Quick question about Free porting

[Phaedrus]

Hey folks,

I'm researching lightly porting a saw. I've some limited experience/success with earlier saws some years ago. But I'm currently out of practice. So I've been catching up and reading old forum posts etc. Once or twice I've seen the term Free porting mentioned, typically in a bad context. Can someone help me out here and explain what this term actually means?

thanks P

 

[Lightning Performance]

Hey folks,

I'm researching lightly porting a saw. I've some limited experience/success with earlier saws some years ago. But I'm currently out of practice. So I've been catching up and reading old forum posts etc. Once or twice I've seen the term Free porting mentioned, typically in a bad context. Can someone help me out here and explain what this term actually means?

thanks P

The port is simply beyond the pistons ability to cover the area.


Most times it occurs from too wide of a port or the exhaust skirt coming up short at the exhaust port floor when your piston is at top dead center. MS660 big bore cylinder tends to just cover the exhaust floor at TDC as cast. Once you machine the cylinder to drop it down a "free port" hole is at the bottom of your port.

I've been trying to practice getting my thoughts onto paper. Most times I miss the ball so I do hope that helps some. Many times I'm just dismissed because of poor writing skills or poor spelling. My skin is as thick as you could ever imagine but it doesn't change the fact many many people enjoy others struggles and like to say so.

Enjoy your port work and do ask more questions because learning never ends in the real world unless you're a know it all like so many eloquent writers out there enjoying others shortcomings.

 

[Phaedrus]

Thanks LP,

Yes I did some more internet research and excessive exhaust was mentioned there.

I'm actually building a 044 with an MS460 Big bore kit. Is this the right board to get help on this project?

P

 

[Sierra_rider]

Thanks LP,

Yes I did some more internet research and excessive exhaust was mentioned there.

I'm actually building a 044 with an MS460 Big bore kit. Is this the right board to get help on this project?

P

This or the chainsaw sub. The chainsaw sub gets more traffic, but the people in this sub are probably going to be more knowledgeable about modding.

Is it a big bore kit or an actual 460 cylinder? I've built saws with both, I just did a hybrid 044/46 saw a couple weeks ago. If you're doing the hybrid, the basic mods are going to be milling out the cylinder bolt holes to fit the 044 bolt spacing. Shaving the material on the outside of the cylinder(on the outside of the transfer tunnels) and on the outside of the base to fit onto the 044 cases. Finally, you gotta make a spacer for the exhaust, as the exhaust will now be about a 1/2" in front of the cases. Some people just throw nuts/washers to make up the difference. CCC chainsaw parts makes an adapter too. I made my own adapter out of some round bar I milled and some flat stock.

 

[Phaedrus]

Thanks @Sierra_rider

Actually it is/was a 460 BB (hyway) on a 044. My second hybrid, first being just a straight OEM ms460 P&C. Yes, I did the narrowed bolt pattern, the exh spacer, and also for me I clearanced the crankcase mouth, and cut away the internal ribbing within the top cover/shroud.

Because the hyway piston on the BB kit is so short, in order that the exh floor would not be ridiculously high above the piston crown at BDC, I ended up with a very slight (about 1mm) free porting situation at TDC. However, I fired the beast up earlier this week, and it seemed ok, but I think I may need to play with the carburation a little over the next few days.

What's worrying me currently is the longevity, whilst I did chamfer the ports (I widened the exh to 33mm and the inl to 32mm), I have heard some bad stories about the 460BB kits not lasting long. Time will tell.

P

 

[Sierra_rider]

Thanks @Sierra_rider

Actually it is/was a 460 BB (hyway) on a 044. My second hybrid, first being just a straight OEM ms460 P&C. Yes, I did the narrowed bolt pattern, the exh spacer, and also for me I clearanced the crankcase mouth, and cut away the internal ribbing within the top cover/shroud.

Because the hyway piston on the BB kit is so short, in order that the exh floor would not be ridiculously high above the piston crown at BDC, I ended up with a very slight (about 1mm) free porting situation at TDC. However, I fired the beast up earlier this week, and it seemed ok, but I think I may need to play with the carburation a little over the next few days.

What's worrying me currently is the longevity, whilst I did chamfer the ports (I widened the exh to 33mm and the inl to 32mm), I have heard some bad stories about the 460BB kits not lasting long. Time will tell.

P

I'm curious how the 460bb cylinder runs on an 044 bottom end? If you don't mind doing some machine work, I'm betting an 066 piston is an option to run. I'm going to do the same thing with an 064 piston in my standard 044/46 hybrid. It's a taller skirt, so I should be able to not worry about free-porting...I also should be able to machine the top of the piston into a dome. I believe the wrist-pin is narrower on the 064, so that will have to be addressed, but it's easy enough.

 

[sean donato]

There are a few trains of thought with free porting. Bad don't ever do it. (I typically subscribe to this thought.)

Then there's the its ok if it's just a little. Like the oops barely kinda thing.

Then there's the purposeful free port. The best way it was described to me. (Mind you it was rc 2 stroke reed engine with tuned pipe) was the free porting allowed the exhaust to pull more air/fuel mix into the bottom end and the exhaust, then when the pressure wave from the pipe came back around it forced more fuel/air mix into the cylinder then just what case volume alone could. I've heard of guys saying it works great, but I've never seen proof of concept. Hit a dyno, do timed runs or anything of the like to prove it works.
You could be shoving a bunch of burnt air/fuel mix back into the cylinder, or loose fresh mix out the exhaust and never get it back in the cylinder.
Really it's just best to avoid it Imo.

 

[Sierra_rider]

There are a few trains of thought with free porting. Bad don't ever do it. (I typically subscribe to this thought.)

Then there's the its ok if it's just a little. Like the oops barely kinda thing.

Then there's the purposeful free port. The best way it was described to me. (Mind you it was rc 2 stroke reed engine with tuned pipe) was the free porting allowed the exhaust to pull more air/fuel mix into the bottom end and the exhaust, then when the pressure wave from the pipe came back around it forced more fuel/air mix into the cylinder then just what case volume alone could. I've heard of guys saying it works great, but I've never seen proof of concept. Hit a dyno, do timed runs or anything of the like to prove it works.
You could be shoving a bunch of burnt air/fuel mix back into the cylinder, or loose fresh mix out the exhaust and never get it back in the cylinder.
Really it's just best to avoid it Imo.

Some saws it's just hard to avoid. My 044/46 hybrid is slightly free-ported...for that matter, any 044 or 046 it's hard not to free-port. The 064 I've been working on is really close to being free-ported...I would've actually liked to drop the cylinder even more, but I just couldn't swing it.

 

[Phaedrus]

There are a few trains of thought with free porting. Bad don't ever do it. (I typically subscribe to this thought.)

Then there's the its ok if it's just a little. Like the oops barely kinda thing.

Then there's the purposeful free port. The best way it was described to me. (Mind you it was rc 2 stroke reed engine with tuned pipe) was the free porting allowed the exhaust to pull more air/fuel mix into the bottom end and the exhaust, then when the pressure wave from the pipe came back around it forced more fuel/air mix into the cylinder then just what case volume alone could. I've heard of guys saying it works great, but I've never seen proof of concept. Hit a dyno, do timed runs or anything of the like to prove it works.
You could be shoving a bunch of burnt air/fuel mix back into the cylinder, or loose fresh mix out the exhaust and never get it back in the cylinder.
Really it's just best to avoid it Imo.

Well mine only free port by probably no more than a mill. So any possible seepage is shut off in a couple degrees.

Another way to look at it, I think, is to think of what is actually going on in the crankcase of the engine in those few degrees. It's mainly the intake cycle. Now for the couple degrees of freeport just before TDC, I assume that gaseous momentum/inertia keeps the charge flowing, and for the couple degrees after TDC, you already have the situation when you could lose some charge back through the inlet (!). So I'm guessing that the effect of having the EXH open for just a fraction aswell at that point is negligible.

P

 

[Phaedrus]

I'm curious how the 460bb cylinder runs on an 044 bottom end? If you don't mind doing some machine work, I'm betting an 066 piston is an option to run. I'm going to do the same thing with an 064 piston in my standard 044/46 hybrid. It's a taller skirt, so I should be able to not worry about free-porting...I also should be able to machine the top of the piston into a dome. I believe the wrist-pin is narrower on the 064, so that will have to be addressed, but it's easy enough.

When I get some decent wood (probably November) I'll try to remember to upload a cutting clip.

Generally I did find that I had to clearance the mouth a frightening amount to feel safe at BDC. I forgot to use model clay when I had the old bearings and seals unfortunately in, and I wasn't fully aware of the situation until later on. So I ended up clearancing it by eye really.

The other problem with the BB kit I found was that the hyway one I had ain't machined very true where the bolt holes go (comes up a slight amount, I could see v slight air gap after a bit of tightening, I ended up using 12Nm torque and sealant, no BG, for my base joint) the only bit which looks real true and sealable is a precious little couple mil right near the bore. And of course you are grind off the jug sides too in order to drop the thing down nice. So I'd advise to be v careful you remove the bare minimum from the jug sides, and be v fussy what BB jug you buy.

P

 

[sean donato]

Like I said, the purposeful free port was a discussion in a very different 2 stroke set up.
I agree a little bit open for only a few degrees can't hurt much if anything.