Wanna Talk About Port Shapes?

[Mastermind]

We talk a lot about compression, port timing, muffler mods, etc. We don't talk too much about the shape of the port, and how the shape relates to performance.

Here's a few pics of an 064 I just did.......and there's some pics of another 064 jug in here too. I sanded through the plating when I was getting the transfer off the second 064 jug.

This pic really shows the difference between a "performance shape" and a stock port. Notice how the corners have been opened up to increase time area.........but the port height is unaffected.



In this pic the rough in of the intake port is nearly complete......I'll tighten the corners a little more and work on out to my marks. Notice that this port is only a little wider than stock, and at the stock height. It should be plain to see that this port will be more conductive to flow than a stock port.



Here is a stock 064 intake port for comparison.



And here they are side by side.

 

[watsonr]

how did you determine the width to widen to and what did you do to draw the lines on the jug? The intake roof is pretty flat, why?

 

[Mastermind]

Now, for the transfer ports.

Here's a shot of the stock ports.



And here's a fully finished transfer.

Notice the corners. They are even tighter than the intake port. When the transfers first crack open, you want to be fully flowing across the entire area of the port.



Here's another shot that shows how tight the corners are......and that shape is carried back into the port a good ways.



Here's another......note the lower opening of the transfer. I don't open this down as far as I once did. I just take the lip off to prevent fuel puddling.

 

[Mastermind]

how did you determine the width to widen to and what did you do to draw the lines on the jug? The intake roof is pretty flat, why?

I just widen a little past the factory bevels Randy. I see no need to overly widen the ports.....

I use a pencil and a business card to draw the lines. I sit the card against the squish band so my lines will be square with the bore.

The roof being as flat as it is comes from the corners being tightened up. I didn't lower the floor, or raise the roof. Just tightened the corners.

 

[watsonr]

thank you Sir... great post!

 

[Mastermind]

Let me throw this pic in again. In it you can see the bevels that I put on the ports. Notice that the intake port is heavily beveled....

 

[SquareFile]

When you connect the upper transfer to the lower is that considered a boost port?

 

[Mastermind]

Same treatment on the exhaust port.......just raise it to the height I want, and tighten the corners up a bit. Not as much as on the intake though. The rings ride across the exhaust port so you want a nice arch to the port to keep from beating the rings up.



Here's another thought. In a two stroke engine, it's all about the time/area of the ports went it comes to performance. You can widen the port to gain T/A, or you can raise it. If the rings travel across the exhaust would it be wiser to get the T/A you want by setting the height, or widening the port?????

 

[deaves61]

Hey mastermind I have maybe a dumb question, do you know if a 455 husky piston may have a bit more compression distance than a ms 311 stihl thought about trying to do a swap. Not really sure if wrist pin is there same size.

 

[CapitaineHaddoc]

When you connect the upper transfer to the lower is that considered a boost port?

I was going to ask the same question...

 

[Mastermind]

When you connect the upper transfer to the lower is that considered a boost port?

No......that's what I call a bridge port. It crosses the bridge in the bore.

It's more for direction than flow. Notice that it meets the very corner of the transfer. The flow from the bridge port changes the direction of the incoming fuel/air charge......forcing it upward and into the combustion chamber.

This is the sort of stuff that makes two saws with the same timing numbers so different. Just because you follow the numbers that one builder would use, does not mean that the unit will perform the same way.

 

[Mastermind]

Hey mastermind I have maybe a dumb question, do you know if a 455 husky piston may have a bit more compression distance than a ms 311 stihl thought about trying to do a swap. Not really sure if wrist pin is there same size.

I've never worked on either of those saws.

 

[deaves61]

Thank you

 

[Mastermind]

Here's the last pic I have for now.

I don't normally enlarge the exhaust flange area on these saws. It's plenty big enough. I just deburr, and smooth it out. It's more important the match the muffler, heat shield, and gasket to the jug perfectly than it is to hog stuff out and weaken the flange.....

Note the arch across the roof of the exhaust port.........easy on the rings is key here.

 

[CapitaineHaddoc]

No......that's what I call a bridge port. It crosses the bridge in the bore.

It's more for direction than flow. Notice that it meets the very corner of the transfer. The flow from the bridge port changes the direction of the incoming fuel/air charge......forcing it upward and into the combustion chamber.

This is the sort of stuff that makes two saws with the same timing numbers so different. Just because you follow the numbers that one builder would use, does not mean that the unit will perform the same way.

So basically, this bridge port is used to accelerate the flow?

 

[Mastermind]

Exactly. It being a small passage does just that. I see in my mind, the mixture squeezing through there at a high rate of speed.

 

[CapitaineHaddoc]

Exactly. It being a small passage does just that. I see in my mind, the mixture squeezing through there at a high rate of speed.

Thank you for the explanation. Just one last question: It's not the first time i see a ported cylinder wit theses "bridge ports", but as fas as i remember, those cylinders have only 2 transferts (i see this often on 288 cylinders). Is this something you will do on a quad transferts cylinder too?

 

[cgraham1]

We talk a lot about compression, port timing, muffler mods, etc. We don't talk too much about the shape of the port, and how the shape relates to performance.

...And here they are side by side.

Well said, and great pictures!
Thanks for sharing, Randy!

Now, do you have any Husky cylinders to show us?

 

[Trx250r180]

Them little bridges make a noticeable difference in the one i had .

 

[Mastermind]

No need to add anything to the quad port jugs.