Another Porting Question

[drf255]

I've read through a bunch of threads on this already. The problem is DOING is quite different than reading. I have some limited experience porting auto cylinder heads. This is like doing microsurgery.

Currently playing with a burned out ms230 Cylinder. Hoping to open up an ms250 cylinder in the near future. Was actually thinking of mig weld in some material to the top of the cylinder or tigging a small dome on the piston. I prefer the chamber weld to increase compression as it won't increase rotating mass. The problem is how tight the cylinder is. I can't get a tig torch in there, and the nozzle of my Spoolgun is barely smaller than the cylinder.

So now for the questions.

I understand that the critical areas to leave alone are the bottom of the intake port and the top of the exhaust port. I will double check things, but I believe that the piston rings never even hit the top of the intake port. The exhaust port is fully cleared by the piston on the downstroke.

Does opening the port larger than the actual opening in the cylinder allow any increased flow at at all?

Does changing the angle of the ports affect anything? My intake port is angled down (following flow) making relieving the top the natural spot to increase port area. The exhaust floor sharply angles down as it moves away from the cylinder.

If I open the intake port area and the intake boot diameter remains the same (smaller than the stock port as is), will it affect flow at all?

Does raising the intake roof or lowering the exhaust floor have any effect on flow, or has most of the flow already occurred by the time these are opened?

I started with a Dremel and carbide cutters. The definitely take a lot of material off quickly. I can see how someone who doesn't know what they're doing (ME) could destroy a cylinder very quickly.

Thanks for taking the time to look at this.

 

[rmh3481]

Take your time and use an old piston ring to mark the height of the port. Set your squish first and get one of the cheapo degree wheels or printout one off the web to tell you where things are at. Both intake and exhaust can be safely widened to 60% of the bore. Good luck with the project.

 

[old-cat]

You picked an almost impossible design of engine to do porting on. A Husky 350 would be a whole lot more realistic. CHEAP and easy!
Compression increase is where you get MOST power increase, that means lowering the cylinder on most saws, then re-time the port openings.

 

[AKDoug]

I agree with Old Cat, I can't imagine a more difficult cylinder to mess around with.

 

[drf255]

I have a bunch of 025/250's.

Gonna play.

Would you attempt welding a dome on the piston or trying to close up the combustion chamber?

The piston would be easier. Have an AC tig and I'm OK with aluminum welding.

No lathe though. I'd have to shape the dome by hand.

 

[Moparmyway]

If you can adjust the balance on your machine, then I would give the welded dome a shot
If you cant adjust the balance, then its an easy choice; cut the squish and base

 

[drf255]

I can adjust balance and AC frequency. 300 amp watercooled.

I keep reading about preheat of the piston which is opposite to most I've learned.

Reduced heat input is achieved by using more amps up front. Only heats a small area at once.

Will likely try on a Chinese AM P&C kit I have here. If it grenades, I'll deal. If it works, others can try it as well.

There's no lowering the jug on a clamshell.

The 025/250 is a great saw in terms of weight to power. The saw only has 155 psi of compression if you're lucky. If I add a small popup, I can probably bring that up to the 180-190 range.

Whatever I do, I'll post some pics so others can learn what not to do.

 

[mdavlee]

You preheat to try to get even expansion and contraction on the piston. We preheat pipe all the time to control the temperature fluctuations from welding and then cooling off. Welded popup pistons aren't new. There's a bit of info on here about them already in a search.

 

[Moparmyway]

There's no lowering the jug on a clamshell.

Forgot its a clamshell

 

[BBP]

Anybody try using a longer reach plug to increase compression?

 

[wcorey]

On FeePay you can find (depending on what torch you have) extended gas cups up to around 4.5" that will get you close to the bottom of a cylinder.
I'd drop a rolled up piece of sheet metal in the bore to shield the plating, something goes Pop and you could end up with tungsten stuck on your nikasil.

Assuming I understand your original questions...

Does opening the port larger than the actual opening in the cylinder allow any increased flow at at all?
If I open the intake port area and the intake boot diameter remains the same (smaller than the stock port as is), will it affect flow at all?

Opening up the intake port at the cylinder wall increases time/area, no advantage that I know of to opening the rest of the tract (if you even could) to be larger than that opening.
On the exhaust side however, an expanded area past the cylinder wall opening, matched to the muffler, does seen to be beneficial.

Does changing the angle of the ports affect anything? My intake port is angled down (following flow) making relieving the top the natural spot to increase port area. The exhaust floor sharply angles down as it moves away from the cylinder.

Directing flow by changing entry/exit angles only gets you much of anything significant (good or bad) in the upper transfers, which would be difficult to accomplish, even more-so on small 025 cylinder, without a right angle handpiece.
So yes, you could probably use that part of the intake to get more volume without affecting much else.
Really though most of what you want to do there is just blend the wall opening up toward the boot.

Don't expect big gains in just widening the ports without also altering the timing, usually (but not always) this involves lowering intake/raising exhaust and transfers. Blow down timing should not be ignored!
Timing wheel and lots of research on your particular saw are your friends there...

Don't forget, especially if your just playing around with aftermarket pistons, that you can accomplish most of the timing alterations on the piston rather than the ports.
Not the best way but is easy and can be effective...

Does raising the intake roof or lowering the exhaust floor have any effect on flow, or has most of the flow already occurred by the time these are opened?

Lowering the exhaust/raising intake will likely get you nothing as the piston typically blocks that space anyway and can get you in trouble otherwise (freeporting on the exhaust side and exposing a ring end in the intake, for example).

Don't forget to try a bit of ignition advance, easy to do and easily reversible...

Of course on a work saw, most of your gains will be in the compression increase, muffler mod and ign adv, the porting part is a lot of the work and really just the icing on the cake. And I've had to scrape less than stellar icing off of an otherwise good cake...

 

[drf255]

Thanks for the detailed response. Exactly what I was looking for.

I forgot about those long cups. I ordered a few and will attempt combustion chamber mods as well now.

Will post pics.

 

[Chris-PA]

Clamshells can be ported, there are just some limitations. I've had good success porting several 46cc and 42cc Poulan clamshells - a couple run far better than you might expect. The advice to measure port timing and read up on what timing mods do is good - you can get gains by guessing but often you end up doing counterproductive grinding.

I've wanted to add a popup to increase compression on my 42cc Poulan clamshells, but I don't have the equipment so I forget about that. I don't mess with the transfer height either because I've never been able to grind them with any accuracy using my cheap Dremel tools. I set the port widths at 55%, I usually don't raise the exhaust port much unless it has a really short duration (because I cannot raise compression), but I often increase the intake to 155° - 160°, depending on other timing.

Then a mild muffler mod and sometimes a bit of timing advance, and maybe a bigger carb. The results can be surprising.

 

[wcorey]

I don't mess with the transfer height either because I've never been able to grind them with any accuracy using my cheap Dremel tools. I set the port widths at 55%, I usually don't raise the exhaust port much unless it has a really short duration (because I cannot raise compression)

Aside from the loss of compression, another disadvantage to raising the exhaust alone is the resulting increase in blowdown, where most saws seem to benefit from a bit of a decrease if anything.
Before I got a rt angle handpiece, I'd use a disc shaped cutter (like a thick cutoff wheel) to come in from the side and take a 'slice' from the roof of the upper transfer.
If its about the same width as the port it goes in pretty deep and leaves a nice even finish, seemed easier to control too, than chewing away at it with a smaller burr.

 

[Chris-PA]

Aside from the loss of compression, another disadvantage to raising the exhaust alone is the resulting increase in blowdown, where most saws seem to benefit from a bit of a decrease if anything.
Before I got a rt angle handpiece, I'd use a disc shaped cutter (like a thick cutoff wheel) to come in from the side and take a 'slice' from the roof of the upper transfer.
If its about the same width as the port it goes in pretty deep and leaves a nice even finish, seemed easier to control too, than chewing away at it with a smaller burr.

I've been thinking of trying that again, but my Dremel is clumsy big and it can be hard to see in through the exhaust port while working. And the direction of rotation can make it hard to get one side to match the other, plus it only takes one slip....

I have engines that run very well at 25deg blow down, it all depends. When the transfers should open is not just dependent on the BD/angle from the exhaust opening, but also on the the closing of the intake and the desired case compression angle.

 

[redfin]

Chris, I have been kicking around getting a pop up cut for my 4218. Of course I need to find a taller piston first. My question, how do I go about figuring what height is needed for desired compression?

 

[Chris-PA]

Chris, I have been kicking around getting a pop up cut for my 4218. Of course I need to find a taller piston first. My question, how do I go about figuring what height is needed for desired compression?

Gee, I don't really know - you'd have to measure the combustion chamber volume really carefully if you wanted to try to calculate it, but even then there are an awful lot of variables. If you were able to find a taller piston, then you could just cut the top of the piston flat and set the squish at accepted clearances (0.020" or so), and then any pop up would be based on how big the combustion chamber is - maybe look at other cylinders to compare combustion chamber size and see how much you want to reduce it?.

 

[redfin]

Gee, I don't really know - you'd have to measure the combustion chamber volume really carefully if you wanted to try to calculate it, but even then there are an awful lot of variables. If you were able to find a taller piston, then you could just cut the top of the piston flat and set the squish at accepted clearances (0.020" or so), and then any pop up would be based on how big the combustion chamber is - maybe look at other cylinders to compare combustion chamber size and see how much you want to reduce it?.

Assuming a taller piston and acceptable timing numbers are available would be not be easier to just cut squish and port from there?

 

[mdavlee]

Assuming a taller piston and acceptable timing numbers are available would be not be easier to just cut squish and port from there?

Yes

 

[redfin]

Badda Bing. Thanks man.