Progressive Modding of a Dolmar 7900

[B_Turner]

Also the saw is breaking in. About half the saws I've bought gained alot the first 3 to 15 tanks depending on the saw. My 346 was a turd the first tank.

And one of my 3120s got ALOT stronger after a few tanks. Night and day and I changed nothing.

 

[4mocajuns]

<a href="http://www.sweetim.com/s.asp?im=gen&lpver=3&ref=11" target="_blank"><img src="http://cdn.content.sweetim.com/sim/cpie/emoticons/000202C4.gif" border="0" title="Click to get more." ></a>

 

[blsnelling]

<a href="http://www.sweetim.com/s.asp?im=gen&lpver=3&ref=11" target="_blank"><img src="http://cdn.content.sweetim.com/sim/cpie/emoticons/000202C4.gif" border="0" title="Click to get more." ></a>

I'm grinding right now

 

[4mocajuns]

I'm grinding right now

Brown CHicken Brown Cow.......




Oh wait you mean grinding on the cylinder!<a href="http://www.sweetim.com/s.asp?im=gen&lpver=3&ref=11" target="_blank"><img src="http://cdn.content.sweetim.com/sim/cpie/emoticons/000203F2.gif" border="0" title="Click to get more." ></a>

 

[blsnelling]

She's done and back together. Unfortunately, it'll be tomorrow afternoon before I get to cut with it.

 

[blsnelling]

How's the compression and squish?
To me, the new muffler side port looks much better than a frontal dill job.

Squish is .022. Compression is 153. I left the squish where it was. This a work saw and will likely see some heavy duty use from what MCW has been showing us of his job.

Quite honestly, this 7900 doesn't look much different inside from a 361, 357, or 372. It does have a single ring, so that made port layout simple. Skirt width will not allow 70% port width. I left transfer and exhaust port timing stock. I lowered the intake a couple degrees. I widened both the exhaust and intake, and widened the rear transfers towards the intake. I then port matched the muffler, heat shield, and gasket. We'll see tomorrow if it worked

 

[brncreeper]

Squish is .022. Compression is 153. I left the squish where it was. This a work saw and will likely see some heavy duty use from what MCW has been showing us of his job.

Quite honestly, this 7900 doesn't look much different inside from a 361, 357, or 372. It does have a single ring, so that made port layout simple. Skirt width will not allow 70% port width. I left transfer and exhaust port timing stock. I lowered the intake a couple degrees. I widened both the exhaust and intake, and widened the rear transfers towards the intake. I then port matched the muffler, heat shield, and gasket. We'll see tomorrow if it worked

After seeing MCW work pictures it sounds like a wise move on the compression. Too bad about the skirt. That surprises me on just one ring, it should be a real screamer! Looking forward to the pictures.

 

[tdi-rick]

[snip]
Too bad about the skirt. That surprises me on just one ring, it should be a real screamer! Looking forward to the pictures.

Well known limitation for porting the 7900 using the OE piston. Skirt width is 32mm across the chord, OE port width is 26mm.

Good squish measurement straight up, I wouldn't have played with it either.
Did you measure the port timing before starting ?
How did you approach the lower transfers, bridge between them and how the piston windows feed them ?

Much lightening on the piston, or only casting dags removed ?

 

[TraditionalTool]

Now you tell me, lol. Looks like I proved your point. I gutted the internal cage. I then opened up the factory exit and replaced the screen. I also added a second port and used a Husky deflector. It could be that it's not tuned perfectly. Back to the drawing board.

Time wise they were very similar, and the first video when it was stock has one of the fastest runs as the 2nd run, just over 12 seconds.

Curious about the muffler mod, is this the style you would do on the 359?

This looks like it wouldn't require any brazing or welding, just cut out the sides and use a screen. You mention those are Husky vent covers, what type of saw were they off of?

Cheers,
TT

 

[Zombiechopper]

Time wise they were very similar, and the first video when it was stock has one of the fastest runs as the 2nd run, just over 12 seconds.

Curious about the muffler mod, is this the style you would do on the 359?

This looks like it wouldn't require any brazing or welding, just cut out the sides and use a screen. You mention those are Husky vent covers, what type of saw were they off of?

Cheers,
TT

+1 on the vent info. Is there a part number?

 

[outdoorlivin247]

Brown CHicken Brown Cow.......




Oh wait you mean grinding on the cylinder!<a href="http://www.sweetim.com/s.asp?im=gen&lpver=3&ref=11" target="_blank"><img src="http://cdn.content.sweetim.com/sim/cpie/emoticons/000203F2.gif" border="0" title="Click to get more." ></a>

We were just talking about that the other night...LOL

 

[tdi-rick]

Husky deflector # 503 07 83-01 (off a 281 ??) seems the popular choice.

 

[MCW]

This a work saw and will likely see some heavy duty use from what MCW has been showing us of his job.

Best bit is that it will be run in when I get it (not that I've ever taken any notice of "running in") so it'll be straight to work. I'll get a few vids and photos up ASAP when it arrives to show you it's new home and me giving it a hammering
I hope it's not too good though as I don't want to retire my old 7900 just yet! I do have this compulsive habit type thing going on with chainsaws...

 

[wendell]

I asked this on another thread but never got a response (I think I killed the thread ) What is "squish"?

 

[MCW]

I asked this on another thread but never got a response (I think I killed the thread ) What is "squish"?

This is my saw and I'm not even sure! I just do the cutting and saw maintainence. I leave saw mechanics to the experts.
I think it is a term used for determining piston clearance, could be wrong?

 

[angelo c]

I asked this on another thread but never got a response (I think I killed the thread ) What is "squish"?

Wendell,
Try this link.

http://www.sacoriver.net/~red/squish.html

 

[tdi-rick]

I asked this on another thread but never got a response (I think I killed the thread ) What is "squish"?

Short answer first.

In this instance it's a bit of a misnomer, what we are actually talking about is piston to head clearance, or more precisely, piston to squish band clearance.

The squish band is the area of the head nearest the cylinder wall that closely follows the piston crown shape, having minimal clearance between the two. The combustion chamber is then set somewhere in the middle of this band. The chamber can either be centralised or offset, depending on what the design engineer is tying to achieve, but that band around the outside of it is still a squish band, even though it may vary in size.

In simple terms, the narrower that gap, the more efficient your combustion process will be and it dramatically reduces the chances of detonation occurring (very unlikely in a saw anyway) as the end gasses have been 'squished' back into the chamber, not lingering out near the cylinder wall where they may self ignite, a long way from where ignition is supposed to start occurring.
The reason for the gain in efficiency is that you are squeezing the maximum amount of charge (fuel/air mixture) into the combustion chamber where it can be ignited by the plug.
If the squish clearance is too great, anything up to 10% of your charge will be wasted, igniting far too late to get any real work out of it as it's so far from the plug and chamber proper.

This squish clearance can also be too little, leading to piston and squish band hitting as things stretch under the extreme forces or even through just a bit of carbon buildup.

 

[blsnelling]

Well known limitation for porting the 7900 using the OE piston. Skirt width is 32mm across the chord, OE port width is 26mm.

Good squish measurement straight up, I wouldn't have played with it either.
Did you measure the port timing before starting ?
How did you approach the lower transfers, bridge between them and how the piston windows feed them ?

Much lightening on the piston, or only casting dags removed ?

Is there a known replacement piston with wider skirts?

I did not measure port timing since I knew I was going to be working with the stock timing.

I did sharpen the divider in the transfer intake.

I do quite a bit of work to the piston. All four holes are tapered and flowed from the inside out. A lot of material is removed.

Curious about the muffler mod, is this the style you would do on the 359?

This looks like it wouldn't require any brazing or welding, just cut out the sides and use a screen.

I use this deflector and spark arrestor screen on many of the mufflers I mod, including the 359/357.

+1 on the vent info. Is there a part number?

503589901 EXHAUST OUTLET $8.74
503405601 SPARK ARRESTOR $4.97

 

[TraditionalTool]

I use this deflector and spark arrestor screen on many of the mufflers I mod, including the 359/357.

503589901 EXHAUST OUTLET $8.74
503405601 SPARK ARRESTOR $4.97

I just punched them into Bailey's but nothing comes up. Maybe this is something that has to be special ordered from them???

It looks clean and I think that would work out well for me,

Cheers,
TT

 

[fredmc]

Is there a known replacement piston with wider skirts?

Anybody got an answer for this question? inquiring minds wanna know.