Part Three: The Transfer Ports

[MustangMike]

I've only done port work on dual port saws, and I'm just a Rookie, but I like Bridge Ports, and think they help accomplish the same result.

 

[mettee]

 

[mettee]

Not my pic, but instead of drawing it...

Primaries are lower, and the secondaries seem to go up a little higher.

 

[mettee]

Same on this one

 

[huskihl]

View attachment 184560

Same on this one

Can't speak for the 1st pic, but in the 2nd one the primaries are 4° higher than the secondaries

 

[Redfin]

Dolmar 7910...

Talk to me about staggering the transfer ports vs not. I've had suggestions both directions. Im a rookie but I wrapped my head around the porting theory a little bit.

I figured the stagger would continue to direct the charge up into the squish/combustion chamber better than having them flat.

But I'm also considering the timing and the charge going out the other side...

Ive been dorkin around with my 7900 this week. If you look down into the tunnel from the case feed. The primary side has a nice direction but right at the corner it has an odd bump out. I spent quite a bit of time clearing that to get that feed equal at the corner.

My mains are almost a flat entry to the port wall.

 

[Nutball]

I'd think you could ramp a dual port or step it half way to get a similar effect

 

[mettee]

Can't speak for the 1st pic, but in the 2nd one the primaries are 4° higher than the secondaries

Thats what I observed as well, but aren't the secondaries supposed to follow the primaries?

I think maybe I'm a little confused on the order in regard to the cycle timing

 

[huskihl]

Thats what I observed as well, but aren't the secondaries supposed to follow the primaries?

I think maybe I'm a little confused on the order in regard to the cycle timing

Primaries are on the exhaust side. I opened them 4° earlier than the secondaries in that 2nd pic

 

[mettee]

Okay, so, higher out of the jug is actually earlier...

So is the piston on the way up or down, I'm trying to see it. My cylinder is in transit I can't look at it.

As far as the secondary and primary I knew the primary was on the exhaust side. I think my confusion is I'm looking at the jug upside down. Piston is going down to compress the case, as it goes down seems like when the transfer is active.

 

[huskihl]

Okay, so, higher out of the jug is actually earlier...

So is the piston on the way up or down, I'm trying to see it. My cylinder is in transit I can't look at it.

Higher would be as it sits on the saw. Depending on the individual 7900, after the piston comes up to tdc, on it's way down, the ex side transfers open at around 126° after tdc. The secondaries would then crack open about 4° later

 

[mettee]

Yep okay, I connected what I thought to what I'm seeing, that makes better sense.

So why stagger over level? Don't get me wrong it makes sense to me, but what are the pros and cons of both approaches.

 

[huskihl]

Yep okay, I connected what I thought to what I'm seeing, that makes better sense.

So why stagger over level? Don't get me wrong it makes sense to me, but what are the pros and cons of both approaches.

Level seems to promote higher peak hp. But for a narrower rpm range. A stagger seems to lower, but flatten, the hp curve, and add torque. It's barely noticeable to me. A dyno would probably shed more light as to what I felt

 

[mettee]

That is really making me think...

Right off I wonder if the cc of the saw can cover for the peakiness you might create running flat transfers.

Then I also think I want a saw with good torque and a broad power band.

 

[Nutball]

So, the primary is the exhaust side, and you want the exhaust side higher? Just clarifying

 

[huskihl]

So, the primary is the exhaust side, and you want the exhaust side higher? Just clarifying

I do.
Not that it's right. Or wrong. Just theory

 

[Lightning Performance]

I like a bit of roof angle on the entry to the cylinder. Flat on both with quads does make it sing up top. I did factory angles on an 046 to wear a 28". He likes it but it has a fresh slug and two tanks run off, scuffed not honed. I ran it in red oak 25" rounds, pulled good. Stihl RS round new. Gasket delete with mild port work. All oem. 120 100 78/ no ignition advance. Squish 18 muff mod Hyway dually no baffle with screens. Built for stumps and flush cuts. He bucks with an 880 he bought new. I modded it. It wears a 31"

 

[mettee]

I do.
Not that it's right. Or wrong. Just theory

See now I'm all bass akwards.

My thought was that the primary does the heavy lifting and opens first(126-127-128), and that the secondary follows the primary continuing on a few more degrees higher(129-130-131-132). Or something of the sort.

Which is on the down stroke after TDC. TDC is 0, and going down you hit the first T port at X, the piston goes down 4 more degrees and that is where the secondary opens.

 

[huskihl]

TDC is 0, and going down you hit the first T port at X, the piston goes down 4 more degrees and that is where the secondary opens.

This part is how I do it

 

[Wonkydonkey]

‘‘Tis good to see this thread more active and shedding more light in the darkness of them transfer tunnels.

Cheers