Real talk about saw porting theory (no arguments)

[wcorey]

Does anyone else feel that strato saws need to be approached differently? Seems to me from staring at them that adding fuel into the strato ports is 2-fold. One being increase life of saw by bringing a bit more oil up top, two being if you're upping performance you can take advantage of using it like a larger intake.

Seems to me that all the fuel/mix that goes in through the crankcase has to go through the top also, however if you split it to route some through the stratos then you just have less through the crankcase where it's most needed.

And it is a larger intake either way, still can have more air going through, the air/fuel ratio stays the same either way by having a richer/leaner mixture in the carb tract. With the strato the final mix just happens further downstream and hence the "stratification".

I see the possible advantage strato has, whether combined or not, is in how it routes the incoming charge into the combustion chamber as compared to traditional transfers.

 

[Brian Carlson]

I really love all this porting talk. I am new to porting as a hobby and to help some friends get a little more out of their saws. All the ideas and techniques are greatly appreciated. Keep it coming guys.

 

[EvilRoySlade]

Bill, that's a very interesting point about stealing some lubrication from the bottom end. I'm working on a 365xt that I'm going to use the stratos for part of the charge. We'll see how it goes, baby steps for me on this one

 

[XP_Slinger]

Wouldnt having too little blowdown also cause exhaust to enter the upper transfers? In my mind, a lot of the scavenging of exhaust takes place as the exhaust port is opening as the rapid escape of pressure would "suck" exhaust from the cylinder. There will still be some but there should be very little remaining exhaust that would need to be pushed out by the incoming transfer charge. And with that thought, if blowdown is too short, there would be more pressure in the cylinder for the transfer charge to fight against and could lead to back feeding to the uppers.
Another issue with short blowdown would be the lack of time the crank case has to pressurize. Which of course would put the new charge at a disadvantage to overcome any pressure remaining in the cylinder when the transfers open.
I'm not saying that insufficient blowdown would be the only cause of back feeding, but in my head it seems that it could be a major player. Much like an intake that closes too late. If I'm off on this please let me know...always ready to learn.
Any body have numbers from the saws that had carbon build up in the uppers? I'd be curious to know how much blow down they had.

 

[XP_Slinger]

Bill, that's a very interesting point about stealing some lubrication from the bottom end. I'm working on a 365xt that I'm going to use the stratos for part of the charge. We'll see how it goes, baby steps for me on this one

I ported my 372 X-torq last year and it turned out very strong. If you gut the strato system I would recommend leaving the carb divider in place. Tuning adjustments weren't as responsive with it removed. If I could do it again I would leave my saw with about 22 degrees of blowdown...but it still runs strong at 98/116/78. Like guys say all the time, you learn by experimenting...happy grinding!

 

[mdavlee]

Wouldnt having too little blowdown also cause exhaust to enter the upper transfers? In my mind, a lot of the scavenging of exhaust takes place as the exhaust port is opening as the rapid escape of pressure would "suck" exhaust from the cylinder. There will still be some but there should be very little remaining exhaust that would need to be pushed out by the incoming transfer charge. And with that thought, if blowdown is too short, there would be more pressure in the cylinder for the transfer charge to fight against and could lead to back feeding to the uppers.
Another issue with short blowdown would be the lack of time the crank case has to pressurize. Which of course would put the new charge at a disadvantage to overcome any pressure remaining in the cylinder when the transfers open.
I'm not saying that insufficient blowdown would be the only cause of back feeding, but in my head it seems that it could be a major player. Much like an intake that closes too late. If I'm off on this please let me know...always ready to learn.
Any body have numbers from the saws that had carbon build up in the uppers? I'd be curious to know how much blow down they had.

Stock 346 will do it. 18 degrees or so if I remember right.

 

[Mastermind]

Some older 046s had as little as 14° of blowdown. They also had very open flowing mufflers.

 

[XP_Slinger]

So instead of increasing blowdown by raising the exhaust I wonder if back stuffing could be decreased by a muffler mod accompanied by widening the exhaust port. Or just one or the other? I would think increasing port area (widening) would improve the efficiency of a short blowdown period. My 372 has what I consider to be a short blowdown time when compared to other numbers I've seen. But I didn't see any evidence of back stuffing the last time I had it apart. I assume it's because of the widened exhaust and MM.

 

[mdavlee]

18 degrees are pretty normal on a 372.

 

[XP_Slinger]

18 degrees are pretty normal on a 372.

Thanks Mike...good info. So much for my thought on exhaust efficiency...lol

 

[mdavlee]

Thanks Mike...good info. So much for my thought on exhaust efficiency...lol

I've went as short as 15 on them.

 

[MustangMike]

Some older 046s had as little as 14° of blowdown. They also had very open flowing mufflers.

How did it run like that Randy? The 460/046-D you did for me is a very strong saw.

 

[mdavlee]

How did it run like that Randy? The 460/046-D you did for me is a very strong saw.

Those were usually the very early cylinders

 

[MustangMike]

Those were usually the very early cylinders

Weren't the early cylinders also supposed to be very strong? I believe Brad's Dad had one.

 

[mdavlee]

Weren't the early cylinders also supposed to be very strong? I believe Brad's Dad had one.

Yes. Short blowdown works on them. Long blowdown will too. Just depends on who's porting it.

 

[XP_Slinger]

Me brain is thinking that with a short blowdown a lower (numerically higher) exhaust/transfer opening would be better...gives the intake more time to close and build higher pressure in the crank case to combat the higher pressures remaining in the cylinder when the transfers open. Yes no? I'm trying

 

[Keith Gandy]

Weren't the early cylinders also supposed to be very strong? I believe Brad's Dad had one.

My buddy John, (LaslabJohn), had a 1st yr thats very strong

 

[mdavlee]

I've seen a pattern of soot in the exhaust port from flow. The bigger exhaust time would lower the pressure in the cylinder.

 

[XP_Slinger]

I've seen a pattern of soot in the exhaust port from flow. The bigger exhaust time would lower the pressure in the cylinder.

Exactly what I was thinking...thanks.

 

[EvilRoySlade]

I ported my 372 X-torq last year and it turned out very strong. If you gut the strato system I would recommend leaving the carb divider in place. Tuning adjustments weren't as responsive with it removed. If I could do it again I would leave my saw with about 22 degrees of blowdown...but it still runs strong at 98/116/78. Like guys say all the time, you learn by experimenting...happy grinding!

I have read and thought about that build very much in the last few months.