Part Three: The Transfer Ports

[Michpatriot]

Years ago a guy brought me a pair of heads to put on his engine and they had been ported with a machine that flowed an abrasive slurry through them. One of the nicest port jobs I'd ever seen (before CNC) and I've always wanted to try that with a saw especially in the transfers, I forgot the name of the process but it wasn't cheap.

Its called Extrude hone.

 

[AKJonsereds]

Been rereading this series and came up with a question.

In the series, roughing up the intake was mentioned to create a type of vortex generator. I am only halfway through this thread and haven’t seen an answer so here it goes:



Vortex generators create a boundary layer that helps flow “stick” to a surface. Has anyone tried adding some kind of vortex generator to the top of the transfer?

Seems like it would let us make the upper roof flat (better flow per the two stroke manual drawing in this thread) but still direct the flow around the upper roof corner and into the combustion chamber.

 

[AKJonsereds]

Or what about on the inside corner of the full transfer radius? Mostly thinking about piston ported duals on that one….

 

[Nutball]

Been rereading this series and came up with a question.

In the series, roughing up the intake was mentioned to create a type of vortex generator. I am only halfway through this thread and haven’t seen an answer so here it goes:



Vortex generators create a boundary layer that helps flow “stick” to a surface. Has anyone tried adding some kind of vortex generator to the top of the transfer?

Seems like it would let us make the upper roof flat (better flow per the two stroke manual drawing in this thread) but still direct the flow around the upper roof corner and into the combustion chamber.

I have thought of doing that, I just haven't tried it. I feel like it may cause too much resistance to the flow, or the vortex would mix too much exhaust into the fresh mix.

 

[Mastermind]

I use a diamond burr in the uppers....and usually leave the lowers as cast with the exception of removing casting flash and lines.

 

[AKJonsereds]

The lowers on my current project have a light stipple from casting which is what started the thought.

The VGs we use in aviation are tiny…. Bet it is similar effect

 

[Ketchup]

592. Not my photo.

 

[Ronie]

Anyone have a theory why duel port Stihl cylinders have the intake side of the transfers opening a couple of degrees before the exhaust side? If you did nothing else to the ports except leveling out the transfers, how would that effect the way the saw runs.

 

[huskihl]

Anyone have a theory why duel port Stihl cylinders have the intake side of the transfers opening a couple of degrees before the exhaust side? If you did nothing else to the ports except leveling out the transfers, how would that effect the way the saw runs.

I believe it’s because there is still exhaust inside the motor when they open and they’re keeping the fresh charge further away from it by opening the intake side first

 

[David Young]

Ronie great question with complex answers. The goal is to sweep/scavenge the cylinder before the exhaust closes without blowing too much out of the fresh charge out. Flatter takes longer. Straight up is faster.

People look at the burn pattern on the piston which is part of the story. But. Also look at the combustion chamber is it washed enough? The up angle can help sweep that out

 

[Ronie]

Thanks guys, I was thinking that it might be to give a little extra push from the rear to help get the burnt gasses out. Has anyone done a before, slanted rear opening first, and after, straight across? The reason I'm asking is that I got this 460 cylinder and it has the slanted transfers that I was going to leave that way but if there's a noticeable improvement when they are straight, I'll take a few minutes and grind them straight.

 

[huskihl]

Thanks guys, I was thinking that it might be to give a little extra push from the rear to help get the burnt gasses out. Has anyone done a before, slanted rear opening first, and after, straight across? The reason I'm asking is that I got this 460 cylinder and it has the slanted transfers that I was going to leave that way but if there's a noticeable improvement when they are straight, I'll take a few minutes and grind them straight.

View attachment 325216

Raising the exhaust side raises torque

 

[huskihl]

Or maybe it just reduces hp. Hell idk. Just what I feel

 

[Ronie]

Raising the exhaust side raises torque

That's what I after, I'll flip it around and have the exhaust side open first.

 

[Ketchup]

I like a ramped transfer. Throttle seems snappier.

 

[huskihl]

That's what I after, I'll flip it around and have the exhaust side open first.

I guess I should’ve said “leaving the intake side down seems to add torque “

 

[Ronie]

I guess I should’ve said “leaving the intake side down seems to add torque “

I think I understood what what you were saying.

Better torque

 

[farminkarman]

I feel like I have a good handle on quad port transfers....not so much the duals.

 

[huskihl]

I think I understood what what you were saying.

Better torque


View attachment 325220

Yes.
But the way I worded it was kinda wrong. When transfers need to go up, typically raising them will move the rpm range up, adding hp. It also adds peak torque, but it’s less forgiving. Which is why I explained what I was thinking.

 

[Ronie]

Does the size of the upper transfers matter, top to bottom? If I raise the upper transfers a lot and I end up with this huge hole, will it affect the way the saw runs?