Porting the dolmar 421...

[Red97]

I messed with a 401 and a I can't remember the numbers used in it but it was torquey

Must be the model before the 420/421?

This saw seemed to handle the 18" low pro pretty well, even in some noodling cuts.

What do you think about the numbers I perposed?

 

[mdavlee]

Yeah it was running a 16 or 18 lopro. I think you're on the right track.

 

[Red97]

Yeah it was running a 16 or 18 lopro. I think you're on the right track.

Thank you.

Now if I could figure a way to shave 1 lb off this saw we would really have something....

 

[mdavlee]

Yep. The 401 was a little lighter. Kind of wish I still had it around.

 

[huskihl]

That big wide black cover kinda gives the impression it has a fat ass. More placebo effect. Looks bigger so it must be heavier lol

 

[Red97]

That big wide black cover kinda gives the impression it has a fat ass. More placebo effect. Looks bigger so it must be heavier lol

Well much ultra precision calibrated arm can't tell the deference between 9.9, 10.6 and 10.8... to me at 3 saws are the same physical size. Just slightly different price tags

 

[CoreyB]

Maybe it is just me but a lot of the lighter saws feel cheaper and fragile. The 421 doesn't feel heavy to me it just feels solid.

 

[huskihl]

Maybe it is just me but a lot of the lighter saws feel cheaper and fragile. The 421 doesn't feel heavy to me it just feels solid.

10-4 on that. Work an 066 all day and grab an 026. Sure it feels much lighter, but also feels like it might break in half when you dog in and pull up

 

[drf256]

I like your numbers Joe.

Exhaust ports still confuse me.

You managed that coil dilemma very well.

 

[Red97]

I like your numbers Joe.

Exhaust ports still confuse me.

You managed that coil dilemma very well.

Thank you.

This is the way I am thinking about it.

exhaust controls compression, the size and shape has a bit of influence on the power characteristics.
Just have to keep in mind, the distance from the squish to the exhaust opend is your "power stroke" get that too short, and you start losing torque. But will make more peak hp long as the induction tract is up to the task.

Everybody has different preferences, porting is just a compromise of working with what you are given, while trying to find the power left on the table.

Since I can't widen the ports, I have to raise them.

My new compression goal is mid 180's since that coil kind of shot me in the foot.

 

[drf256]

I've read that 90% of exhaust pressure in vented in the first 10* of opening. Does that relate to volume as well? This I don't know.

The shape of the roof matters also Joe. A squarer roof will vent faster/better but will be tough on the rings.

Ever see how square Stihl makes the 261 exhaust roof? It really flat, much more than a 26 roof.

I've also seen some contradictory info from builders on a few sites. They open the exhaust to 65-70% of bore. But then when they make a really fast saw with fingers requiring the piston to be rotated 180*, the go 60% on the exhaust and make more power.

 

[Red97]

I've read that 90% of exhaust pressure in vented in the first 10* of opening. Does that relate to volume as well? This I don't know.

The shape of the roof matters also Joe. A squarer roof will vent faster/better but will be tough on the rings.

Ever see how square Stihl makes the 261 exhaust roof? It really flat, much more than a 26 roof.

I've also seen some contradictory info from builders on a few sites. They open the exhaust to 65-70% of bore. But then when they make a really fast saw with fingers requiring the piston to be rotated 180*, the go 60% on the exhaust and make more power.

I think the square roof can make a saw "peaky". Combined with extra wear on the rings. I try for about 3* arch in the exhaust roof from center to the corners. For a more gradual opening. Seems to keep the powerband broad.

As far as width this piston is 69% wide, I want to leave a little safety margin.

 

[Red97]

Cutting and sanding mandrels done.

Gotta figure out why my compressor took a dump. Think it's the pressure switch. Haven't swapped one of those in a few years...

 

[Red97]

Don't know if it was the angle to the mandrel, or the quality of the hss bit. This jug cuts like butter.

Still have to set the squish, but it is closer than when I started

More to come Tuesday.

 

[Backtroller]

Looks really nice. Big difference from the before shot.

 

[drf256]

Looks really great Joe. Can't wait to see your results.

 

[CoreyB]

Isn't it Tuesday yet? Lol

 

[Red97]

Isn't it Tuesday yet? Lol

Nope not yet...

Momma has Sunday&Mondays off so that is my family time. Little fella growing like a weed. Our first almost 5 months old. Great weekend for his first walk outside.

 

[Backtroller]

Family first.

First nice days of the year. They're only young once the saws can wait.

 

[Red97]

Jug cut and sanded a lot easier than I imagined... so I cut another .012 off and got it down manageable. At .027-28


Marked the ex at 110 set the jug on the case to double check grind till I almost hit the mark on the wheel. Have to leave some for sanding and bevels.




Exhaust to height.

Transfers set at 129( right) and 131 on the left 2

Just Roughed the intake with 80grit, and raised it .045.

By the time I cleaned off the burrs on the bottom of the ex port it left me with .003-4 free port. Don't think it will hurt a thing.

If anyone is wondering, .026 is the max you can take off the bottom before freeport

Had just enough adjustment to make the coil work. Will have to grind the intake area on the case, as the boot holds the jug up.

Hopefully Thursday get it all put back together. And run it friday.