Will Dolmar muffler fit Makita 7900

[David Young]

How about the dolmar muffler on a solo 681?

 

[Greenerpastures]

Hi
I hope someone comes along with an answer, I have no idea,
never even seen a Solo of any size.

If you like to start a new thread, probably get more people
interested that can help.

Regards, john

 

[Poleman]

David, I’m thinking it should or be very close as the Dolmar cylinder fits on the Solo’s and are similar.

 

[mdavlee]

Cylinder mounting holes are the same. Brace for a 6421 muffler might not work

 

[Poleman]

Your correct Mike, I’ve had to shorten or make another spacer when moving mufflers around.

 

[Wilhelm]

Before


Uncrimp with side cutter pliers


Enlarge muffler outlet


Maintain ability to use stock spark arrestor screen


... TO BE CONTINUED (picture count limit)

 

[Wilhelm]

Enlarge all holes in bird cage and ad a few more on the sides


Assemble and recrimp - DONE
After

 

[Wilhelm]

I have a feeling that this mod is pretty close to perfect for a stock Dolmar PS-7900 - just a feeling and personal opinion though.

The sound at WOT at load is awesome without being too loud (never mind the flipped picture, it's a OPE thing).

 

[Wilhelm]

@Poleman
@Greenerpastures

 

[Greenerpastures]

Enlarge all holes in bird cage and ad a few more on the sides
View attachment 94205 View attachment 94206 View attachment 94207

Assemble and recrimp - DONE
After
View attachment 94208

Hi Wilhelm, and thanks for posting.
I was actually contemplating measuring the small holes in bird cage, to see if their total area was
much greater than the current unmodified are a of the muffler outlet, I have not done this yet, but wanted the figure so I could compare with the modded outlet you mentioned, that would still retain the spark arrestor.
Your images show what I will be doing to bring the two dimension, ie, birdcage outlet and final exhaust outlet into synch.

Yery helpfull to see what lays ahead and what is possible.

Thank's Wilhelm.

 

[Wilhelm]

Hi Wilhelm, and thanks for posting.
I was actually contemplating measuring the small holes in bird cage, to see if their total area was
much greater than the current unmodified are a of the muffler, I have not done this yet, but wanted the figure so I could compare with the modded outlet you mentioned, that would still retain the spark arrestor.
Your images show what I will be doing to bring the two dimension, ie, birdcage outlet are and final exhaust outlet are into synch.

Yery helpfull to see what lays ahead and what is possible.

Thank's Wilhelm.

IIRC stock birdcage holes are 5mm, I drilled them open to 7.5mm.
I didn't remove the birdcage in order to prevent the hot exhaust gases from directly hitting the outer muffler half.
Also, You want to keep the two pipes and the bottom socket from the birdcage as they serve as spacers between the two muffler halves.
Just opening the holes up seemed easier to me at the time and I'm satisfied with the end result.

 

[Greenerpastures]

Those enlarged holes are more than adequet
for gas flow, and you added side ones and
a cutout too, definately nothing restrictive
left.
I will of course be leaving the cage as it clearly
is a structural component keeping the two halfs
from collapsing.
Im modifying a pair of pliers to act as a tool
to open up the muffler, one fine sharp face
to get behind the fold, and the other face will
be flat, and I may even managt to cut a chanell
in it to support the bottom edge of the fold and
keep it straight while the sharp edge get dug in
where it needs to be.

I have a funny feeling that the heat synching
effect of the tail and all the other bulk of the
metal inside my original muffler are casuing
more of a problem because they retain heat
that would actuall be dissapated much more
effectively if it reached the outer casing of the
muffler, which it does in the Dolmar
7900 type exhaust.

And the biggest bottleneck regarding flow
has got to be the spark arrestor.

Im going to calculate the area of that screen
that actually blocks the outlet, it has got to be
30 - 40 percent at least.

So will be weighing up the merrit of cutting
another outlet and adding a Screen to it also.

I see no point in having a huge area inside
the muffler that allows flow, and then blocking
it at the outlet.

Thanks for the images and info.

Regards, john

 

[Greenerpastures]

Just calculated the area of the round holes
insde the birdcage, and the area of the muffler
outlet hole.
Birdcage hole area = 373.064 sq mm
Muffler outlet area without screen interferance = 165.129 sq mm

So, there is a massive difference in what can
get through the standard birdcage,
and what what can get out the Muffler hole,
207.934 sq mm, and thats not factoring in
the severe restriction caused by the spark screen,
or taking into account of the square hole in the
bottom of the birdcage that on its own is larger
than the muffler outlet.

Heat storage in the area of the exhaust port is
to me a pretty big problem because of all that
metal in the vicinity, and the restricted mufler.

Drilling the birdcage like Wilhelm has done
definately reduces this issue.
And enlarging the muffler outlet helps
breating.

These Two issues when sorted should definately
prolong the life of these saws.
And we can thank the EPA for them in the
first place.
Nice heatsink below

 

[Wilhelm]

Nice heatsink below
View attachment 94263

That's NOT a heatsink, that's a heat accumulator - the exact opposite of a heatsink!

As You enlarge the muffler outlet hole under the stock spark arrestor screen, the flow area through the screen itself increases as well!

Stainless steel wire mesh at various mesh sizes can be had fairly affordable on eBay, making a custom screen if desired shouldn't be an issue.

 

[Greenerpastures]

Hi Wilhelm, quite true, it accumulates heat,
I call it sinking, but it has the handicap in
that it is inside another container, so insted
of getting rid of heat, it retains it.

I have a few stainless steel fine wire strainers
sitting here, kitchen ware, that will sort out
any future scteens, that will be the easy part.

Either going to look at a few tractors tomorrow,
or tackle that muffler, don't know which I don't
want to do the most.

I liked the old days, when things were made right
from the word go, no messing about.

Regards, john