Porting 101

[Tzed250]

A chamfer is a flat plane, normally cut at a 45° angle on the corner formed by two perpendicular planes. Chamfers are specified by a distance parallel with one of the original planes. If there is any rounding, then the term radius comes into play. Since the edge finish on a port is not usually made with a straight sided tool, it normaly becomes a chamfer with radiused corners.

 

[SWE#Kipp]

Thanks everybody for the info about beveling ,, got a 346xp cyl 45cc that i'm working on and have been insecure about how much bevel is needed but now i got a good idea how much is needed
The pictures from TW was well needed !!!

 

[Terry Syd]

Cute pock

Tzed250, I suppose we could call it a 'Cute Pock' - which is a bastardised phonetic pronounciation of the acronym CWTPOAUC (chamfer with the profile of a used cylinder)

 

[edisto]

Bell does say 1mm/0.3 deep on all the ports other than the exhaust as the ports are smaller.

Indeed, and Jennings makes a similar recommendation. My question was about TW's exhaust port, but I should have elaborated.

 

[edisto]

A chamfer is a flat plane, normally cut at a 45° angle on the corner formed by two perpendicular planes. Chamfers are specified by a distance parallel with one of the original planes. If there is any rounding, then the term radius comes into play. Since the edge finish on a port is not usually made with a straight sided tool, it normaly becomes a chamfer with radiused corners.

But the chamfer is what does the work. Ideally, the radius should only be on the inside edge of the port chamfer to improve flow.

 

[nmurph]

CWTPOAUC

this is not an acronym.

 

[timberwolf]

From a very little bit of flow bench testing I have done I found it quite surprising just how much improvment to flow can be had by a very slight softening of a square edge. There is much more to be gained in softening an edge where flow enters a duct from free space though than where it leaves a duct into free space.

 

[BigJ]

From a very little bit of flow bench testing I have done I found it quite surprising just how much improvment to flow can be had by a very slight softening of a square edge. There is much more to be gained in softening an edge where flow enters a duct from free space though than where it leaves a duct into free space.

I recall learnin' this back in fluid dynamics as well. So, any reason to not put a small radius on the sides of the exhaust port? Though, I guess that it's already a fairly large angle exiting the side of the port...

 

[timberwolf]

A radius on the sides is just going to extend the length of the unsupported ring. But the main reason is that 90% of the exhaust flows in the first 10% of port opening or something like that. So by the time the sides of the exhaust port are exposed the transfers are open and any significant exhaust flow is essentially over and done with. What limited flow there will be as part of the end of scavenging will be very low velocity not significantly affected by the coefficient of discharge.

 

[Tzed250]

From a very little bit of flow bench testing I have done I found it quite surprising just how much improvment to flow can be had by a very slight softening of a square edge. There is much more to be gained in softening an edge where flow enters a duct from free space though than where it leaves a duct into free space.

This is why many four strokes respond well to a full radius valve job, as done with a Serdi valve seat cutter. I the '60s and '70s it was the 3-angle valve job, with an angle on each side of the seat. The '80s saw the rise of the 5-angle valve job. The full radius valve job became very popular once the technology to cut them became common.

 

[TraditionalTool]

Don't forget the limiter tabs on the carb screws!

To add a point to this thread, do not forget that the carb screws often have tabs on them to limit the screws, and they need to be cut off.

I have mine cut off now, but need to run out and attend to some errands, so won't be able to get around to this until later, to see how I can tune it.

timberwolf,

Seems what your saying is that the intake is more important, with the flow going in, or am I misunderstanding you?

 

[Terry Syd]

Diffusion

“From a very little bit of flow bench testing I have done I found it quite surprising just how much improvment to flow can be had by a very slight softening of a square edge. There is much more to be gained in softening an edge where flow enters a duct from free space though than where it leaves a duct into free space.”

TW, this goes to my point about the diffusion of the flow caused by excessive chamfer on the transfer ports. As you mentioned, “there is much more to be gained in softening an edge where flow enters a duct from free space”. This rounding helps the flow bend around into the exhaust port. The converse applies for the transfers, that is, the transfer flow is allowed to bend around the transfer port as it exits.

If we concentrate too much on bulk flow, we can loose sight of the direction of the flow.

The Stihl cylinder is a case in point. The generous chamfers around the entire transfer port will tend to diffuse the flow on the edges. Granted, the bulk flow through the port could possibly be increased, but at what cost to mixing and short circuiting of the mixture.

If we milled the head of the jug, put a glass plate over the cylinder, installed a vacuum pump through the plate, we could watch smoke streams as they entered the cylinder. The generous chamfers should show diffusion around the ports. If the chamfers were tightened up to the edges of the ports we would see less diffusion.

However, in this case, we are seeing the cylinder in isolation – without the valve (the piston) in position. If we add the valve to the ports and move it to where the transfers are just cracking we would see even more diffusion. The piston would still be above the level of the actual port, but the chamfers would be exposed. The chamfers would direct the flow up into the departing exhaust gases and back towards the exhaust port itself. This results in mixing and short circuiting of the incoming fuel mixture.

Thus, on a transfer port we want to port the transfers for the desired direction of flow and to keep the chamfers to a minimum, essentially, just enough chamfer to accommodate the ring/s.

 

[tdi-rick]

Good point Terry.
With gas flow, any transition is vitally important in terms of flow velocity and volume, but that initial change of direction is most important, and following what Terry said it's possibly worth noting that there is NO bevel on Dolmar/Makita PS7900 transfers at all, they are sharp edged as cast and honed and I'm guessing it and the 681 Solo probably have the best BMEP's of stock saws ATM.

When you look at the transfers in those jugs a lot of time has been spent on maintaining velocity and direction of flow, with a very generous radius in the duct closest to the exhaust to shoot the stream across the top of the piston with a hell of a twist before the horizontal transition so it's angled straight at the back wall of the cylinder. Without a bevel/chamfer, whoever designed it is making sure that the charge goes straight across the piston crown towards that back wall. There won't be any diffusion and short circuiting going on.

 

[timberwolf]

Seems what your saying is that the intake is more important, with the flow going in, or am I misunderstanding you?

Not exactly, I don't think.

What I am saying is if you take a pipe squared on both ends you will gain much more by bell mouthing the intake than the outlet side of the pipe.

It's all due to a viena contracta effect. If the end of the pipe is just a square edged stub the air strams come from all angles and the ones in particular that are comming from 90 deg off to the side of the pipe need to make a sharp bend and the higher the velocity the more they are forced wide in making the turn into the pipe. This creates an eddy just inside the pipe at the wall constricting the cross section of flow to less than the diamiter of the pipe. A bell mouth helps steer the air streams on the outside edge of the pipe into the pipe without them pushing into the center constricting flow.

 

[Jacob J.]

Good point Terry.
With gas flow, any transition is vitally important in terms of flow velocity and volume, but that initial change of direction is most important, and following what Terry said it's possibly worth noting that there is NO bevel on Dolmar/Makita PS7900 transfers at all, they are sharp edged as cast and honed and I'm guessing it and the 681 Solo probably have the best BMEP's of stock saws ATM.

Same thing if you look at the early Stihl 044, 064, and 084 jugs. Squared off upper transfer ports with no chamfer, and those saws ran really good. Much better than the updated EPA versions.

 

[edisto]

Same thing if you look at the early Stihl 044, 064, and 084 jugs. Squared off upper transfer ports with no chamfer, and those saws ran really good. Much better than the updated EPA versions.

It just depends on the width...the transfers are narrow relative to the exhaust, so a bevel is less important there because the cylinder wall keeps the rings from protruding. If it's narrow enough, I suppose you can get away without a bevel, but in any event, you should use less bevel than you would for the exhaust.

As TW pointed out, flow is into the exhaust port, so a small radius inside the bevel effectively increases port area, and improves flow into the port.

The chamfering has to balance 2 different phenomena. If easing the rings back into the cylinder was the only concern, a bevel would do the job, but this has to be balanced with flow considerations.

Like anything you do with a 2-stroke, it's a matter of balancing the tradeoffs.

 

[crmyers]

Porting tools

What kind of bits do I need to use in my Dremel for this kind of work? Grinding stones seem to load up in the aluminum.

 

[TraditionalTool]

What kind of bits do I need to use in my Dremel for this kind of work? Grinding stones seem to load up in the aluminum.

I showed this in post #30, it is what I used.

 

[tdi-rick]

[snip]
Like anything you do with a 2-stroke, it's a matter of balancing the tradeoffs.

That goes for any engine.

I'd written a long waffle on four stroke porting and engine building in an above post but scrapped it as not relevant but it was saying exactly that.

Everythings a compromise, and for best results you have to try and balance each area.

 

[blsnelling]

Here's what I do most of my porting with. Go HERE and look at item 8175A23.