Part One: The Exhaust Port

[Mastermind]

Liking this thread already, having just "ported" my first saw, all I was able to do was widen the intake a little, the exhaust a little bit more and deepen and smooth the transfers. The exhaust was a main focus, polished and port matched with a modified muffler, advancing the timing, set squish and piston mods pretty much wrapped it up. Not knowing where to begin with the numbers, I never even tried to degree the saw, as they did not mean much at the time. Having a basic guide will be a monumental step forward if I'm ever to make a true ported saw and not just one that's been dickered with. Surely had I been armed with a basic target guide for the numbers this saw may have turned out different. I'm hoping I'm at least on the right track, what do you fellas think?
Here's my first attempt at porting - Husqvarna 51 open port .325" - 20" bar 16" Norway Pine log.

Oh, and for the record, you guys are definitely wearing off on me!

I love the sound of those saws. Sorta deep and throaty.......

Holds RPM pretty well Steve. Do you have a tach?

 

[Stump Shot]

I love the sound of those saws. Sorta deep and throaty.......

Holds RPM pretty well Steve. Do you have a tach?

I do, but just tuned it by ear.

 

[Stump Shot]

Question, without going too further ahead, how is the balance, or maybe a better term is well rounded, the whole job(entire engine) figured in to each modification, this case the exhaust height?

 

[Stump Shot]

This may sound stupid of me, but it took a lot of saws for me to get to the "51" job. Been taking the baby steps approach to the whole thing. The saw prior just had the exhaust widened a bit polished and matched, a few before that were just polished and matched, before that just matched, and before that just a muffler modification, before this working on other mods, not focused on here. I'd did impress on me though that while gains were usually made with any one modification, it wasn't until combined with others that something good could really stand out in a saw motor. This is why I asked the question above. Surely there must be a way to match the goal of the saw to the modification, at least I hope so. In other words, you surely wouldn't want a racing intake matched up to a firewood exhaust in a saw for felling.

 

[Mastermind]

Question, without going too further ahead, how is the balance, or maybe a better term is well rounded, the whole job(entire engine) figured in to each modification, this case the exhaust height?

This may sound stupid of me, but it took a lot of saws for me to get to the "51" job. Been taking the baby steps approach to the whole thing. The saw prior just had the exhaust widened a bit polished and matched, a few before that were just polished and matched, before that just matched, and before that just a muffler modification, before this working on other mods, not focused on here. I'd did impress on me though that while gains were usually made with any one modification, it wasn't until combined with others that something good could really stand out in a saw motor. This is why I asked the question above. Surely there must be a way to match the goal of the saw to the modification, at least I hope so. In other words, you surely wouldn't want a racing intake matched up to a firewood exhaust in a saw for felling.

I think it is too easy for us to disregard is how much thought went into the build by the original designers. Before any changes are made, we should carefully map, and try to understand why the port timing numbers were chosen to begin with.

I look at the intake timing, and the piston skirt length before I decide on a plan. That tells me how much I can take off the base.

Yeah.......the whole deal has to work together, and as we go thru each port I hope we can start piecing that part of it together too.

 

[Johnmn]

Boy this is gonna be great!

 

[Deets066]

Size of engine, original port timing, and piston skirt are where I start. I want to machine my jug as close as possible to my target numbers. Sometimes you have to sacrifice a bit to get enough comp. other saws you get plenty of comp but struggle to hit your numbers.

 

[Deets066]

It's not perfect, but does really have to be to really run

 

[p61 western]

Great thread subbed and looking forward to learning from this.

 

[p61 western]

View attachment 68310

It's not perfect, but does really have to be to really run

What grit is that finished with? Looks great btw.

 

[mdavlee]

I use transfer height and desired blowdown more to set exhaust height any more. Some of my stuff is in the 170-176° of duration on exhaust. Some with the high exhaust pull better than low exhaust.

 

[RIDE-RED 350r]

Definitely subbed on this thread. I have some thoughts I will share when I get home and on my laptop.

Great idea for a series of threads Randy

 

[Redfin]

Size of engine, original port timing, and piston skirt are where I start. I want to machine my jug as close as possible to my target numbers. Sometimes you have to sacrifice a bit to get enough comp. other saws you get plenty of comp but struggle to hit your numbers.

I believe we must consider the bore/stroke also in equally sized engines. Perfect example is 661/660.

Both are almost equal in cc size but have that volume using different bore/stroke combinations.

 

[leadfarmer]

Ok, you ask for this Boss man so here goes. In your video you were showing how to determine ex height, so if you take off the base that would change that right? So for this dumb guy what determines what to cut base?

Sent from my SM-G900V using Tapatalk

 

[Mastermind]

I use transfer height and desired blowdown more to set exhaust height any more. Some of my stuff is in the 170-176° of duration on exhaust. Some with the high exhaust pull better than low exhaust.

I believe we must consider the bore/stroke also in equally sized engines. Perfect example is 661/660.

Both are almost equal in cc size but have that volume using different bore/stroke combinations.

That's a point worth exploring more......

Case in point is the Solo hybrids for me. I tried them with a shorter exhaust duration, and never felt like they had any "WOW" factor. Once I started taking the exhaust higher they really began to come alive. That tells us that there really isn't any one rule that works in all engines, or even with all builders.

 

[TreeLife]

So for this dumb guy what determines what to cut base?

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I would think depending upon what one wants from the saw, factory timing, and compression from the factory comes into play.

 

[mdavlee]

That's a point worth exploring more......

Case in point is the Solo hybrids for me. I tried them with a shorter exhaust duration, and never felt like they had any "WOW" factor. Once I started taking the exhaust higher they really began to come alive. That tells us that there really isn't any one rule that works in all engines, or even with all builders.

Less you grind on transfers the better off you are normally. I like my exhaust heights higher than most and blowdown is longer normally.

 

[Mastermind]

Ok, you ask for this Boss man so here goes. In your video you were showing how to determine ex height, so if you take off the base that would change that right? So for this dumb guy what determines what to cut base?

Sent from my SM-G900V using Tapatalk

Squish clearance is the main determining factor for me now.......but I've done a bunch of most models, and have been able to put together build specs for each one.

For instance......I know that on a 395XP I need to take .030" out of the squish band with a 100° EX height and a .025" squish clearance to make 200psi at my elevation. So, after I take the .030" out, I check the squish clearance to see how much I need to cut off the base to get my clearance to .025"

 

[Mastermind]

Less you grind on transfers the better off you are normally. I like my exhaust heights higher than most and blowdown is longer normally.

I agree with that in most cases. Some of the newer Stihls gotta go way the heck on up to come alive though. A pop-up would be better in those really.

 

[mdavlee]

I agree with that in most cases. Some of the newer Stihls gotta go way the heck on up to come alive though. A pop-up would be better in those really.

Yep. Popup has their place.