Porting 101

[Terry Syd]

That is an already high exhaust port duration, I doubt you want to take it any higher unless you know that the engine needs it. As far as the chamfer, yes, the chamfer should be greater in the center. However, on an exhaust port the flow is going out the port so a chamfer can actually assist the flow as it enters the port (unlike a transfer port where the excessive chamfer diffuses the flow).

Since the engine is already running with the existing chamfer, I'd leave it alone at the edge of the cylinder lining and trim a bit off the inside of the port where the chamfer is excessive. That should give you a 'flatter' opening of the exhaust port.

 

[Tohya]

I still don't like the looks of the factory chamfers on the jug, but maybe that's just me.

I remeasured the ports on the 066 BB I have using a depth gauge and a small inspection mirror. I made two measurements, one at the cast port height, and one from the edge of the chamfer.

The roof of the exhaust is 25.96mm from the top of the cylinder and the chamfer is 1mm wide. The top of the transfers are 35.3mm from the top with 1.3mm wide chamfer. The bottom of intake is 63.5mm from the top and had a 1.1mm chamfer.

I used http://www.modelenginenews.org/design/tcalc/TCalcForm.php to calculate the port times, using 111.5mm as the distance from the crank to top of cylinder, and 68mm for rod length, 41mm piston height, 23mm piston Pin offset.

As cast, without the chamfers:
Exhaust: 165
Transfer: 98
Inlet: 174
Blowdown: 33.5

Durations with chamfers
Exhaust: 171
Transfers: 110
Inlet: 181
Blowdown: 30.5


Would fixing the shape of the chamfers and raising the roof of the transfers and exhaust to narrow the size of the chamfers hurt anything? The way I see things, fixing the transfers should reduce the diffusion of the incoming charge. And fixing the exhaust would give a stronger pulse, but that may not make any difference on a saw with a factory muffler.

 

[Terry Syd]

Crikey, who ever put those chamfers on the transfers should be shot. Heck, they are even bigger than the ones on the exhaust port.

The blowdown period on the exhaust is not only related to time, but also to area. Since the pressure differential between the exhaust gases and the ambient air pressure is greatest when the exhaust port first cracks open, it helps to have a large area for flow to quickly vent down the cylinder pressure. You mentioned a 'stronger pulse' - yep, that's what a larger area will give you, that's good, go for it.

You may want to try the jug with the existing transfer ports. Yeah, there will be some diffusion, but the timing change to clean it up would probably make more difference (could be a good or bad move). Try the jug the way it is and see if you like the power. If you think it could use some more transfer area, then you can go back in and clean everything up.

One problem I've noticed with overbored two-strokes over the years is that many times the mod results in insufficient transfer tunnel area to feed the transfer ports. You could have that problem in the existing BB kit. If you raise the transfer ports to clean up that chamfer you could be compounding the problem with the too small transfer tunnels.

It would be a good idea to compute the area at the outlet to the transfer port and see if the existing transfer tunnels can feed that area at BDC. You may find that you need to open up the existing tunnels to feed the transfer ports that you already have. Note that the cross-sectional area of the tunnel should increase as it moves towards the crankcase.

If the stock tunnels are presently proportioned to the ports and then you open up the port window to get rid of the excessive chamfer, you will likely have to open up the tunnels an equivalent amount to take advantage of the increased port area.

 

[bowtechmadman]

Thanks guys...good read.

 

[Tohya]

Wouldn't the area of the transfer tunnel have more to do with the velocity of the charge? Well, the crankcase volume would affect that as well.

High velocity in the transfers being good for high rpms, but bad for low rmps as it would blow the charge out the exhaust. Since I'm not looking for really high rpms, opening the transfers within the limits of the chamfers and enlarging the tunnels some to keep the velocity down might work out for me. Or maybe just enlarging the tunnels some and leaving the transfers where they are.

 

[Terry Syd]

Note in my previous post that I said that 'the cross-sectional area of the tunnel should increase as it moves toward the crankcase'.

In other words, the 'pinch point' of the least cross-sectional area is the port opening. That will be where the velocity is the highest, at the port window. All you are doing in the tunnel work is to make sure that the ports are adequately fed so that the port windows can do their work.

Think of the transfer port window as the nozzle on a garden hose. The nozzle will direct and control the shape of the water spray - that's what a transfer port window does, it directs and shapes the flow into the cylinder.

If you put too small of a garden hose on the nozzle, you won't get the pressure or/flow of water at the nozzle. The same thing with the transfer tunnel, you will decrease the delivery of fuel/air mixture AT THE PORT OPENING.

There may be an advantage to moving the pinch point to just before the port opening, but that is a different concept to having too small of transfer tunnels to feed the transfer ports. All I'm suggesting is that you check the cross-sectional area of the transfer tunnels on your BB.

 

[Tohya]

Oh, I see what you are saying.

The tunnel is rather short and the transfers operate under pressure rather than vacuum. So I wouldn't of expected that the area would make much of a difference. It'll be hard to measure, maybe I'll just stuff some silly putty in there and measure that.

 

[edisto]

Pressure or vacuum doesn't make a distance. Sounds to me like you guys are talking about 2 different aspects of the same thing.

All fluids, including air, 'wet' the surface over which they flow. Velocity of flow decreases as you approach the wettable surface, and reaches zero at some point adjacent to the surface. This creates a boundary layer of zero flow (that's why moving fan blades accumulate dust).

If the diameter of a pipe increases, the boundary layer increases in depth and flow slows. What Terry is saying is that for optimal flow, you want the diameter of the transfer (cross-sectional area) to gradually decrease (too rapid of a decrease reduces flow as well) as it approaches the transfer port.

As far as velocity goes, an overall increase in cross-sectional area (maintaining the same tapered profile) will reduce the velocity of the charge. This tends to help mid-range power, not top-end power, but might be a necessary evil if you need more area from the transfers to keep up with changes in the exhaust and intake.

The pattern of flow from the transfers is much more important than time-area for proper scavenging. If the flow is not balanced, scavenging (and therefore performance) will suffer (although this seems to be less noticable at high rpms, so it is possible to see no real ill effects of playing with the transfers, and just not realize the potential optimum).

 

[Tohya]

New question, after doing all that work on the ports in the jug does anyone try to do anything with the rubber intake boot?

The opening in the jug is where the boot connects is ~24mm in diameter. But the inside of the rubber intake boot is about 20.5mm, with a bumpin that narrows it to 17mm for the carb studs.

The two carbs I have have to following specs.

WJ34 Choke: 22.19mm, Venturi: 18.25, Throttle: 21.43

WJ35A Choke: 22.19mm, Venturi: 19.05, Throttle: 21.43

So the area inside the intake is somewhere between the size of the venturi and throttle.

The inside of a 084 intake boot is 26.6mm and it will connect to the jug, seems the intake ports on 084 and 066 jugs are the same size.

So my current thoughts are, I could either try find a broken 084 tank and try graft the larger 084 carb on to the 066 tank. Or try to bend up a short piece of pipe to fit inside a trimmed 084 boot with tabs under the carb studs to hold it in place and use a little RTV to seal things up.

 

[Outlaw5.0]

I tried to put the 066/660 intake boot onto the 046/460, but the angle and length is wrong and kinks the boot.

 

[Tohya]

Length is the same, the angle is off by about 2 degrees between the 084 and 066.

The problem is that the 066 boot is for a carb with 31mm bolt centers and the 084 uses 46mm bolt centers.

 

[Terry Syd]

As long as the inside area of the boot is more than the area of the venturi you should be fine. Going to a larger boot without increasing the carb size probably won't get you anything, in fact, you will loose some velocity in the intake.

The reduction in velocity could make the engine more peaky, especially if you have increased the intake duration.

 

[parrisw]

Finally made up a nice degree wheel holder today. Was getting real tired of the crappy mount I half arsed together, and it never ran true which will throw off your measurement. Here are a few pics I think its pretty self explanatory.

 

[blsnelling]

Now that is ingenious! I love it!

 

[parrisw]

Now that is ingenious! I love it!

Ya, its great. Easy and fast to use, fits any saw, and very accurate!!
0 runout.

 

[edisto]

Ya, its great. Easy and fast to use, fits any saw, and very accurate!!
0 runout.

Even lets you measure timing with the cylinder off!

Seriously though...that's bloody clever. If my Riders had won, this would be a landmark day for Canada.

 

[parrisw]

Even lets you measure timing with the cylinder off!

Seriously though...that's bloody clever. If my Riders had one, this would be a landmark day for Canada.

Ya its that good EH!!

Thanks

 

[edisto]

Ya its that good EH!!

Thanks

Just saw my typo...maybe if the Riders had had ONE they would have WON.

 

[parrisw]

Just saw my typo...maybe if the Riders had had ONE they would have WON.

ahh, ok, gotcha now.

 

[Tzed250]

Nice work Will....!!!

P.S.

Here is a link to my degree wheel setup thread:

LINK


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