Part One: The Exhaust Port

[RIDE-RED 350r]

OK, so I am NOT a professional porter but have just done a couple of my own saws thus far, so take my thoughts for what they are worth...

With the exhaust, in my mind the quicker you can get that exhaust out and start reducing the combustion pressure in the cylinder, the easier and more efficiently the new fuel/air charge can then come in, the better. And if that result is achieved it seems to me that lower blow-down numbers will then be tolerable thus getting the transfers bringing in that fuel/air charge in quicker and more efficiently.

So in my mind, I think most engines should benefit from wider (more efficient) exhaust. Now I have tried a couple of different things on two of my saws and both run very strong. I widened the exhaust to 70% of bore diameter on my 372XPW along with raising it to 101 deg ATDC if I recall, with a BG delete. That saw will spool 14.4k and still 4-stroke, and is quite strong in the cut taking to dogging in well.

OTOH, my 394 had the base and squish cut. Ex is at 98 but I left the width stock just to try and see. That saw is a brute with 210lbs compression and just 18 degrees of blowdown. I think it might spool a bit higher with wider exhaust but I'm not ready to pull it down and grind on it some more just yet. I have it tuned to a safely rich 12.9-13K and it is brutal on hardwood with a 36" bar.

Those are just two examples of what I have tried and both work well. However I know that those are two drastically different saws and what works for one might not necessarily work for the other.

But in the end, my theory is that the more you can un-cork that exhaust taking all things into consideration like ring life and what the piston skirt will allow for, the better you will be set up to make a very much improved saw in terms of overall power and speed.

IDK if any of my above ramblings will make an ounce of sense, but that's where my brain leads me when I consider these things.

 

[Mastermind]

So.......we really haven't found any real solid answers to the question. "What port heights should I use?"

OK........what about width?

Shape?

Outlet size?

Finish?

 

[spencerpaving]

View attachment 68304

Soon hawt!!

 

[Mastermind]

OK, so I am NOT a professional porter but have just done a couple of my own saws thus far, so take my thoughts for what they are worth...

With the exhaust, in my mind the quicker you can get that exhaust out and start reducing the combustion pressure in the cylinder, the easier and more efficiently the new fuel/air charge can then come in, the better. And if that result is achieved it seems to me that lower blow-down numbers will then be tolerable thus getting the transfers bringing in that fuel/air charge in quicker and more efficiently.

So in my mind, I think most engines should benefit from wider (more efficient) exhaust. Now I have tried a couple of different things on two of my saws and both run very strong. I widened the exhaust to 70% of bore diameter on my 372XPW along with raising it to 101 deg ATDC if I recall, with a BG delete. That saw will spool 14.4k and still 4-stroke, and is quite strong in the cut taking to dogging in well.

OTOH, my 394 had the base and squish cut. Ex is at 98 but I left the width stock just to try and see. That saw is a brute with 210lbs compression and just 18 degrees of blowdown. I think it might spool a bit higher with wider exhaust but I'm not ready to pull it down and grind on it some more just yet. I have it tuned to a safely rich 12.9-13K and it is brutal on hardwood with a 36" bar.

Those are just two examples of what I have tried and both work well. However I know that those are two drastically different saws and what works for one might not necessarily work for the other.

But in the end, my theory is that the more you can un-cork that exhaust taking all things into consideration like ring life and what the piston skirt will allow for, the better you will be set up to make a very much improved saw in terms of overall power and speed.

IDK if any of my above ramblings will make an ounce of sense, but that's where my brain leads me when I consider these things.

Great Post !!!!

 

[Magic_Man]

I've always wondered about outlet size. I see some guys try to keep the port walls straight so outlet flange opening is the same shape and size as it is at the cylinder wall. Others I see open the flange up as far as possible, most porters on here I see land someplace in between. Is there a guide or rule of thumb as to how far you should bell an exhaust port if you should do it at all.

 

[spencerpaving]

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 agree...you worded it better than I could

 

[RIDE-RED 350r]

So.......we really haven't found any real solid answers to the question. "What port heights should I use?"

OK........what about width?

Shape?

Outlet size?

Finish?

To be perfectly honest with you and in my humble and inexperienced opinion, I really don't think there is a set number any given exhaust port should be raised to. I like your earlier post about the guidelines for ex height based on engine displacement. In my limited bit of porting I have done, I unknowingly followed that almost to a "T".

I think though that one of the bigger contributors in where to set the exhaust as far as height might be what if any compression mods are done and what kind of power curve you are looking for.

With width, like I said before I did 70% on my XPW. On my 357 I did 65%. No real rhyme or reason for those width numbers, just trying things. I think one thing to consider before deciding how wide to make the ex port is how much you raised it. If you raised it alot, say 5 degrees or more, maybe you don't widen it out so much. If only raised a little bit, maybe you push the envelope on width a little bit. I don't really see any downside to widening the exhaust port other than a bit more wear on the rings.

 

[mdavlee]

I've always wondered about outlet size. I see some guys try to keep the port walls straight so outlet flange opening is the same shape and size as it is at the cylinder wall. Others I see open the flange up as far as possible, most porters on here I see land someplace in between. Is there a guide or rule of thumb as to how far you should bell an exhaust port if you should do it at all.

I like it to be bigger past the outlet at the cylinder wall. Hot gasses are still expanding.

 

[spencerpaving]

View attachment 68310

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

That's a really nice port...gradually increasing away from the cylinder...a flat roof is optimal but in the real world it's not good on rings...when exhaust leaves the cyl it's hot and wants to expand..with the right shape the gas will pick up speed as it expands hopefully causing a slight vaccum in the cyl from momentum thus helping to suck up charge though the Transfers...jmho

 

[RIDE-RED 350r]

I like it to be bigger past the outlet at the cylinder wall. Hot gasses are still expanding.

A reverse funnel effect, very low if any restriction....till you get to the muffler.

Another thing to consider and there is probably no way to determine this... at what point in the combustion cycle is the optimum point to start allowing those expanding gases out the exhaust? As combustion happens, pressure builds and drives the piston down. So where is the point that expansion starts to diminish and ceases to make power and should be allowed out the exhaust port? I have no idea how to determine such a thing. But I would think you still want some expansion/pressure to get things flowing out the exhaust.

 

[Magic_Man]

A reverse funnel effect, very low if any restriction....till you get to the muffler.

Another thing to consider and there is probably no way to determine this... at what point in the combustion cycle is the optimum point to start allowing those expanding gases out the exhaust? As combustion happens, pressure builds and drives the piston down. So where is the point that expansion starts to diminish and ceases to make power and should be allowed out the exhaust port? I have no idea how to determine such a thing. But I would think you still want some expansion/pressure to get things flowing out the exhaust.

That's the million dollar question we are all hoping to answer I believe.

 

[TreeLife]

So.......we really haven't found any real solid answers to the question. "What port heights should I use?"

OK........what about width?

Shape?

Outlet size?

Finish?

Square, beveled inlet.

Outlet, probably slightly smaller than the inlet for velocity.

Finish? Mirror polished. The intake should be rough though

 

[spencerpaving]

I tried this design based on the old megaphone exhaust on snowmobiles in the early 70s....it works well...but it takes a ton of fuel to feed it...oh and very loud....not good for a work saw...but it gives a good idea

Sent from my SCH-I545 using Tapatalk

 

[srcarr52]

As far as shape you have to address that the exhaust is making a 90 deg turn to exit the cylinder so the more you can do to lessen this angle the better. I like the idea of the step exhaust port floor to allow the exhaust to make the turn a little more gradually. Early Stihl cylinders had a more divergent exhaust port floor than most other saws of the era, other manufactures caught on like the Dolmar 9000/9010 and now we see tilted back cylinders on Husky 550/562 to lessen the exhaust exit angle.

 

[mdavlee]

I tried this design based on the old megaphone exhaust on snowmobiles in the early 70s....it works well...but it takes a ton of fuel to feed it...oh and very loud....not good for a work saw...but it gives a good idea

Sent from my SCH-I545 using Tapatalk

I like the exhaust port shape.

 

[Stump Shot]

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.

Excellent point! I would expand upon that just a little bit not to derail but to go with your statement and backing up as a place to start, that for me; the first place I like to start with any type of engine, is with what is there. In other words, do the finish detail work that the factory could not spend the time on, bearings are perfect and resistance free, casting defects are removed, crankshaft is well centered to the bore of the cylinder etc., etc., so everything is true, square and right as rain. This allows free power that would otherwise be wasted to be utilized and considered a gain over what the factory was able to produce, not design. OK enough about that back to exhaust ports...

 

[spencerpaving]

One of these days I'm going to weld up the exhaust port on the sides...keep the standard port width and add boost ports on the side at the top but only half the duration. ..then it would have 3 ex ports....possibly this summer on a project saw

 

[Stump Shot]

Question; Lets say we have new saw that has no history and unfamiliar design as compared to what you're used to. Where to start... Mention was made that the setting of the squish band to it's desired clearance after widening the squish band to gain desired compression will yield the desired exhaust port height in degrees of crankshaft rotation. So, are we generally trying to return to our original height before other alterations were made, or are we also trying to improve this from the original factory height? So if originally the number of degrees is close to your chart at the beginning of this thread stay with that, if not close to the chart shoot for what's on the chart? So say the cylinder was dropped .040", do I want to gain that .040" back in the exhaust port automatically? Or what other factors come into play to a different decision? In other words, how do we go about picking this starting number to shoot for for exhaust port height. Also how close should we be + or - to be in the ballpark.

 

[RIDE-RED 350r]

Stump, I would say that initially and most times, you would want bring exhaust at least back to stock timing after making a compression mod be it cutting squish and base, or deleting the base gasket.

 

[Stump Shot]

Sorry if it sounds like I have a thousand questions, it's only because I really have 10 thousand questions. Lol

Question; So if raising the exhaust port height, raises the RPM of the peak power band, while also lessening the amount of trapped fuel/air mixture. Are we really shooting for this peak part of the power band, and if so, if gone past, will this seriously diminish power because of the lost trapped air/fuel mixture, or is it because the peak power band is now unobtainable or both? In the same respect the act of dropping the exhaust like in a gasket delete or base cutting, lower the peak power band and increase grunt or torque by having a larger air fuel charge to burn? This is seaming like a more serious decision for the application of the power head if any type of success is to be achieved.
Please bear with me guys, I'm almost there on "getting it", I can feel it. Lol