Part Two: The Intake Port

[XP_Slinger]

Exactly Stump Shot. If the intake is open too long case compression will not build to an efficient level resulting in a lazy charge entrance as it comes through the transfers. I get why some guys trim the skirt but it's very easy to go too far. In my limited experience, a base gasket delete will net you around 3 degrees of additional intake time if I recall correctly. That's just lowering the cylinder by roughly .020" give or take. Now imagine you're trimming your skirt so it's not protruding into the port window at TDC ( not a problem I've seen yet), lets say its 1/8" that you're trimming off, that's .125". Translate that to time gained in degrees using ".020 = 3 degrees gained" and you would have gained a whopping 18.75 degrees of intake time. Translation = Intake closing much later producing what would most likely be very poor case compression. What is the right intake time to get the right case (primary) compression? Question of the ages! Different with every saw most likely but I'm speculating on that. Keep in mind that the trimming number I used as an example is highly impractical but I hope I've helped further illustrate the importance of Intake Time vs Area.

 

[Mastermind]

Intake Time vs Area.

And that brings us to another good topic.

If we decide that we want to run a short intake duration (less time), can't we just use bigger port to get more area?

I remember someone once saying........"If we don't have time, we gotta have area".

Thoughts?

 

[Mastermind]

I use the camshaft as an example.

Port height is duration, and size is lift.

When I built SBCs I used short duration, high lift cams in light weight street cars and four wheel drive trucks.

 

[RIDE-RED 350r]

Oh boy! Part II here we go!!


I'll be in here later when I get home

 

[tree monkey]

And that brings us to another good topic.

If we decide that we want to run a short intake duration (less time), can't we just use bigger port to get more area?

I remember someone once saying........"If we don't have time, we gotta have area".

Thoughts?

a small carb needs more time to fill the case then a large carb. adding width to the port will not help fill the case if using a small carb.

 

[XP_Slinger]

And that brings us to another good topic.

If we decide that we want to run a short intake duration (less time), can't we just use bigger port to get more area?

I remember someone once saying........"If we don't have time, we gotta have area".

Thoughts?

That logic seems sound to me. However, we are limited by our rubber intake boots and such with how big the intake tract can be made before pickup truck turbulence is created. Strange analogy I know but that's how I see the charge flowing when it goes from an intake boot that is more narrow than the intake it's connected too. When air flows over the roof of the cab of your truck it creates lift in the bed right behind the cab. A little vortex action.
A contradicting line of my own thinking is that a slightly enlarged intake tract beyond the rubber boot would create a "plenum" of sorts. Storing atomized fuel and air for immediate ingestion as soon as the intake cracks open. Would certainly slather a little oil on the intake skirt of the piston lol

 

[XP_Slinger]

a small carb needs more time to fill the case then a large carb. adding width to the port will not help fill the case if using a small carb.

Can you define small? Is small simply the stock carb whereas a large carb would be an upgrade like putting a 394 carb on a 288? thanks

 

[Stump Shot]

So is it like the garden hose analogy, by adding a larger diameter hose to an existing one, only slows down the water out of the end of the hose, because no more water could flow through the first hose that the second larger hose could handle?

 

[Dolmar Junkie]

I don't feel like I graduated part one yet. Lol

( Feel free to skip this long winded post,sorry if not appropriate. )Me neither, but that is the best part about this approach, if I was standing beside him and he showed me exactly how to do it. I would probably not retain enough of it to have a clue (kinda slow like that but gimme a break I got a metal plate in my head) with the thread I can go back and study it until I get it and if I stihl don't retain it some threads are detailed enough to basically use as a step by step instruction manual. Don't forgot the pictures,you know I love a storybook with pictures... And since Apprenticeships are basically a thing of the past with the exception of maybe Europe or Japan? This is the kind of place I seek knowledge. I joined this forum for these types of mind expanding adventures. I have been researching tools and plan on buying at least the basic necessities, but I told another member the other day that I was even thinking about a " inexpensive used or entry level mill, like
Something from Grizzly or? Perhaps it's a mid life crisis or maybe gas? But it is an appealing hobby if nothing else.
So Thank you Randy and any of you other builders for providing this type content Matt....

 

[Stump Shot]

( Feel free to skip this long winded post,sorry if not appropriate. )Me neither, but that is the best part about this approach, if I was standing beside him and he showed me exactly how to do it. I would probably not retain enough of it to have a clue (kinda slow like that but gimme a break I got a metal plate in my head) with the thread I can go back and study it until I get it and if I stihl don't retain it some threads are detailed enough to basically use as a step by step instruction manual. Don't forgot the pictures,you know I love a storybook with pictures... And since Apprenticeships are basically a thing of the past with the exception of maybe Europe or Japan? This is the kind of place I seek knowledge. I joined this forum for these types of mind expanding adventures. I have been researching tools and plan on buying at least the basic necessities, but I told another member the other day that I was even thinking about a " inexpensive used or entry level mill, like
Something from Grizzly or? Perhaps it's a mid life crisis or maybe gas? But it is an appealing hobby if nothing else.
So Thank you Randy and any of you other builders for providing this type content Matt....

Feels like I have a quarter million dollar Ferrari sitting in the driveway and I can't find the darn keys to actually drive it. Lol

 

[Al Smith]

Another work saw builder here .I might widen them a tad bit and taper them outward inside the crankcase but I don't cut the piston nor shave the bottom of the piston skirt .In my way of thinking lowering the cylinder opens the intake up enough .

 

[Mastermind]

a small carb needs more time to fill the case then a large carb. adding width to the port will not help fill the case if using a small carb.

Thanks for sharing Scott. In my humble opinion you are the "Godfather Of Port Work".

I think you've forgotten more about this stuff than I will ever know.

 

[Hedgerow]

a small carb needs more time to fill the case then a large carb. adding width to the port will not help fill the case if using a small carb.

Time x Area...

If you got's no area, you need some time.

But if your saw has too much time on it's hands, it'll mis-behave.

Best to give it some area and keep it's time on a leash.

 

[XP_Slinger]

Time x Area...

If you got's no area, you need some time.

But if your saw has too much time on it's hands, it'll mis-behave.

Best to give it some area and keep it's time on a leash.

I took a screen shot of that. Great way to remember it

 

[RIDE-RED 350r]

Being that we are working with little engines that have no form of intake valve (reed or rotary) and no tuner pipes (well, for the most part, we are mainly talking work saws here) we are pretty limited on how aggressive we can go with the intake. If there were an intake valve of some kind that closes and traps that charge in the case the instant the piston starts moving downward rather than approaching closure of the intake port itself, we could really have some big power mills. Think about the HP a 125cc dirt bike makes compared to a 3120. Yeah there is more than just intake valves and tuner pipes involved, but it certainly plays a role I think. But I digress a bit and we have what we have.

I understand the intake enough to know that we don't want too much duration for fear of low case compression inevitably creating a slow and lazy transfer action as well as some loss of fuel atomization. But at the same time we want that case to get a good gulp of fuel/air too. All I can really say with confidence on this subject is that generally speaking most of us are looking at between 70 and 82 degrees BTDC plus or minus a degree or two for the most part. I can definitely understand why we need to be cautious not to give it too much intake duration. Besides freeporting, transfer is the first in line to suffer a loss in performance and efficiency. How to determine what is too much or how far one can take it, I do not yet fully know. Standing by to listen and learn.

 

[Mastermind]

I'm fixin to go spray 2-4-D on a couple of fields.

Y'all keep em in line.

 

[Dolmar Junkie]

Feels like I have a quarter million dollar Ferrari sitting in the driveway and I can't find the darn keys to actually drive it. Lol

Let's hot wire it, who cares if we blow it up you only made one payment. Theoretically speaking...

 

[Dolmar Junkie]

I'm fixin to go spray 2-4-D on a couple of fields.

Y'all keep em in line.

Hay fields?if so what you cutting for cool season grass? Orchard grass, Timothy, Some really nice Johnson grass...

 

[Chainsaw Jim]

Why not drop the floor, cut and window the skirt, and then poke a reed valve somewhere in there?

 

[mdavlee]

I like to keep a narrower intake with around 160° of duration in most work saws. If they go to 170° on some I can deal with that. 180° on a few was too much and they were lazy. I do like to put bigger carbs on when possible without too much work.

A reed valve let's you have crazy duration without spitback. I want to say the 880 is somewhere around 190° total with a reed valve.