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The first one is better. You must not change the angle to point too much upwards. A and B transfers should point around 20 degrees up. Actually race cylinders have it so that A port has more upward angle(20deg) and B port less (5-10deg). This is of course very hard to measure. Just form the epoxy so that they point "slightly" up 😀

Edit, I think the 40mm cyl is actually better. Even though 44mm has more capacity, the advantage will be nullified from less port area relative to capacity (bore to stroke ratio gets worse) and also the transfer ducts get even smaller on the 44mm cyld.
 

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First side is better, transfer should only aim a few degrees over the piston, boostport should aim toward the sparkplug.
 

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Discussion Starter #123
The first one is better. You must not change the angle to point too much upwards. A and B transfers should point around 20 degrees up. Actually race cylinders have it so that A port has more upward angle(20deg) and B port less (5-10deg). This is of course very hard to measure. Just form the epoxy so that they point "slightly" up 😀

Edit, I think the 40mm cyl is actually better. Even though 44mm has more capacity, the advantage will be nullified from less port area relative to capacity (bore to stroke ratio gets worse) and also the transfer ducts get even smaller on the 44mm cyld.
Alright, i do it as the first picture then.
Boost port could be done like the second picture to get it aiming at sparkplug. Most grind the boost port deeper first.
I think I leave the boost port as is for now.
First side is better, transfer should only aim a few degrees over the piston, boostport should aim toward the sparkplug.
Thank you, i leave boost port as is right now but might get it done later
 

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Discussion Starter #124
Oh and about machining cylinders and cylinderhead, the place where I we're to get it done are closed this week because of covid-19, so it might take a couple of weeks before I can get it done unfortunately.
But then i may have purchased a second cylinderhead that also needs to be machined.
 

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Discussion Starter #125
I have thought about the boost port. Mostly on the 40mm cylinder I've got, cause there is no boost port yet.
Instead of make the boost ports open like the transfer are.
Why not making the boost port closed.

I'm thinking of jb weld the back of the cylinder in case of drilling/grinding it too far out.
Then start with a drill bit and drill at the angel of the sparkplug, then grind the port to shape.

Then grinding the opening in the case matching to boost port opening on the cylinder.



Jb weld inside the markings.
IMG_20201209_143729.jpg


Drill and grind the port
IMG_20201209_142943.jpg

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Drill the boost port duct (not as near the bore as I've marked.
IMG_20201209_142950.jpg


Just did the drawings quick for pictures

What do you think?
Do you think it will work?
Is it any benefit to do it like this? Except the less wear on the piston?
Would it be any performance gains between this design and a open boost port?
 

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You could try adding a third port but it might increase the case volume too much and not work properly.

Remember boost ports function by charging the air/fuel mix that's usually trapped under the piston that's why there's no advantage to having two or three boost ports when one single boost Port charges just as well...

Bosted306 the guy that came up with boost porting these engines his final revision of his stage 3 kit that he sold back in the day had one single boost port. Even though he thoroughly tested two and three port designs.

The two boost Port design that we buy off of eBay is a copy of a PBU design it was called the killer bee kit it had the two boost ports a yellow powder coated cylinder and an aluminum two-piece head. The Chinese got the copying wrong though and the triangular cutouts in the Piston are actually upside down causing the negative side effect of blow by if you want to improve the design throw the Piston with the triangular cutout Windows into the garbage and get a stock 44 mm piston and drill two round holes in it.
 

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Discussion Starter #127
I think you misunderstand me.
I'm just adding one boost port but instead of having a open port I'm planning on doing a closed one.
So it would be more crankcase volume than if i do the open boost port.


Like this but I'm just doing one wide and not two like in the photo
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Edit.
I'm talking about the 40mm cylinder I've got with no boost port but planning on getting a real boost port
 

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The boost is the cylinder being force charged by the parachuting effect of the piston. the air/fuel mix from under the piston is being pushed through the hole in the piston into the slot and around the rings into the chamber - the rest of the feeding is going up the main transfer ports.

When the Piston comes back up it starts pushing all the mix out the exhaust port 😭 as the piston becomes level with the bottom of the exhaust port with any luck you're exhaust system is tuned to the right resonance and the residual sonic wave is coming back at the exact time too help push back with a wall of sound energy sealing the leanest and best part of the two-stroke mix to be burnt in the compression duration.

Your idea is adding a third closed charging port it won't boost but it would charge in the usual manner.

I would definitely sit down and crunch the numbers before spending a lot of time building a closed port just to find out it would make the same power as a standard slot style boost port.

But then normally on the 40 mm cylinder most people including me are building for max RPM because the displacement is so small there's no reason to add a boost port and slow the engine down to increase torque. For the most part I just port out the transfers anti-parachute piston mod turbo crank mod ceramic crank bearing on the clutch side only advance the timing balancing everything really well and a nice lean tune. You will find on the 40 mm cylinder that the crankcase not breathing so well is a lot less apparent than on the 44 mm cylinder because as the smaller piston is going down to bottom dead center the air/fuel charge is allowed to feed through a wider part of the crankcases feed ramps.
 

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Discussion Starter #129
Continued to work on the 44mm cylinder.
Jb welding did not turn out well and I think i will grind the jb weld off and start over again.

Here's some pictures

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In theory the closed boost port is a good idea but I'm afraid there is no benefit from doing it in a cag cylinder, or if there is it will be very small. Since the boost port should aim upwards toward spark plug, the inner wall does not have so much effect on guiding the flow on the duct. The main transfers would benefit from the inner wall much more but pretty hard to add it there ha ha.. Also I believe the poorly flowing cases will prevent any great power gains that could be had from perfecting the cylinder ducts. Also what will happen is that if you cut cooling fins off the cylinder and replace with bunch of JB weld, it will severely reduce heat dissipation which will result in power loss and increased risk of piston seizing. Actually this is the biggest reason why I wouldn't do it.

But then normally on the 40 mm cylinder most people including me are building for max RPM because the displacement is so small there's no reason to add a boost port and slow the engine down to increase torque. For the most part I just port out the transfers anti-parachute piston mod turbo crank mod ceramic crank bearing on the clutch side only advance the timing balancing everything really well and a nice lean tune. You will find on the 40 mm cylinder that the crankcase not breathing so well is a lot less apparent than on the 44 mm cylinder because as the smaller piston is going down to bottom dead center the air/fuel charge is allowed to feed through a wider part of the crankcases feed ramps.
Forget the "parachute effect", there is no such thing. From what I have understood, "anti-parachute" holes would be holes on the piston skirt connecting to the transfers. This does not do anything for the pressure difference between crankcase and cylinder. When the piston is coming down and transfer ports are not yet opened, there is no mixture flow from the crankcase anywhere. When the ports open, the mixture already has a large area to flow via the crankcase mouth to the transfers. There will be no "overpressure" at the crankcase which would "parachute" the piston at this stage. All it will do is weaken the piston.
Also I seriously doubt the effect of a ceramic crank bearing.. A normal metal 6202 bearing is rated to 22 000 rpm: SKF bearing specification . These engines have trouble to exceed even half that. I doubt there is any difference in bearing friction that could be felt in power output.
 

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In theory the closed boost port is a good idea but I'm afraid there is no benefit from doing it in a cag cylinder, or if there is it will be very small. Since the boost port should aim upwards toward spark plug, the inner wall does not have so much effect on guiding the flow on the duct. The main transfers would benefit from the inner wall much more but pretty hard to add it there ha ha.. Also I believe the poorly flowing cases will prevent any great power gains that could be had from perfecting the cylinder ducts. Also what will happen is that if you cut cooling fins off the cylinder and replace with bunch of JB weld, it will severely reduce heat dissipation which will result in power loss and increased risk of piston seizing. Actually this is the biggest reason why I wouldn't do it.



Forget the "parachute effect", there is no such thing. From what I have understood, "anti-parachute" holes would be holes on the piston skirt connecting to the transfers. This does not do anything for the pressure difference between crankcase and cylinder. When the piston is coming down and transfer ports are not yet opened, there is no mixture flow from the crankcase anywhere. When the ports open, the mixture already has a large area to flow via the crankcase mouth to the transfers. There will be no "overpressure" at the crankcase which would "parachute" the piston at this stage. All it will do is weaken the piston.
Also I seriously doubt the effect of a ceramic crank bearing.. A normal metal 6202 bearing is rated to 22 000 rpm: SKF bearing specification . These engines have trouble to exceed even half that. I doubt there is any difference in bearing friction that could be felt in power output.
I like adding the extra holes when I'm not putting in a boost port I feel that it does help performance even though it may be placebo it's a very old school pocket bike modification.

I add the ceramic bearing on a super high performance engine on the clutch side only to deal with the heat off the clutch it has nothing to do with RPM.
 

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Discussion Starter #134
In theory the closed boost port is a good idea but I'm afraid there is no benefit from doing it in a cag cylinder, or if there is it will be very small. Since the boost port should aim upwards toward spark plug, the inner wall does not have so much effect on guiding the flow on the duct. The main transfers would benefit from the inner wall much more but pretty hard to add it there ha ha.. Also I believe the poorly flowing cases will prevent any great power gains that could be had from perfecting the cylinder ducts. Also what will happen is that if you cut cooling fins off the cylinder and replace with bunch of JB weld, it will severely reduce heat dissipation which will result in power loss and increased risk of piston seizing. Actually this is the biggest reason why I wouldn't do it.
I think i will try the closed boost port. There are enough material so no need to add jb weld.
If it not works as intended it would be easy to make the closed boost port to an open one.
 

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Discussion Starter #135
So did some work on the cases for the 40mm cylinder.
And some porting of the cylinder.
Hard to make the ports exactly equal.
Do you think they are close enough?
I haven't sanded them yet in the pictures.

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Discussion Starter #136 (Edited)
More pictures
Cut the sides of the piston skirt so it's follows the crankcase better.
I hope it increases the flow, i think that's the losses in crankcase volume are worth the increased flow.
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Looks really good. Better than mine atm.
I think enlarging the crankcase is only good, or at least it cannot make performance any worse. Larger case will provide more mixture for the pipe action = more power. Although this effect may be restricted by the reed valve which is in a bad position and far from the cylinder but can only guess how significant that restriction is.
 

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Discussion Starter #138
Thank you!
Yes with a good pipe the cylinder increased crankcase volume should make more power.
And I agree the reed valve is in a bad location and i have thinking of changing the position but it's not the easiest task to do and i have not come up with something that will work yet.

And i was wrong about the cylinder wall, it is too thin to make closed boost port i think.
The wall is only 3mm thick. So there is no way to make a deep closed boost port.
2mm maximum depth and that's leaving only 0.5mm bore wall.
So i think that's not going to happen.

It might work if jb welding the backside of the cylinder without removing any cooling fins.
But I suppose it will have a bad effect on cooling anyway.
Or if tig welding the back of the cylinder. But that might ruin the plating.

But I think to get more knowledge on these cag engines and how to get the most power it's important to not just relaying on others work.
New ideas, experiment with different hypothesis will make progress and hopefully more power.

It would be really great if we can come up with a way to move the reed cage to a better location.

I should probably need to get a better reed cage for my engine too.
Or at least modify it so the air/fuel mixture enters the crankcase in the right direction.
Stock cag engine reed valve are upside down, so that should decrease the velocity as well and turn it to the right direction should improve the velocity.

@asoftaaja i really like your work and your informative youtube videos.
 

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Before you guys waste time turning the Reed cage around I would strongly advise reading this thread...


also remember that these CAG engines are actually a copy of the Robin Subaru NB411 brush cutter engine my guess is they put the read cage back there number one to save space and allow room for the gas tank and reason number two plenty of lubricant for the crank bearings and connecting rod they weren't worried about cylinder charge efficiency and on a weed eater you don't want an engine that will rev unlimited RPMs and then blow up.
 
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