Chainsaw Porting Theory

[Brewz]

measure twice, cut once

 

[Keith Gandy]

measure twice, cut once

Maybe even measure 4 times or more and ponder on it a few days??!! Ive learned that

 

[jmssaws]

It will run better at 101 anyway and if it don't throw it away and do another one it's just a cylinder.

 

[Brewz]

When it comes to exhaust porting, I am interested in thoughts on the size and shape of the muffler end of the exhaust port compared to the piston end.

Is it best to hog it out as much as possible to get the fastest pressure drop over distance to help exhaust gas exit volume or can this be counter productive for some reason I cant think of.
My brain says pressure and push behind the escaping gas will be aided with less restriction.

Also, shape.

Will squaring out the lines towards the muffler be beneficial for flow or does it creat high and low pressure zones that may hamper gas flow compared to a more rounded similar ratio port from piston to muffler?

 

[drf256]

Port was measured. Not hogged out. At about 65% of bore width.

I ground down to the line in the center too much.

It's called the Heineken parallex refractive error of the middle aged eyes.

 

[Brewz]

Oh I wasn't implying you hogged it mate, and I am Australian so no need to explain alcohol induced incidents. It is culturally acceptable here

When i said hogging, I meant the muffler side, opening it right up as far as the muffler and cylinder will allow.
Is there an advantage of a bigger outer side of the exhaust port over the actual cylinder port size?

 

[jmssaws]

Port was measured. Not hogged out. At about 65% of bore width.

I ground down to the line in the center too much.

It's called the Heineken parallel refractive error of the middle aged eyes.

If you need to loose compression raise it.

 

[Mastermind]

Do share...... I have one coming I am going to build up for my brother

Nothing special. What do you want to know?

 

[shorthunter]

Why is a stock width port OK in a super high performance mod, but not in a woods port?

Who builds a race saw with a stock width exhaust port?

 

[Brewz]

Nothing special. What do you want to know?

Nothing like specific numbers as I probably wo t be doing any machine work on it.
Just interested in what basic work to ports and transfers has proven to be advantageous with this model

Also what sort of squish number should I aim for?

 

[mdavlee]

Nothing like specific numbers as I probably wo t be doing any machine work on it.
Just interested in what basic work to ports and transfers has proven to be advantageous with this model

Also what sort of squish number should I aim for?

Squish .020". I can't remember the stock numbers to say whether or not to lower and widen the intake a little or just widen. Exhaust widen a little.

 

[Brewz]

I will raise the subject again when I have it rebuilt and a degree wheel on it

 

[drf256]

Who builds a race saw with a stock width exhaust port?

Ok, worded incorrectly.

Let's try again.

Someone wants to do finger ports or boost ports on a Stihl that has the ring pins on the intake wall. So they rotate the piston 180* to allow this.

Now they have the pins on the exhaust wall. So the safest limit of how wide the port can be widened has been reduced because there is now more of a threat of catching a ring on the exhaust port.

This type of engine should have a higher output, if done correctly, than a less modified engine. But somehow it's able to do so with a narrower exhaust port than an engine with different mods.

Why?

My point is whether or not widening the exhaust really does that much?

 

[Keith Gandy]

Ok, worded incorrectly.

Let's try again.

Someone wants to do finger ports or boost ports on a Stihl that has the ring pins on the intake wall. So they rotate the piston 180* to allow this.

Now they have the pins on the exhaust wall. So the safest limit of how wide the port can be widened has been reduced because there is now more of a threat of catching a ring on the exhaust port.

This type of engine should have a higher output, if done correctly, than a less modified engine. But somehow it's able to do so with a narrower exhaust port than an engine with different mods.

Why?

My point is whether or not widening the exhaust really does that much?

More blowdown?

 

[Brewz]

Well a wise 066 man did tell me that you don't need wide ports to build a monster.

Thinking laterally, by the time the piston gets to a point where the wider ports are taking effect, the evhaust 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?

It will be in play while the fresh charge is pushing into the top over the piston so I guess a wider ports main advantage is to reduce resistance to the incoming charge from the transfers

 

[Deets066]

Ok, worded incorrectly.

Let's try again.

Someone wants to do finger ports or boost ports on a Stihl that has the ring pins on the intake wall. So they rotate the piston 180* to allow this.

Now they have the pins on the exhaust wall. So the safest limit of how wide the port can be widened has been reduced because there is now more of a threat of catching a ring on the exhaust port.

This type of engine should have a higher output, if done correctly, than a less modified engine. But somehow it's able to do so with a narrower exhaust port than an engine with different mods.

Why?

My point is whether or not widening the exhaust really does that much?

Your asking all the right questions Doc, all that you will get is opinions and theory's. There is no hard answer for a lot of these questions. Port geometry can be different on different jugs of the same model saw.

 

[Brewz]

This will also be something that will be very saw dependant, which is why I asked such a general question about the 044 porting

On each saw, widening will have a different effect depending on the motors dynamics and timings

 

[drf256]

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.

 

[Brewz]

Deets beet mee too itt

 

[drf256]

Well a wise 066 man did tell me that you don't need wide ports to build a monster.

Thinking laterally, by the time the piston gets to a point where the wider ports are taking effect, the evhaust 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?

It will be in play while the fresh charge is pushing into the top over the piston so I guess a wider ports main advantage is to reduce resistance to the incoming charge from the transfers

This comes back to squarer roof as well.

More area to vent in the first 10*.

But those figures may be pressure vs Volume. With less pressure as the port opens, it may need to be larger to vent the rest under lower pressure.