Screen or no screen, always need to adjust the carb. I run half 100ll gas and half 90+ no ethanol at about 35 to 40 :1 mix ratio using stihl ultra.
Then adjust so it 4 strokes out of the cut and smoothes out in the cut. On a big saw like this it will sometimes 4 stoke in smaller softer wood say less than 16". Remember this is a mill and usually it is ripping 22-30" hardwood. I have a tach to double check my tune and pull the plug to see too rich or lean.


I'll get some pics when I get it finished.

 

[mtngun]

Not to cut in but it was mentioned no spark arrest screen, I too cut in wet woods or bucking in the gravel shop yard no worry of fire, does removing the spark arrester improve performance and does carb need to be adjusted?

Spark screen open area is 25% of screen area, but most Stihl screens are several times larger than outlet.

Generally Stihl's internal spark screens are not a bottleneck with the factory muffler outlet, but if you enlarge the outlet, then the spark screen may become the bottleneck.