Part Two: The Intake Port

[Deputyrpa]

It's as close as the width of a sharpie line. It looks the same size as stock because I raised the two lobes on either side. It's a beeatch beveling the inside without an angled tool with all of that curve action. I will make a tool from a broken mini-file.

 

[Stackowood]

I have a degree wheel set up on this MS193T block, looking for simple ways to get a little more power without getting into a lot of port work.

So I'm looking at intake duration and thinking about trimming the piston skirt rather than grinding on the cylinder (black sharpie marks in 2nd pic). If I mess up a piston it's easy to replace.

I'm getting about 144 deg. intake duration, and about 145 deg. on exhaust. Does that sound right?

Thoughts please.

 

[XP_Slinger]

Interesting shape you've drawn on the piston. Looks like you'd be taking a significant amount out of it. You would gain some serious time on the intake, I personally wouldn't take that much out. Full disclosure being I have no experience on the saw you're working on, but I know them strato pistons don't give us as much room to work on the intake as the traditional cylinders. How much time were you looking to gain? 144 and 145 sounds like a good square set up too me. I know you said you're looking for gains with minimal grinding but I bet increasing the area of the intake and exhaust while maintaining the square duration numbers would run nice. But then we're back to the strato piston limiting your potential for increasing area on the intake. Talking circle complete...lol

 

[Stackowood]

The marks on the piston are just for reference- not the amount I was planning to remove. I was thinking maybe 0.010" and see what that does. The exhaust port on these would be hard to change because it takes a right angle as it goes out to the muffler.

 

[Stackowood]

If I increase intake duration and do nothing with exhaust, what problems (if any) does that create?

Since this is a clamshell, the "crankcase" volume is pretty small. Is there a point where stuffing too much air/fuel into the crankcase is not a good idea?

 

[XP_Slinger]

If I increase intake duration and do nothing with exhaust, what problems (if any) does that create?

Since this is a clamshell, the "crankcase" volume is pretty small. Is there a point where stuffing too much air/fuel into the crankcase is not a good idea?

My mistake on thinking you were intending to take out that much material. .010 is a very reasonable amount to remove in my opinion. That amount is about half of what you would get by deleting the base gasket. .010 should open the intake about 1.5* sooner giving you an additional 3* of duration. The main concern when increasing intake time is ensuring it closes soon enough to generate the crank case compression that forces the charge through the transfers when they open, which is knowledge gained through experimentation. I believe you are no where near that "lazy" charge risk with what you're intending to do.

 

[Stackowood]

Thanks! I'll give it a try and report back.

 

[kingOFgEEEks]

Thanks! I'll give it a try and report back.

I'm by no means an expert, but I recently cleaned up a toasted 550 XP, and the one thing I had noticed was that the OEM piston had a cutout just like you are showing, but it didn't completely clear the intake port, so it was shortening intake duration a little bit. I got a Golf piston from HL Supply, and when I installed it, I noticed that the cutout cleared the intake port at TDC, so I would be getting just that much more intake duration (a few degrees?). When I reassembled the saw, the only other thing I did was a mild muffler mod. This 550 is noticeably stouter than another one I rebuilt last year, so there might be something to it?

Like you said, the piston is relatively cheap, so give it a try.

 

[Mastermind]

The marks on the piston are just for reference- not the amount I was planning to remove. I was thinking maybe 0.010" and see what that does. The exhaust port on these would be hard to change because it takes a right angle as it goes out to the muffler.

.010" ain't gonna do anything.

Take .040" of in the shape you drew. Then widen the exhaust. Watch the ring ends......I don't recall where they land on that model.

 

[Stackowood]

Yeah, I guess 0.010" isn't much. Looks like 0.040" off of the piston skirt will give me roughly 6 degrees more intake duration.

 

[Fairways_and_Greens]

I'm just a hobbyist porting my MS 241. I've checked out @Mastermind's build and reviewed @Keith Gandy and him go back and forth on the build.

• It seems that @Deets066 and @mdavlee advocate for a rounded rectangle. Stock is squarish with some kind of weird nub in the corner. I can't figure out what nub is for. I channeled my inner ape, and I can't figure anything on the piston or lower that makes the nub necessary.
  
  
• Right now I think I have 145* of opening. The port is currently .560 wide, and the piston skirt is .742, leaving me .182 to split. How much support does the intake need on each side?

 

[Canadian farm boy]

Intake port shape is likely designed that way to keep the rings tucked in when the piston is at BDC. Check out the ring pin locations and compare to intake port. Last thing you wanna have happen is have the saw hang a ring in a port.

 

[huskihl]

I'm just a hobbyist porting my MS 241. I've checked out @Mastermind's build and reviewed @Keith Gandy and him go back and forth on the build.

• It seems that @Deets066 and @mdavlee advocate for a rounded rectangle. Stock is squarish with some kind of weird nub in the corner. I can't figure out what nub is for. I channeled my inner ape, and I can't figure anything on the piston or lower that makes the nub necessary.
  View attachment 158351 View attachment 158352
  
• Right now I think I have 145* of opening. The port is currently .560 wide, and the piston skirt is .742, leaving me .182 to split. How much support does the intake need on each side?

The nub keeps the bottom ring ends from hanging in the intake roof

 

[Stump Shot]

I'm just a hobbyist porting my MS 241. I've checked out @Mastermind's build and reviewed @Keith Gandy and him go back and forth on the build.

• It seems that @Deets066 and @mdavlee advocate for a rounded rectangle. Stock is squarish with some kind of weird nub in the corner. I can't figure out what nub is for. I channeled my inner ape, and I can't figure anything on the piston or lower that makes the nub necessary.
  View attachment 158351 View attachment 158352
  
• Right now I think I have 145* of opening. The port is currently .560 wide, and the piston skirt is .742, leaving me .182 to split. How much support does the intake need on each side?

I would be checking the relationship of the rings at BDC with the "nub".

 

[Fairways_and_Greens]

I would be checking the relationship of the rings at BDC with the "nub".

I'll have to double check. I should be able to see the ring of that's it's purpose, right?

 

[huskihl]

I'll have to double check. I should be able to see the ring of that's it's purpose, right?

Not if it's doing it's job.
Mark the piston at bdc and remove the cylinder to see where the ring is

 

[Canadian farm boy]

Being able to see the rings in part of the port is normal. You wanna make sure that the ring ends at the ring pin are still well tucked.

 

[Dub11]

Intake port shape is likely designed that way to keep the rings tucked in when the piston is at BDC. Check out the ring pin locations and compare to intake port. Last thing you wanna have happen is have the saw hang a ring in a port.

The nub keeps the bottom ring ends from hanging in the intake roof

I would be checking the relationship of the rings at BDC with the "nub".

Ya'll about the nub tonight....

 

[huskihl]

Ya'll about the nub tonight....

Do we win anything?

 

[Stump Shot]

I'll have to double check. I should be able to see the ring of that's it's purpose, right?

Depends on how much the cylinder is moved down. Then maybe the "nub" can be raised accordingly. Check first like Kevin said to before proceeding.