044 project

[MattG]

+1. Flatten the floor of the intake port too.

Sorry Brad, and anyone else for that matter. But why exactly? Are you saying this because seeing as the piston skirt bottom is cut straight, then by straightening the port floor you are maximising the delivery of mixture once the opening angle is reached?

I'm not trying to contradict anyone here.... just looking for the rationale involved.

Anyway if my (above) guess is correct, why not actually cut at the bottom of the skirt and thus match to the inlet port floor that way?

I'm just wondering, like why did Stihl go to the bother of curved port roof and floors, there will be a reason, Stihl being who they are. Presumably it is for good flow.....it can't be for ring life can it? The rings don't pass the inlet floor do they?

Sorry for the excess of the question marks.....I'm trying to write this out fast....wanna go away and order some parts in a minute!

 

[blsnelling]

Flat is always the most efficient. You achieve maximum area and flow as early as possible. However, the exhaust can't be flat because the rings ride over it and would snag. That is not the case with the intake floor.

 

[MattG]

Flat is always the most efficient. You achieve maximum area and flow as early as possible. However, the exhaust can't be flat because the rings ride over it and would snag. That is not the case with the intake floor.

Ok fair enough, see what you mean, I guess I was just curious kind of wondering essentially what the difference between putting the same profile curvature on the bottom of the skirt (as there is on the inlet floor), and getting the maximum area open once the open angle happens that way, compared to the slightly more orthodox way of flattening the port bottom to match the stock skirt profile.

Some people do cut skirts don't they?

Thanks for the tip, anyway.

 

[MattG]

I did find some gossip on good ol' AS, on the subject of skirt cutting

https://www.arboristsite.com/community/threads/milling-down-the-piston-skirt.81785/

but didn't get around to seeing if anyone was mad enough to contemplate matching it's curvature to the (inlet) port's floor.

 

[blsnelling]

I've trimmed the intake skirt on many saws, but always kept it flat.

 

[MattG]

I've trimmed the intake skirt on many saws, but always kept it flat.

Yeah, I know what you mean Brad. I'll probably play it safe TBH and just flatten the floor a little as you suggest earlier.

 

[MattG]

So I just saw this on AS: from drf255, who I'm guessing is @drf256 on here

You increased case volume. You didn't touch the intake floor, thank god, because that's where you build case compression. So you made less velocity/inertia in the intake port. Make sure your skirt edges and bottom aren't freeporting in the intake and exhaust.

It's from https://www.arboristsite.com/community/threads/im-a-porting-idiot.305197/page-2#post-6123873

So what's the about "not touching the intake floor"? Some of you guys are talking of dropping the floor to 76* (and MMs ms460 spec sheet see #317 even mentions dropping it to 82* !).

Surely we don't mean "not touch the intake floor" - we just mean don't over do it?

Sorry to obsess a bit about all this stuff - I just like to get things properly thought through before attacking a brand new cylinder with my dremel!

 

[MattG]

Another note-to-self:

https://www.arboristsite.com/community/threads/port-shape-summary.212378/

 

[Wonkydonkey]

Oh, I just read the next page of your link @MattG page 4, I'm a porting idiot,
I,m glad the I don't read that site much,

 

[Adamski]

Oh, I just read the next page of your link @MattG page 4, I'm a porting idiot,
I,m glad the I don't read that site much,

yeah the stuff gets 'fun' real quick on some threads...

 

[drf256]

So I just saw this on AS: from drf255, who I'm guessing is @drf256 on here

You increased case volume. You didn't touch the intake floor, thank god, because that's where you build case compression. So you made less velocity/inertia in the intake port. Make sure your skirt edges and bottom aren't freeporting in the intake and exhaust.

It's from https://www.arboristsite.com/community/threads/im-a-porting-idiot.305197/page-2#post-6123873

So what's the about "not touching the intake floor"? Some of you guys are talking of dropping the floor to 76* (and MMs ms460 spec sheet see #317 even mentions dropping it to 82* !).

Surely we don't mean "not touch the intake floor" - we just mean don't over do it?

Sorry to obsess a bit about all this stuff - I just like to get things properly thought through before attacking a brand new cylinder with my dremel!

The intake floor and the exhaust roof are where the ports functionally close off to do their thang.

Case compression of charge begins when the piston closes off the intake port and before the transfers open. You generally want a minimum of around 40* of case compression.

 

[Adamski]

The intake floor and the exhaust roof are where the ports functionally close off to do their thang.

Case compression of charge begins when the piston closes off the intake port and before the transfers open. You generally want a minimum of around 40* of case compression.

So if you keep the numbers the same then it is logical to just flatten the intake floor as much as possible. Gain maximum flow as soon as the port opens?

 

[MattG]

So if you keep the numbers the same then it is logical to just flatten the intake floor as much as possible. Gain maximum flow as soon as the port opens?

Ha ha! Maybe.........I think you'd need a either a wind tunnel or a computer simulation to be quite sure.

Check out #241

There's a PhD in fluid dynamics in there if you look hard enough.

Mechanically speaking ports need to comprise between i) production costs ii) ring life and iii) air flow.

Straight/flat roofs floors - you're a CAD boy are they cheaper? However flat floors/roofs will torture (and then snap rings). I know I tuned a 175cc motorbike nearly 30 years ago. New top ends was needed every 400 miles since I made the exhaust marginally too wide, with insufficient rounding/bevelling. Sure it won't matter for port edges where there's no ring movement (inlet floor). But air flow wise, why should a fluid want to flow quicker through a square section, or round or oval section. Lots of considerations there, I guess, but to match a straight piston skirt presumably a flat floor is easier.

 

[MattG]

The intake floor and the exhaust roof are where the ports functionally close off to do their thang.

So guessing that drf255 is you, then, you were warning the OP in that AS thread that he was fortunate not to have dropped the floor by too much. Because if he had, he could have inadvertedly lost case compression.

Case compression of charge begins when the piston closes off the intake port and before the transfers open. You generally want a minimum of around 40* of case compression.

Ok help me out here, so I don't get confused, I'm not looking at an engine right however, by my earlier numbers.

of 103/122/72.....

does that mean that I've currently got 122 - 72 = 50 degrees of case compression? Sorry to be seem stupid, I'm just trying to get my head around BTDC/ATDC and piston crown and skirts etc. (whilst staring at my office-based day-job!)

 

[drf256]

So if you keep the numbers the same then it is logical to just flatten the intake floor as much as possible. Gain maximum flow as soon as the port opens?

Yes. It’s port area x time = flow. Most intake flow occurs on the floor. But the wider and squarer the port is made, the more the piston skirt wears. The thrust area on a piston is mostly on the intake skirt, it’s where you will find the most wear on any piston in a saw.

So guessing that drf255 is you, then, you were warning the OP in that AS thread that he was fortunate not to have dropped the floor by too much. Because if he had, he could have inadvertedly lost case compression.


Ok help me out here, so I don't get confused, I'm not looking at an engine right however, by my earlier numbers.

of 103/122/72.....

does that mean that I've currently got 122 - 72 = 50 degrees of case compression? Sorry to be seem stupid, I'm just trying to get my head around BTDC/ATDC and piston crown and skirts etc. (whilst staring at my office-based day-job!)

Yes, you’ve got 50* of case compression. And you’ve got a small tight 10mm case IIRC. My bet is that 76* would be your best bet. 78-80 usually works well on these hybrids. But your case is a bit tighter. If you open the intake floor more, I believe you will get more spit back and just lose case compression.

I’d keep the uppers at 122. A hybrid is different than a 460. You’ve got a smaller case. Raising them too much will cause overscavenging with fresh charge right out the Ex port.

So 102-3/122/76 would be my personal goal on the saw.

 

[MattG]

I’d keep the uppers at 122. A hybrid is different than a 460. You’ve got a smaller case. Raising them too much will cause overscavenging with fresh charge right out the Ex port.

So 102-3/122/76 would be my personal goal on the saw.

Thanks my friend (BTW is your name Al?).

I've been out in the workshop already this morning! I think the exhaust is already at 101*. I guess I should leave it there! I'm not gonna touch the upper transfers though I reckon I could pull off if I was very very slow with the dremel.

The intake is a bit a best , weird shape an' all. And it had some minor casting imperfections to smooth out. I'm probably at 75* on that now.

Once my head torch has charged a bit more, I'm going back out...........poor eyesight is my weakness alas....

Get some piccies later

 

[MattG]

@drf256

So definitely no more off than 101* for exh. roof for the kinda mild tune I'm aiming for?

 

[MattG]

Just got a couple of piccies. Sorry the quality is NOT that great. Lots of different lights etc. in w/shop, so tried to hold in one hand and shot with other

Before the dremel this is best I could get of my squish band:


And this is the best I could find on my phone of the exh. It's now at 101* and I reckon I took off about 0.6-0.8mm to get there.


I'll bevel the roof in a bit.

Anyway I hope my dremel skills turn out better than my photographic ones!

 

[MattG]

Ok, so I've done the exhaust port bevel. Pretty tricky to do (i.e. getting the angle sharp enough)with a dremel if you ask me. I thinking that it's quite a pronounced one, since I can still remember my first motocross port attempt which smashed up it's rings due to my exhaust port being too wide.


Anyway I've got another question now. Though I'm leaving the upper transfers alone, what should be done with the lower ones? I've noticed a really obvious ridge, left by the manufacturer, as indicated by the arrow:


What is to be done? Should I endeavour to smooth out the ridge and work out a slight smoother lead-in? i.e. grind around the area within the dotted line:



Actually there's a lot debate on this lower transfer issue... #72 to fill or to grind?

 

[blsnelling]

Grind the lip out.