Part One: The Exhaust Port

[David Young]

Piston velocity is pretty similar to a sine wave. So the highest velocity is when the crank pin is at 90 or 270 deg and it has 0 velocity at TDC and BDC (0 and 180 deg).

With maximum piston speed at 90 degrees btdc and atdc. The closer to 90 means it opens and closes faster? So the exhaust is covering more linear distance in a shorter period of time then when the transfers open.

Does this mean closer to 90 degrees port opening the arc matter less?

 

[srcarr52]

With maximum piston speed at 90 degrees btdc and atdc. The closer to 90 means it opens and closes faster? So the exhaust is covering more linear distance in a shorter period of time then when the transfers open.

Does this mean closer to 90 degrees port opening the arc matter less?

I think at the speed the piston is traveling when it passes the exhaust port the arch of the roof is almost negligible. The corner radius would have a bigger effect on the time area than arch, still I think you could substitute a deg of exhaust timing for a rounder corner and not be able to tell the difference.

 

[RIDE-RED 350r]

I think you have it backwards. For one thing, the larger the port is that the rings have to pass over, the more need for an arc to it. I would surmise the reason transfer ports are not arced is due to the fact that they are much smaller in size than the exhaust port. Having an arc to the window is not as crucial. The closer to TDC, the less linear piston movement per degree of crankshaft rotation.

 

[mdavlee]

I think at the speed the piston is traveling when it passes the exhaust port the arch of the roof is almost negligible. The corner radius would have a bigger effect on the time area than arch, still I think you could substitute a deg of exhaust timing for a rounder corner and not be able to tell the difference.

Probably 2 or 3 degrees.

 

[Deets066]

Well, linear speed of the piston does change. It is slowest when at and near TDC and BDC, fastest halfway between the two.

As to the mirror polishing of the exhaust, I think there may very well be some benefit to it, but I also think it is temporary unless one cleans the carbon from the exhaust tract regularly. Carbon inevitably will start to accumulate no matter how exotic the premix oil one uses or the mix ratio. Some oils are cleaner than others, but in the end carbon deposits will start to form. That is the main reason I have never tried it on my 3 work saws I have ported. Maybe a different story on a race saw that only sees racing action and regular cleaning of carbon is reasonably expected to be done.

Carbon doesn't seem to build up with a mirror finish on the ex port. My 066 has at least 30 tanks through it with 0 carbon on the port

 

[huskihl]

Carbon doesn't seem to build up with a mirror finish on the ex port. My 066 has at least 30 tanks through it with 0 carbon on the port

Mine either.



OIL THREAD!!!!!

 

[RIDE-RED 350r]

Carbon doesn't seem to build up with a mirror finish on the ex port. My 066 has at least 30 tanks through it with 0 carbon on the port

So the principle then is that with a polished finish, there isn't anything for the carbon deposits to grab on to in the first place?

 

[Deets066]

So the principle then is that with a polished finish, there isn't anything for the carbon deposits to grab on to in the first place?

That's the gain I see in polishing the ex

 

[RIDE-RED 350r]

Hmm.. I hadn't really thought of it that way. I would surmise that like me, you run a bit more oil than 50:1 in your ported saws too??

 

[Deets066]

Hmm.. I hadn't really thought of it that way. I would surmise that like me, you run a bit more oil than 50:1 in your ported saws too??

32:1 klotz

 

[Stump Shot]

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

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

Just got the 51 back from being demo'd, so I drug the Tach out to see what was what. It's tuned a little fat at 11,500 no load RPM and is turning 10,000 RPM cutting that same log like I was in the video. Max is 12,500 for this saw, might could be leaned out just a hair yet after it's broken in.

 

[RIDE-RED 350r]

I run 32:1 as well... Husky brand.

Maybe I'll have to give that polished exhaust a whirl.

 

[RIDE-RED 350r]

...Max is 12,500 for this saw...

Maximum RPM????

 

[Stump Shot]

Maximum RPM???? View attachment 68480

Per Husqvarna. Lol Just showing that what I did to the saw did nothing to alter the top no load speed, but the cut speed, maybe not so bad.

 

[RIDE-RED 350r]

Per Husqvarna. Lol Just showing that what I did to the saw did nothing to alter the top no load speed, but the cut speed, maybe not so bad.

I gotcha... Just having a little fun... That's what popped into my head when I read your post

 

[paragonbuilder]

Okay so back to port shape for a second time. If some curvature is required on the port roof at the cylinder wall to prevent ring snags, why not match the edge of the piston to match the curvature of the port, thus allowing the port to open wide instantly rather than gradually with the port curvature? Feasible?

I've thought a lot about doing this.
@Terry Syd mentioned it a few months back.


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[Deets066]

I like a nice even, wide squish band. Not sure it would be worth the gain when you can get the roof relatively flat anyway

 

[Terry Syd]

If you have an engine that needs a fair bit of blowdown, it works quite well. The curvature of the top of the piston to the curvature of the exhaust port roof gives it the equivalent of about 2 degrees extra blowdown. So, if you have an engine with 12 degrees of blowdown, re-working the piston can make the engine think it has 14 degrees of blowdown. However, if you have an engine that already has good blowdown - there ain't nothing in it (just a lot of fiddly work).

The reason it works so well in those situations is that the opening is not only INSTANT , but it is even BIGGER than a flat opening. The shortest distance between two points is a straight line. The curvature on the piston is a curved line - it is LONGER.

The longer window upon opening is just like widening the port, it gives it more area to flow. The first part of the opening of the exhaust port is where most of the flow occurs as that is where the pressure differential is the greatest. I've read that most of the blowdown occurs in the first 10% of the port opening, but I've never been able to confirm that. However, considering the pressure differential, I wouldn't be surprised if it was true.

 

[paragonbuilder]

If you have an engine that needs a fair bit of blowdown, it works quite well. The curvature of the top of the piston to the curvature of the exhaust port roof gives it the equivalent of about 2 degrees extra blowdown. So, if you have an engine with 12 degrees of blowdown, re-working the piston can make the engine think it has 14 degrees of blowdown. However, if you have an engine that already has good blowdown - there ain't nothing in it (just a lot of fiddly work).

The reason it works so well in those situations is that the opening is not only INSTANT , but it is even BIGGER than a flat opening. The shortest distance between two points is a straight line. The curvature on the piston is a curved line - it is LONGER.

The longer window upon opening is just like widening the port, it gives it more area to flow. The first part of the opening of the exhaust port is where most of the flow occurs as that is where the pressure differential is the greatest. I've read that most of the blowdown occurs in the first 10% of the port opening, but I've never been able to confirm that. However, considering the pressure differential, I wouldn't be surprised if it was true.

So would it be fair to say that if you're going to port a saw, and didn't need the extra blow down, you could leave the exhaust a couple degrees lower and shape the piston to the port, and possibly gain some power.
I've been planning to do this for a while, I need some time...


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[Terry Syd]

So would it be fair to say that if you're going to port a saw, and didn't need the extra blow down,

I don't understand the question. If you don't need the extra blow down, then the mod doesn't do anything. I tried it on a couple of racing bikes that already had significant blow down and noticed no change, nada, zip, zero.

If you do need the extra blow down, then the mod can work. Instead of raising the exhaust port two degrees, you shape the piston crown to the curvature of the exhaust port roof. That gives you the needed blow down PLUS you retain two more degrees of TRAPPING EFFICIENCY and two more degrees of the POWER STROKE.

It takes a fair bit of work to get the shape right. You shine a bright light in the spark plug hole and then use a flat needle file to trim the edge of the piston. Just keep trimming the dark spots until the entire port cracks open at the same time.

I like a nice curve on the top of the exhaust port. By using this mod, I can use the curve I want and actually gain more port area on opening than if I used a flat roof.