Chainsaw Porting Theory

[Brewz]

And this brings me back to my question on exhaust size on the muffler side

Are we better off making the port mouth outlet bigger to get a faster drop in pressure over distance as the gas leaves the chamber as opposed to making the port wider

Perhaps porting is better done after the piston side port?

 

[drf256]

And this brings me back to my question on exhaust size on the muffler side

Are we better off making the port mouth outlet bigger to get a faster drop in pressure over distance as the gas leaves the chamber as opposed to making the port wider

Perhaps porting is better done after the piston side port?

Whatcha talking about Willis?

 

[Keith Gandy]

At least in a 4 stroke, the exhaust valve is much smaller than the intake because of the pressures that pushing gasses through them. The exhaust flows by pressure, the intake vacuum. Even though the volume of gas is much more, it can flow though a smaller area.

In a chainsaw, I've seen it quoted that 90% of the exhaust gas pressure/flow is gone in the first 10* of port opening. If I'm wrong on this, please offer the correct info.

So if 90% is gone in 10*, why risk catching a ring with a wider exhaust?

I guess I'm trying to ask if it actually does much and if the benefit of widening outweighs the risk for this port.

Square the top up as much as possible, increase port window duration just short of freeport, add a stepdown exhaust if needed? Just ideas

 

[drf256]

Depends on engine. Almost all I've worked on,IIRC, had a piston crown above the exhaust floor at BDC. So lowering the ex floor would do zero.

 

[Keith Gandy]

Depends on engine. Almost all I've worked on,IIRC, had a piston crown above the exhaust floor at BDC. So lowering the ex floor would do zero.

I have a 390xp with a stepdown exhaust thats opened up pretty good that is a runnin son of a gun but also pulls 28 404 buried very well. Exhaust hasnt been widened

 

[mdavlee]

A wider exhaust will evacuate the exhaust gas quicker than a taller narrower one. It won't be much of a difference but the last 10% is hardest to gain.

 

[Keith Gandy]

A wider exhaust will evacuate the exhaust gas quicker than a taller narrower one. It won't be much of a difference but the last 10% is hardest to gain.

Im believer in at 1st crack of the ex. Port is where most of the flow will happen with the most velocity so widening the top or squaring as much as safely can be done is where its at

 

[Keith Gandy]

Wonder how reeds in exhaust would work for top and bottomend power??? Lol!!

 

[mdavlee]

Power valve set up?

 

[Keith Gandy]

Power valve set up?

Man thoso things were a nightmare in my kx125!! Carbon buildup was always a problem

 

[Keith Gandy]

U could definitely tell if that darn valve was lazy or stickin

 

[Keith Gandy]

BelRay MC1 must have been the culprit!!

 

[Brewz]

So pressure V's volume would indicate more compression will benefit from a wider port

 

[Moparmyway]

And this brings me back to my question on exhaust size on the muffler side

So far, on any Stihl 70cc-90cc that I have ever seen, you really cant go to big on the exhaust opening in the muffler.
They like them big, matter of fact, the bigger you go, the more power they have in the cut
The ported ones really come alive with a second large opening, like @Red97 makes

 

[Mastermind]

A wider exhaust will evacuate the exhaust gas quicker than a taller narrower one. It won't be much of a difference but the last 10% is hardest to gain.

Mike says a lot here.....and with only a few words.

I've not been posting much. But I'm still reading along. Just trying to get some work done.

 

[Barneyrb]

Will do. Numa I push chips in a linerboard plant for International PaperView attachment 12353

I've been in that mill numerous times, don't go anymore but have been on Hollywood & Vine

 

[Keith Gandy]

I've been in that mill numerous times, don't go anymore but have been on Hollywood & Vine

Campti Red River Mill? We process, reclaim/inventory about 450 to 550 tons/hr of softwood chips

 

[shorthunter]

Thinking laterally, by the time the piston gets to a point where the wider ports are taking effect, the exhaust port it probably 10 to 20% open and the transfers are opening or open.

So is a wider evhaust port going to increase performance at this point?

Yes

 

[Redfin]

A wider exhaust will evacuate the exhaust gas quicker than a taller narrower one. It won't be much of a difference but the last 10% is hardest to gain.

Ive never measured but now im wondering in a stock saw how much the physical area the ex and trans relate to each other.

In my head the ex area doesnt need to be to terribly much larger than trans. Considering most is expellend very soon at first opening.

 

[Barneyrb]

Campti Red River Mill? We process, reclaim/inventory about 450 to 550 tons/hr of softwood chips

Yup, that one too. I had thought you were at Mansfield. I've been in most papermills within 200 miles of Monroe.