Real talk about saw porting theory (no arguments)

[XP_Slinger]

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

Glad you enjoyed it and I hope it's been helpful...it was fun.

 

[XP_Slinger]

So here's a question...how many degrees before the transfers opening should the intake be closed? There's lots of talk about crank case pressure and I'm wondering what timing is needed to make the saw happy. I don't remember what my saws have otherwise I'd share...just another question to hopefully keep this thread rolling.

 

[mdavlee]

30 minimum is what I go for but prefer 34-40

 

[Al Smith]

Just depends on how you look at blow down .If say you crank 2000 more RPMS out of the engine you've changed it's design parameters .Port time open .It has less of time to do what it normally did at lower speeds so it either has to stay open longer or increase in size.If you hog it out too much larger you decrease the differential pressure which isn't usually good.

So take many Stihls you have something like 20-22 degrees of blow down stock exhaust port height .So you deck the cylinder and maybe drop the ex by 2-3 degrees to say 102 103 after .You've already raised the comp and lowered the intake ,the transfer remains the same only in a slightly different portion of rotation.Now say you raise the ex port to stock being 100 maybe you've added 2-3 degrees more blowdown . I've raised them to 96-97 after which doesn't kill the torque as far as I've seen .Now you got around 27 or so blowdown that purges the cylinder better ,Yes some short circuits right out the exhaust,so what .You pick up the speed .If you like a more torquey engine leave the ex long or split the diff .

Just remember it's easy to remove metal and quite the opposite trying to put it back on.It's not a race .---works for me ---

 

[Keith Gandy]

30 minimum is what I go for but prefer 34-40

Thats what I was thinkin. 75 to 80 on intake and 117 to about 125 on transfers

 

[jmssaws]

The 036 I ported yes had 15 degrees of blow down stock but I have no idea how it ran,kinda wish I woulda tried it.

Blow down for me has done a complete opposite,I use to run 20+ on quads and less than 20 on duals but now the only quad I have done in a long time with more than 20 degrees is a 7900 and now I generally use 20+ on duals.

I always heard to run more on quads but for me I find the opposite to be true.
I've timed some stock 395 cylinders with 16 degrees of blow down.

 

[XP_Slinger]

Here goes my brain again...I think a cut chamber would increase the efficiency of a short blow down because of the higher pressure that's being generated. Thus exiting the exhaust port with greater speed.
I'm really enjoying hearing all the different methodologies on this. It's really bringing my level of understanding up a notch from where I was. Maybe I'll work up the courage to ask about fingers soon...lol

 

[jmssaws]

A chamber cut will certainly help with short blow down because your moving the transfers down and then raise the exhaust back up.

 

[jmssaws]

Putting a 056 mag11 cylinder on a 1122 you have to take the cylinder down as far as you can to gain bd to get it to run.
Or at least the few cylinders I've done were this way.
13-14 degrees of blow down stock and 20 is all I could get without raising the exhaust to high.

 

[MustangMike]

The first one I ever saw, and I got to run it at my first GTG, is the one that Bret did. I was very impressed. Their normal cc 660 also ran very strong.

I remember watching Bret cut Black Locust with it, and the Rooster Tail of chips was about 20 feet long.

 

[jmssaws]

The first one I ever saw, and I got to run it at my first GTG, is the one that Bret did. I was very impressed. Their normal cc 660 also ran very strong.

I remember watching Bret cut Black Locust with it, and the Rooster Tail of chips was about 20 feet long.

They run good but it takes some doing.
Muffler is a chore.

 

[MustangMike]

And as Lee famously stated "And it is still not over 100 cc"!!!!

 

[jmssaws]

And as Lee famously stated "And it is still not over 100 cc"!!!![/QUOTE
Lot of work for no more difference between them and a 066 but they are Kool and will flat grunt.
I talked to Bret about this and we agreed stock timing must vary because his was timed better stock than the one I had.
I have 2 more mag11 p&c but haven't checked timing yet.

 

[jmssaws]

I have no idea how I just did that.^^^^

 

[Keith Gandy]

I have no idea how I just did that.^^^^

U started typing before the post had fully loaded

 

[Keith Gandy]

Or inserted in the middle of the previous post

 

[Time's Standing Stihl]

I have no idea how I just did that.^^^^

You should check timing in mine, see if there is any difference

 

[jmssaws]

You should check timing in mine, see if there is any difference

It stands to reason that there all short blow down because there made to run at a much lower rpm but I'm wrong all the time so I hope I am now.

It takes quite a bit of work to check the timing on one.

 

[EvilRoySlade]

How many of you consider trapped compression as opposed to just standard CR. I know it's discussed in the books and viewed different between Bell and Jennings. I can understand the logic that position of exhaust and transfers will change the trapped CR but the engines standard CR stays the same. That being said, 2 identical engines, one with higher exhaust, will most likely run different but the CR is the same. It then begs the question that a cylinder may be adapted to deal with whatever crankcase compression you given. Vague info but I hope you get my drift.

 

[srcarr52]

I always consider the corrected compression ratio, which is the compression ratio of the volume trapped above the exhaust port.