Boost bottle position??

[Steve Best]

If you shim the cylinder higher with gaskets you will have bring the head down by a similar amount by 1) cutting the top of the cylinder, 2) from the head gasket (limited) and 3) removing metal from the head (limited). I cut the ports to what I want making them very level with each other. A very small dremel with a cutter will do this quick and easily. Slips and gouges are totally your responsibility.

Seriously, if I were you (and I have been!) I would try the cylinder as it is (widened ports and hopefully gasket match), and pull it off again (15 minutes work) and try raising the ports 0.020". Run it again. Keep doing this until you get the power you want.

What if you go too far? I've gone too far many times in life. That is the cost of learning. In this case about $30.

There are 2 ways of getting around cutting too much.
1) have an extra piston with 0.020" cut off the edge at the port:

or 2) do shim the cylinder up and down with an extra base gasket and use the teflon tape instead of a head gasket.

Either will tell you something before you cut metal, 0.020" at a time.
Usually we just raise the exhaust port so the piston trick is best for this.
If you want to try raising all the ports, use the base gasket.
You can sand material off the top of the cylinder, just as you would a head, if you want to go the basegasket route.

Good luck!

 

[Dserna]

If you shim the cylinder higher with gaskets you will have bring the head down by a similar amount by 1) cutting the top of the cylinder, 2) from the head gasket (limited) and 3) removing metal from the head (limited). I cut the ports to what I want making them very level with each other. A very small dremel with a cutter will do this quick and easily. Slips and gouges are totally your responsibility.

Seriously, if I were you (and I have been!) I would try the cylinder as it is (widened ports and hopefully gasket match), and pull it off again (15 minutes work) and try raising the ports 0.020". Run it again. Keep doing this until you get the power you want.

What if you go too far? I've gone too far many times in life. That is the cost of learning. In this case about $30.

There are 2 ways of getting around cutting too much.
1) have an extra piston with 0.020" cut off the edge at the port:
View attachment 94740
or 2) do shim the cylinder up and down with an extra base gasket and use the teflon tape instead of a head gasket.
View attachment 94741
Either will tell you something before you cut metal, 0.020" at a time.
Usually we just raise the exhaust port so the piston trick is best for this.
If you want to try raising all the ports, use the base gasket.
You can sand material off the top of the cylinder, just as you would a head, if you want to go the basegasket route.

Good luck!

Should I lower the intake port any?

 

[Steve Best]

Should I lower the intake port any?

Typically no. There needs to be a difference between the heights of the intake and exhaust and the bottom of both ports does little.
That said, casting quality with these cylinders is poor and you never know what you have to do to fix it.
This shows an approximation of 2 stroke ideal ports, if only we could! Note the heights of intake, transfers and exhaust:

The top of the exhaust is curved to prevent the ring from snagging.
The straighter it is, the stronger the exhaust pulse and noisier too.
The intake does not need to be curved because there is only the piston skirt running by it, no rings.
We cannot do the strange "waisted" shape shown in the above ports, so an ideal exhaust shape is somewhat like this:

The top is the part that does the work, make it as flat as you dare. Radius the corners because fluids don't flow well in square tube, and a radius is gentle on transitioning the ring to the open port. Leave the bottom the stock width because 1) it doesn't do much (only uncovered briefly), 2) to keep flow velocity up especially where its flow is slowest, 3) again to be gentle on the ring. especially if the width is not needed.

Lots of folks like to raise the exhaust port which will often raise the engine's rpm range. How much? Ahhh, that is the rub.
And again, only make the port wider at the top. As for enlarging ports, it often works against you by killing flow velocity. Cross sectional areas need to be gentle transitions where they change. Rapid cross sectional area transitions will kill flow energy and momentum.

My thoughts. Well worth what you paid for them!

 

[Dserna]

Typically no. There needs to be a difference between the heights of the intake and exhaust and the bottom of both ports does little.
That said, casting quality with these cylinders is poor and you never know what you have to do to fix it.
This shows an approximation of 2 stroke ideal ports, if only we could! Note the heights of intake, transfers and exhaust:
View attachment 94743
The top of the exhaust is curved to prevent the ring from snagging.
The straighter it is, the stronger the exhaust pulse and noisier too.
The intake does not need to be curved because there is only the piston skirt running by it, no rings.
We cannot do the strange "waisted" shape shown in the above ports, so an ideal exhaust shape is somewhat like this:
View attachment 94744
The top is the part that does the work, make it as flat as you dare. Radius the corners because fluids don't flow well in square tube, and a radius is gentle on transitioning the ring to the open port. Leave the bottom the stock width because 1) it doesn't do much (only uncovered briefly), 2) to keep flow velocity up especially where its flow is slowest, 3) again to be gentle on the ring. especially if the width is not needed.

Lots of folks like to raise the exhaust port which will often raise the engine's rpm range. How much? Ahhh, that is the rub.
And again, only make the port wider at the top. As for enlarging ports, it often works against you by killing flow velocity. Cross sectional areas need to be gentle transitions where they change. Rapid cross sectional area transitions will kill flow energy and momentum.

My thoughts. Well worth what you paid for them!

Man you're a bad ass!! Thank you so much!!

 

[Dserna]

Typically no. There needs to be a difference between the heights of the intake and exhaust and the bottom of both ports does little.
That said, casting quality with these cylinders is poor and you never know what you have to do to fix it.
This shows an approximation of 2 stroke ideal ports, if only we could! Note the heights of intake, transfers and exhaust:
View attachment 94743
The top of the exhaust is curved to prevent the ring from snagging.
The straighter it is, the stronger the exhaust pulse and noisier too.
The intake does not need to be curved because there is only the piston skirt running by it, no rings.
We cannot do the strange "waisted" shape shown in the above ports, so an ideal exhaust shape is somewhat like this:
View attachment 94744
The top is the part that does the work, make it as flat as you dare. Radius the corners because fluids don't flow well in square tube, and a radius is gentle on transitioning the ring to the open port. Leave the bottom the stock width because 1) it doesn't do much (only uncovered briefly), 2) to keep flow velocity up especially where its flow is slowest, 3) again to be gentle on the ring. especially if the width is not needed.

Lots of folks like to raise the exhaust port which will often raise the engine's rpm range. How much? Ahhh, that is the rub.
And again, only make the port wider at the top. As for enlarging ports, it often works against you by killing flow velocity. Cross sectional areas need to be gentle transitions where they change. Rapid cross sectional area transitions will kill flow energy and momentum.

My thoughts. Well worth what you paid for them!

One more question... Should the piston be flush with the top of the cylinder??

 

[Steve Best]

One more question... Should the piston be flush with the top of the cylinder??

What you really need is a 0.030" gap between the head and the piston.
This is complicated by the piston having a domed head, so now we want the dome of the piston to match the dome of the head with about 2° of draft (or taper) towards the center of the chamber. This is so that we get a "squirt" of air and fuel jetting into the chamber as a sort of donut shaped tornado that keeps fuel and flame in motion:

This keeps the mixture homogeneous and spreads the flame front quickly without detonation or preignition. The flame burns so much faster that less timing advance is used and yet you get a fast clean burn at all rpm. Head shape and squish (quench) clearance really are key to getting a 2 stroke to perform. With the right chamber shape and clearance even the 49cc engine will idle smoothly, then pull away from a dead stop and pull the bicycle to over 40mph at 10,000 rpm. You cannot do that by wildly increasing port duration or compression. Duration increases top rpm power at the expense of low rpm torque. Compression does increase low rpm torque at the expense of piston heating, rod and bearing loads, detonation and reduced engine reliability.

So what does this "perfect" head shape look like? Below on the right is what worked for me:

Left is a stock head (there are many different chambers on these engines). The stock squish area is a bit small and the angle does not match the dome of the piston, so in this case this was happening:

Gases are trapped in a pocket at the outside diameter of the cylinder and preignite, causing detonation, overheating and power loss. The manufacturer does not do this on purpose, it is just that they miss tolerances. These engines make enough hp to do the job so why should they waste time and money to gain more?

Here are some examples of different head chambers that are available for these engines:

Upper left has been my favourite. Airsal middle bottom is pretty darned good too.

So where do you set your piston? Flush is OK. The stock head gasket is about 0.030" so that should give you your squish clearance if all the machining is right (it rarely is). I would look at the squish gap and shape first, machining them right, and then look at modifying the port duration for how you want to run the engine.

Here is a KTM flat topped piston set up flush and the lead that was used to check the squish. Flush was a place to start. The squished lead tells me what I need to correct to. If the port duration is not giving me the power where I want it, I will correct the duration with base gaskets and machine the head to suit.

Whoo! It doesn't need to be this complicated. Correct dome shape, set up the right squish, you will be happy.
Playing with duration can come later and can be done as previously discussed.

 

[Dserna]

Ok I'm going to reshape head and get squish right. But what is your opinion on this mod? I seen this on a YouTube video

 

[Steve Best]

I don't know. Looks interesting. I'm interested to hear how it works.

That is where these engines are so wonderful. You can buy a couple extra heads and cylinders and experiment like crazy.

One trap we have to watch out for is that if we invest a lot of time into an idea, we tend to will it success. We hate to admit if something doesn't work so we tend to be happy with it regardless of the result. Especially if it makes more noise

To avoid this I try to find some way of baselining the performance. Top speed, timed acceleration, how far up an ever increasing hill, top speed on a hill, whatever, as long as it is relevant to the performance I want, and measurable. I measured the performance before I tried a change, did the change, then measured the performance again. This exactly answered my question "Does this work"?

These motors are so simple that most of my experiments could be conducted in a few minutes of a sunny backyard afternoon:

So the question is "what are the chamber volumes of my various heads"? So I gather up all the heads off and on the bikes and compare them, using a bit of grease and flat plate to seal the water in and use a syringe to measure water into the bottom of the spark plug hole. Took me less than 5 minutes to remove the head off my running bike, gear this up and get some measurements. This is just like the big boy NASCAR teams do it but a lot quicker and easier.

Then I notice something. The numbers don't add up. Some chambers are recessed deeper into the head beyond the head gasket surface. This gets back the the question you asked "Should the piston be flush". Hmm, If piston to head is the way we gain performance, shouldn't that be the way I test compression volume? So this is the way I do it now:

I grease the piston and check the actual compression volume, minus the squish gap volume, which I can calculate. Oops, I'm off track.
Well my point was these engines let you try out ideas in minutes, unlike working on more complex engines.

1) Measure the performance before the change, top speed up a steep hill with a speedometer works well for mid rpm power.
2) Do the change, just one change and ideally just a small change that you can increase incrementally to see if if helps or hinders.
3) Re-Measure your performance. Did it improve?

Back when I was younger I'd often try 5 or 6 mods all at once (cuz the magazines all said "this stuff works!") while I had the engine apart, never knowing which ones worked and which ones stole power. Now I will often have the head or cylinder off and on maybe 10 times in an afternoon, trying incremental changes, testing it on the hill in between each change. Wow, I really started to learn stuff. A lot of that "internet wisdom" just did not work.