Porting a 20? (200t)

[Jed]

If you're a professional saw builder, then please disregard this. For the rest, please regard this as a redneck public service announcement. Please do not port your 200t. I had to replace my seals recently, so I took the whole headache as an excellent chance to do some (vey mild, I swear!!) cylinder porting. Dang thing came out having lost at least 10 %. (Even with the new seals.) I know, I know... "Don't quit your day job. lol, and etc." Man, .... forget you guys! I'm a redneck... I HAVE to port saws. Except for the 200.

 

[MasterBlaster]

I wonder why it reacted like that?

 

[woodworkingboy]

Jed, are you sure that you didn't leave any tools inside like surgeons sometimes do?

 

[Al Smith]

Well you did something wrong then because they respond very well .What exactly did you do to screw it up?

Okay here's what I did ,cut the cylinder base so it had about 20-22 thou piston to head deck .Cut about a 12-15 thou raised dome piston widened the intake ,widened the exhaust port and raised it to 96 after .Nothing to the transfers .Some muffler work and that's it .That thing cranks on at 15,600 still 4 cycling.If I leaned it no doubt it could do 16,000 or more .

 

[Jed]

Butch: I dunno.

Jay: Can't be sure.

Al: Ah! But you decked it! That's cheating. I'm thinking that to get any gains from porting a cylinder that small, you'd HAVE to tighten the squish. All I did was raise the intake a hair, hog out the exhaust a bit, and then kinda "cleaned" out the transfers. I did hardly anything on the transfers, but, unless it's all in my head, I definitely didn't do it any good at all.

 

[woodworkingboy]

Maybe you opened up the ports too much. One thing you really have to be careful of, is opening up a port so much that the piston skirt doesn't completely cover it when the port should be closed.

 

[Jed]

Thanks Jay. Yeah, you're right. Funny that I really wasn't thinking about that when I did it, but still, I don't think that I did. If I did, it's cause I raised the intake too much.

The saw still works great.... just feels like it lost a little something, which makes you feel like an idiot when you've deliberately done something that ended up screwing it up.

 

[woodworkingboy]

I know the feeling, too. Still, a part of learning.

 

[Al Smith]

I'm thinking that to get any gains from porting a cylinder that small, you'd HAVE to tighten the squish. All I did was raise the intake a hair,

"Tighten the squish" meaning cutting down the combustion chamber size ?Well that's one option or approach I suppose .Not one I practice but it works for some people .

If you didn't either remove the base gasket or cut the cylinder base to raise the compression by raising the exhaust you've killed the torgue .It might be as simple as removing the gasket and using sealant to bring it back .Leave at least 20 thou clearance and don't fiddle with the ex port .Keep it at least 96 after no higher .

If it takes a gasket you can use about any fairly stiff paper .A standard 020 base gasket is 20 thou thick .Something like a magazine cover is around 15 thou .

Without seeing it I doubt you totaly screwed it up .It just needs a little "adjustment "

 

[Jed]

Yeah, thanks for the encouragement Al. Yeah, there's no way I TOTALLY screwed it up, because the saw still works really well. It definitely didn't lose very much. It SEEMS like it just lost as hair.

I tried the no gasket trick last summer on one of my ported 440s and ran into some problems that I can't now remember (tell the truth, I think that I just tightened the darned bolts down too tight, and then--while one rattled loose on it's own!--I couldn't even get the other ones out without striping about FOUR torx wrenches completely out.) Now you're getting the idea about what kind of a builder I am. Anyway, it definitely scared me away from the no head gasket trick for my 200. But, I do like the idea of the red gasket sealant stuff instead. Might try it. Dang it man, how much could a mandrel possibly cost anyway? BTW... when you said, "cut the base 20-22 thou." then that would mean "tightening the squish," right?

Also, where the heck did you get a "raised dome piston" that would fit a 20? Oh, and... why did you cut it. Sounds tasty though... you don't want to sell that pig do ya?

 

[Al Smith]

I cut the piston myself .The reason as I see it for a raised dome is to direct the sweep from the transfers around the intake side of the piston .It's not to raise the compression as some might think .

Not that's is ultra important just something I do .I got the idea from a Ford 3. liter Vulcan engine .

It's just one way to skin a cat so to speak .I'm strickly an amatuer .There's plenty of actual builders who just use a standard flat top piston and get great results so it's more a preferece than anything .

 

[woodworkingboy]

Jed, I've had very good luck using Threebond sealant in place of a gasket. Long term usage on a few saws and so far no leaks. I have the sealant type number at the shop if you need it, they make a few varieties.

 

[Magnus]

I don't get that talk about lowering the compressed volume at all.
That is were the action is and the more fuel air mix you have compressed will expand. The more the better.
Better is to fill the area with fuel air mix and possible make it bigger so more can get in and give power.

Another thing I think is odd is that there is a lot of focus of the shape of piston top (flat, doomed) but no talk of the heads shape.
If you alter one you should alter the other to match to get the pressures right as fuel/air mix is ignited and expand.
Flat top pistons have a cylinder top that is shaped to allow the pressure to be evenly distributed on piston surface and not side load piston that lead to excessive wear.

 

[Al Smith]

I think the idea of cutting the actual combustion chamber is more for raising the compression in a specific area where combustion will occur .

While flat top pistons are the more prevelant design hemi domed pistons have been used over the years .The Stihl 038 AV for example used both a hemi domed piston as well as an offset combustion chamber .

Regarding modfications to a chainsaw engine it's more along the lines of experimentation using known design criteria .You can pretty well have an idea what specific known practices will produce .Others might sound good on paper but the only real way to know is to do it .That's the fun part for me .

 

[Magnus]

Yes, I try most things I find interesting and this is why I don't get it?

Raising compression alone does not create more power in wood, perhaps some acceleration.

If you lower the cylinder 1mm you get more compression for two reasons.
1) Intake is lowered so it opens faster and stay open longer.
2) Volume that the compressed air/fuel mix is in before ignition is less, fuel air mix compressed more and harder.

Strain on engine is higher as compression is higher than the saw was designed for. Chances of premature ignition is higher as well as a disturbance in the internal pressures in case and transference as well as intake and outlet.

Increasing cylinders compressed volume allow more fuel air mix above piston to be compressed, ignited ans as it expand it do so faster and with more force creating more power.

The switch in 038 and others mostly include a change in cylinders as well as the flow of flame is altered as it expand and pressure too.