Part One: The Exhaust Port

[Deets066]

So on exhaust shape. Flat roof is best because more port is opening at once thus allowing the combustion pressure to force more exhaust out quickly creating kind of a vacuum to pull out the rest? Instead of oval which vents highest combustion pressure through the small gap at the peak of the port when it begins to open. Which doesn't "pull" as much exhaust out with it. Just making sure I'm straight on that

That sure sounds good

 

[Duane(Pa)]

Helluva thread! I waited all winter for this...

 

[huskihl]

Didn't realize this one was this flat, but she's got 10 gallons of mix through it.

034 super

 

[XP_Slinger]

Didn't realize this one was this flat, but she's got 10 gallons of mix through it.

034 super

View attachment 68354

How does it run?

 

[huskihl]

And yeah, it's one of my better runners too. There's something to it, that's for sure

 

[Motorhead]

Anyone have any opinions on port width,For instance what percent of the bore?

 

[Deets066]

Didn't realize this one was this flat, but she's got 10 gallons of mix through it.

034 super

View attachment 68354

Thats pretty flat, it does make a difference. But it could hang at anytime

 

[XP_Slinger]

And yeah, it's one of my better runners too. There's something to it, that's for sure

That's 2 now that are "stronger runners" that have flatter exhaust than most would call "normal" on a woods port saw. Exhaust scavenging or vacuum, whichever we call it is key.

 

[Deets066]

Anyone have any opinions on port width,For instance what percent of the bore?

I would say 65 percent would be safe. 75-80 would be pushing it, if your skirt even allows for that much

 

[Deets066]

That's 2 now that are "stronger runners" that have flatter exhaust than most would call "normal" on a woods port saw. Exhaust scavenging or vacuum, whichever we call it is key.

Over 50 percent of exhaust flow is right when the port opens. The larger the area, the faster it's out

 

[XP_Slinger]

Anyone have any opinions on port width,For instance what percent of the bore?

I've seen 70% talked about in a lot of other threads, but pin location and skirt width need to be considered before widening to any number in my inexperienced opinion.

 

[2manyhounds]

How about exhaust size vs carb size? Can you go to big to be effective?

 

[Deets066]

Guy that ported my snowmobile made the ex. port wider at the top and tapered back down to stock width at the bottom. He said it wasn't necessary to be that wide at the bottom

 

[Deets066]

How about exhaust size vs carb size? Can you go to big to be effective?

I think carb size would affect intake more than ex

 

[RIDE-RED 350r]

I think that the quicker you get exhaust out the better. I also think you will run into mechanical limitations before going too big to be ineffective. Carb is more related to intake volume, case volume, and velocity.

 

[RIDE-RED 350r]

Guy that ported my snowmobile made the ex. port wider at the top and tapered back down to stock width at the bottom. He said it wasn't necessary to be that wide at the bottom

Smart thinking methinks. Helps reduce ring wear too I would wager

 

[huskihl]

I measure card stock to 70% and hold it rounded on the cylinder wall to get my width. Imo, 70% straight across is getting wide for a worksaw. 70% rounded is close to 65% straight across

 

[2manyhounds]

I think carb size would affect intake more than ex

I guess I was meaning isn't the exhaust limited by the carb size? Or does that not really matter?

 

[SOS Ridgerider]

I have to say most of this is way above my pay grade, but it's very interesting as I learn and understand more. The different questions and explanations make it easier for me to understand, as I might not understand something when it's explained one way, but I'll pick it up from someone else. Great thread! Loving it.
I'm not even educated enough to come up with a stupid question, so I'll keep reading until I can. Lol.

 

[Deets066]

I guess I was meaning isn't the exhaust limited by the carb size? Or does that not really matter?

It doesn't matter IMO