Chainsaw Porting Theory

[Keith Gandy]

Wouldnt tapering the exhaust top to bottom decrease the blowdown somewhat?

 

[Keith Gandy]

Remember the picture rattler posted of the exhaust port on a 288 that had a wash pattern in the exhaust floor?

Ive seen that on other saws

 

[Deets066]

Wouldnt tapering the exhaust top to bottom decrease the blowdown somewhat?

I don't think so

 

[mdavlee]

Wouldnt tapering the exhaust top to bottom decrease the blowdown somewhat?

Still opens the same time.

 

[Red97]

Only time I could see a tapered port benifical, would be on a large bridged exhaust port. With a full circle piston, where widening the exhaust would run into the transfers.

But you are not going to see that on many saws.

 

[canepole]

Compression is an easy way to gain torque though........

Best ways to increase compression?

 

[tree monkey]

Best ways to increase compression?

cut base and squish band

 

[Windthrown]

Best ways to increase compression?

Either cut the cylinder base, or just remove the base gasket. Either will increase compression. Now, after you do that you may get piston slap. So you have to compensate by either: cutting the squish band in the cylinder to allow the piston to clear at TDC, or, cut the piston where the squish band is and make a pop up that clears at TDC. You also have the issue that you have dropped all the ports and changed the mechanical timing, unless (as some do) you remove the top ring with the pop up which raises the top of the transfers and the exhaust port back up. Removing the top ring also lowers compression some (by lowering the ring travel height and reducing the seal) but also reduces the ring friction. I like 2 ring pistons myself, and I believe that lessening the weight of a piston with a pop up also changes the stock balance of the piston and crank (not so good thing IMO), though with less inertia it is easier to force the piston up and down. Read: lots of variables here. You also have to contend with cylinder taper, squish taper and ring gap at TDC with a squish cut though, etc. etc.

 

[Magic_Man]

How do you decide on how much to cut from the base ? It makes sense to me that the more you remove from the base the more you have to remove from the band to set squish resulting in a smaller combustion chamber.

 

[mdavlee]

Intake timing or skirt length.

 

[Brush Ape]

Either cut the cylinder base, or just remove the base gasket. Either will increase compression. Now, after you do that you may get piston slap. So you have to compensate by either: cutting the squish band in the cylinder to allow the piston to clear at TDC, or, cut the piston where the squish band is and make a pop up that clears at TDC. You also have the issue that you have dropped all the ports and changed the mechanical timing, unless (as some do) you remove the top ring with the pop up which raises the top of the transfers and the exhaust port back up. Removing the top ring also lowers compression some (by lowering the ring travel height and reducing the seal) but also reduces the ring friction. I like 2 ring pistons myself, and I believe that lessening the weight of a piston with a pop up also changes the stock balance of the piston and crank (not so good thing IMO), though with less inertia it is easier to force the piston up and down. Read: lots of variables here. You also have to contend with cylinder taper, squish taper and ring gap at TDC with a squish cut though, etc. etc.

tl;dr but, I scanned it. lol

You don't have anything to 'contend,' with. When you make a machine work better you train a dog. You don't cajole him, you convince him. It's a mutual thang. A warlord sees war as merely an opportunity to extend his skills through an opponent according to Sun Tzu.

Here's a better way to put it: On the sixth hole of a golf course, a buddy of mine was making a round with his Grandpa. The kid had one terrible hook. I don''t mean he was missing a hand. haha. No he drew the ball in an arc in the direction of the follow-through of his swing. Now, considering the relationship between the insuperable stand of Oaks in the fairway, the tee and the green, he could take advantage of his natural hook to lob the ball around the trees right onto the green. His Grandpa said, "what the hell are you doing?" "I'm playing my hook," said the kid. "Don't play your hook, get rid of it!!!" Said Grandpa. Who was right? Slick runway or furburger? Can you argue with success?

 

[Red97]

tl;dr but, I scanned it. lol

You don't have anything to 'contend,' with. When you make a machine work better you train a dog. You don't cajole him, you convince him. It's a mutual thang. A warlord sees war as merely an opportunity to extend his skills through an opponent according to Sun Tzu.

Here's a better way to put it: On the sixth hole of a golf course, a buddy of mine was making a round with his Grandpa. The kid had one terrible hook. I don''t mean he was missing a hand. haha. No he drew the ball in an arc in the direction of the follow-through of his swing. Now, considering the relationship between the insuperable stand of Oaks in the fairway, the tee and the green, he could take advantage of his natural hook to lob the ball around the trees right onto the green. His Grandpa said, "what the hell are you doing?" "I'm playing my hook," said the kid. "Don't play your hook, get rid of it!!!" Said Grandpa. Who was right? Slick runway or furburger? Can you argue with success?

Wish some saws cooperated well as the dog.
Golf is more confusing than chainsaw porting theory...

But success is success... that is all most of us are searching for with these contraptions.

 

[paragonbuilder]

Intake timing or skirt length.

Can you explain what you look for and how to determine how much can be cut based on this?


Sent from my iPhone using Tapatalk

 

[mdavlee]

Free port the exhaust or drop it so you hit the desired intake number without grinding.

 

[Red97]

Can you explain what you look for and how to determine how much can be cut based on this?

Sent from my iPhone using Tapatalk

By the intake duration, or the degree at which it closes.

If you cut too much off the base the intake may have to be filled back up to get the desired number/duration.

Some saws will free port on the exhaust side with much more than a gasket delete. That is where the skirt length comes into play.

 

[Al Smith]

Perhaps at this time it might be helpful to mention a few reference sources. All of these are on the net,Google is your friend .Macdizzeys, AG Bell two stroke,Gordon Jennings two stroke tuners guide .In addition British motorcycle sites and RC sites .By the way RC sites are not stingy with info .

Fox Valley Kart in Indiana covers a lot of basics .It's out there you just have to search for it .
Now that's the basics,specifics you'll have to play with on your own for whatever you think you might want the engine to do.

For all intents and not slinging mud you'd have a better chance of walking on water than pry specifics from old time kart,motorcycle or racesaw builders .No free woodpile they say you have to do that on your own .--go for it .

 

[Mastermind]

Excellent discussion.

I've been keeping notes since I first started porting saws. As I did more of the same models, I would update my notes to reflect changes to my "recipes". That has been the most valuable resource I have as a engine builder.

When a new model comes out, I start with small changes. Like .020" from the squish to see how much that changes the compression.

On the 661 I bought two new top ends before I learned what works, and how I wanted to do that saw.

That's just how it is sometimes. To really find the gains, you have to do some trial and error testing.

My suggestion is to always study the design. Then run the saw stock to get a idea of what it needs. For example, is it high strung, but lacking grunt? Or maybe it's strong, but lacks rpm...

Look at the design limitations. Like was said above.....how long is the piston skirt....how wide is it? Where are the ring ends? How long is the intake duration stock?

Most of the people that start asking me lots of questions are hoping for a magic recipe that fits every saw. That does not exist. Even different users want different things from the same saw.....

I can share every detail of my notes with another builder, but that doesn't mean they will get the same gains that I get. Port sizes, shapes, finishes, etc, etc. All these things matter to a certain extent.

 

[Deets066]

Well said Randy, I have kept some notes. But some of them are on paper towels, small pieces of paper, business cards, I've written on my bench too. I need to get a decent notebook to write the stuff down in.

 

[idiotwithasaw]

I by no means am looking for a magic recipe, but I however and I'm sure others like me may only ever grind on one saw and won't want to have to buy a new cylinder. They want to get it if not right then close. And having a basic understanding of what changes where can get you in the direction you are wanting to go is helpful. I for one have no illusions that I'm gonna build the fastest saw ever built, but I wanna make one I will be happy to run and if I make a mistake it's MY mistake and no one else's. My hope through this thread and other questions I have asked others that my knowledge base will expand and one day those mistakes will be less frequent.

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[Mastermind]

Deets, I started out keeping note in a notebook.....but it was soon filled, then another...

I use Open Office now and keep my notes on the computer. Easier to find, and to add to as things change.