Chainsaw Porting Theory

[Mastermind]

I've been too busy to post much.

 

[MustangMike]

How is your Brother doing Randy?

 

[bwalker]

A few years ago all the pundits were saying raise the exhaust........always raise it.

My how things change.

People who think that are right in that raising the exhaust almost always increases HP. However, in many cases it also significantly narrows the torque curve and leads to a saw that screams during light load, but falls on its face in a decent cut. The Stihl ms 260 comes to mind. I didn't know the exhaust port timing numbers for this saw, but I could tell that it had to much exhaust port height and not enough transfer duct volume just by the way it felt. Subsequently one of the builders confirmed from this site confirmed my suspicion.

 

[riverrat2]

I've been too busy to post much.

Ye old fart knocker!!!! Cattle baron!!! fence mender, hay stacker,,, bull chip spreader,,,, Etc. Etc.

 

[idiotwithasaw]

Ok I'm sure it's probably been answered but what are we going for when we widen the exhaust and intake? Are we going for a cooler running engine by getting the hot exhaust out quicker and cool air and fuel in at a faster rate? Or are we going for more rpm or torque?

Also how do you go about measuring how wide to open the port. I've heard 65-70% of bore but how is that measured, total actual width or on the curve of the wall?

 

[jmssaws]

Ok I'm sure it's probably been answered but what are we going for when we widen the exhaust and intake? Are we going for a cooler running engine by getting the hot exhaust out quicker and cool air and fuel in at a faster rate? Or are we going for more rpm or torque?

Also how do you go about measuring how wide to open the port. I've heard 65-70% of bore but how is that measured, total actual width or on the curve of the wall?

Most of the stihls have piston skirts that are to narrow to get 70% on the exhaust,
I have found on the saws I build that widening the intake does little but the exhaust is a gain, the more you can get out the more you can get in.

Timing and a bump in compression will yield most of the gains from porting.
Ignition advance is a good gain on most.

If your not careful grinding on the intake all you will do is make it use more fuel.

 

[MustangMike]

The intake and exhaust always have to be narrower than your piston skirts (and have some room on each side). A Meteor MS460 piston has wider skirts than a OEM MS460 piston, giving you a little more latitude. That said, my OEM piston Dr Al ported 460 is a very strong saw.

 

[idiotwithasaw]

So what you are saying is I'm best to leave the intake alone and only widen the exhaust. At this point I'm not looking at raising or lowering anything. Will widening it give me more torque or just higher revs that give you more momentum?

 

[jmssaws]

So what you are saying is I'm best to leave the intake alone and only widen the exhaust. At this point I'm not looking at raising or lowering anything. Will widening it give me more torque or just higher revs that give you more momentum?

That depends on the saw. The intake needs it on some and not others,the exhaust on a some huskys is already Close to 70%

 

[jmssaws]

Some saws need a bunch of grinding and some need almost none, there is no set of rules to go by only experience from doing it. It's hard to tell someone how to do it when there's so many different and equally as good ways.

 

[jmssaws]

That said I'll help anyway I can.

 

[idiotwithasaw]

What I'm working on is a 346 xp jug on a 350 crankcase. I also have a true 346 I may work on as well. They have both been decked with a popup because I was trying to keep my numbers close to stock and I didn't have a degree well at the time. I am aware that by lowering the jug I increased intake duration but most other timing should be close to stock I would guess.

 

[jmssaws]

I think more intake will help you.

 

[idiotwithasaw]

I think more intake will help you.

Which way, widen it? If so by how much?

 

[Chainsaw Jim]

I was trying to keep my numbers close to stock and I didn't have a degree well at the time. I am aware that by lowering the jug I increased intake duration but most other timing should be close to stock I would guess.

You can actually measure all the ports with a micro ruler before you grind so you know how far you've went. If a stock exhaust is 102° then removing the base gasket that's .020 thick will lower it about 1.33 degrees. Removing gaskets, depending on how thick they are, can lower ports anywhere from about 1° to 2.75°. Measure the gasket you're removing and raise the ports that exact amount to maintain stock numbers.

 

[jmssaws]

Which way, widen it? If so by how much?

Lower.
I'll have to look at my notes from the last one but I know I didn't widen much and just raised the transfers no widening and case matched the base.
The exhaust is 104 but I can't remember the rest.
It's very strong.

 

[jmssaws]

It's 104 120 78

 

[bwalker]

Performance gains in a two cycle motor isn't about bulk flow. It's all about timing of the various events and in a saw compression.

 

[Terry Syd]

The best analogy I've heard is that a two-stroke is less like a pump (4-stroke) and more like a musical instrument - especially with a pipe on it.

 

[CR888]

I think a pipe gives a tuner more flexibility as they can tune the pipe to get the power characteristics they desire. A saw tuner has only port timing, shapes and angles to achieve the personality of a saws behavior and must be so precise in changes. Finding the 'right' exhaust height is more important without a pipe that can be changed to do what you want it to do. That's how my migit mind sees it.