2 stroke exhaust port shape

Crankcase and Cylinder porting is probably the most controversial subject among Performance 2-Stroke engine builders. The laws of physics have not changed and the mathematical formulas are readily available.

The computer is one of the tools we use to compare engines and determine the changes required to improve performance. The exhaust port designed by the computer is mapped out on a plastic template.

Other cylinder mods are incorporated into the template. The template provides consistency between cylinders in one engine, and other engines. The template insures a perfectly symmetrical exhaust port. The template also provides a reference if port work needs to be matched later. The cylinder is marked, and the cuts are made to the cylinder.

The exhaust port is smoothed, sanded, then polished, and the exhaust flange matched. Port work involves matching all the mating surfaces and gaskets from the carb boot thru to the exhaust flange. Increasing the airflow thru the engine provides the most useable gains in Torque and HP.

Port work is not Black Magic! Port work IS; tedious, precision work, to perfect air flow!! We will work with your engine to achieve your goals. Then design specific engine mods for your riding, racing, and elevation. A squish test is required before the engine is disassembled. A compression test is recommended as a reference for comparison.

With the engine apart, the parts can be measured, to determine the work required. Accurate and precise measurements of widths, heights, and corner radius, are required for the software inputs.

Precise inputs in the computer give reliable outputs. Crankcase Reed: Remove the reed cage and see if cylinder sleeve blocks airflow?Forums New posts Search forums. What's new New posts New media New media comments Latest activity. Media New media New comments Search media. Log in Register. Search titles only. Search Advanced search…. New posts. Search forums. Log in. JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding.

Port Shape Summary. Oct 26, There are a ton of threads on port timing for nearly every desirable saw. In most all of the threads, it is acknowledged that port shape is equally or more important, but there is much less discussion as to what ideal port shapes are. I think a thread focusing on port shapes would be very helpful in explaining the strategy of the shapes we see in all of the great porting pictures.

For example, the transfers are progressive with the highest part toward the intake Many believe this is really beneficial to that saw.

Anyone recommend a progressive exhaust roof? What about port sides If you want ideal, you'd look at what the gp motorcycles and and twin race karts run. They produce hp per cc cylinder. Here is a 3D representation of what their ports typically look like, though some engines run one large exhaust port with two small auxiliary ports located above the rear transfers.

You do NOT want a flat exhaust port roof or floor. Straight is good on the sides. NPKenny said:. Maybe we're not all speaking the same language, but it seems there area few schools of thought on this It could be what I'm referring to as a very large radius in the port side on an exhaust port is what Brad is referring to when saying not to have a flat roof or floor.

A small radius on a large exhaust port will absolutely kill the life of the ring. Andyshine77 Tree Freak. Having flat or squarer ports gives you more area, but it's also really hard on rings even with a nice chamfer. At that point the rings need to be eased back into the ring grove with a radius and nice chamfer.Every racer of 2-strokes will tell you how important the pipe or expansion chamber, to be more precise is on their bike.

There is no other item on a 2-stroke that will affect the performance so much. So, what is an expansion chamber, and how do they work? The problem with such a simple design as the 2-stroke is that it is relatively hard to improve. In attempts to improve performance, engineers have changed the port timing, carburetor size, compression ratio, and ignition timing many times, but eventually they realized that there was little else they could do to get better, more usable, power.

2 stroke exhaust port shape

As the engineers gained more knowledge of the 2-stroke and its working principles, however, it became obvious that to increase power they needed to have a method of varying the exhaust port timing.

With a piston ported engine the exhaust port is opened and closed symmetrically about TDC top-dead-centerso if you lowered the port to start the compression phase sooner, you automatically kept the burnt gasses in longer, which would then mix with the new charge, for instance. A system for opening and closing the exhaust port at different points about TDC was clearly needed. After much research and development Russian engineer, Michel Kadenacy, discovered how to use the pulses pressure waves from the exhaust to achieve this.

Kadenacy discovered that careful design of the exhaust system could effectively use the pressure pulses to close the exhaust port without needing any additional moving mechanical parts. Taking this knowledge further, he found that the pulses were directly related to the shape, size, length, and diameter of the pipe and muffler.

Further experimentation resulted in an understanding of how and when to change the pulse direction. Although the 2-stroke is very simple in its operation, the interaction between the phases is more complex. For instance, as the piston moves up on the inlet stroke, it is also compressing the previous charge ready to be fired. Therefore, looking at the cycles again, we have the following happening at the same time:. If a returning pulse could push that new charge back into the cylinder just at the right time before the piston seals it offmore power would be produced and less fuel would be wasted.

Although the effect often referred to as the Kadenacy effect will only work over a limited rev range, the useful power gained can be tailored to the application. For instance, a road race bike would need that power in the middle to higher rev range, a MX bike would need it in the low to middle rev range, and a trial s bike at the low to middle end of the rev range. Having discovered the positive benefits of using the pulses, further research concluded that these pulses changed direction when the exhaust pipe or muffler changed size or shape.

These discoveries lead to the expansion chamber system. As the name implies, an expansion chamber exhaust consists of a chamber where gases from the exhaust phase expand into.

However, the change of shape of the chamber, as it reduces in size, sets up a pulse that returns towards the exhaust port. If the returning pulse arrives at just the right time, it will push the unburnt gases back into the cylinder. Although there have been many advances with 2-stroke technology in general, and expansion chambers in particular, the same operating principles remain.

The pioneering work undertaken by engineers such as Kadenacy pushed the performance of 2-strokes to levels that are hard to beat even today. Classic 2-Stroke Racers.Hopefully you guys can help me out here a little bit. I've been told as a rule of thumb, on a high rpm drag motor the exhaust port should be somewhere between 70 and 75 percent of the piston. Do you agree? I've also been told that when raising the rpm the motor was designed to run at, I should raise my ports roughly 1mm for every thousand rpm above stock.

Sound right? If so, do I want to raise my ports via base gaskets, or simply make all my ports taller with a grinder? I'm just looking for some basic pointers here. Thank You, and any help would be appreciated See new innovative product as soon as it gets built.

If your motor is running at 10, you are probably looking at a total exhaust port duration of — degrees and a total transfer port opening of — degrees, again depending on cylinder size and number of ports!

If the cylinder ever needs replating it should be done through the engine builder so he can make sure everything is right. It's far more complicated and involved than that.

Thanks for revisiting this post for me.

2 stroke exhaust port shape

I know there are a lot of variables involved, but I'm not saying I understand them all. I had a motor built by a "reputable" engine builder a couple years ago and was promised about 12 horse more than the dyno showed.

And I was just trying to learn a little more about the thought process that a person uses when porting. The original builder wouldn't give me a map he just told me not to worry about, a simple CDI change and I gained 8 horse! So needless to say I have a new engine builder now, and I'm hoping the results show at the track next year! Thanks A Lot, I'm printing all this stuff off because it's really good info that seems hard to get out of some guys!

I can only speculate on that See, the engine builder wouldn't tell me if he had the box re mapped or not. But on the first pull on the dyno the knock senors went wild and the operator aborted the run, it totally ruined the plugs. After explaining to him that I didn't have a clue what the timing was, he installed a cdi that he mapped for other s he built, and that took care of the problem. So I'd guess retarted The box he had let the motor run a lot smoother and built power a lot faster, and let the motor rev to where it wanted to.

And yes, the sled did burn down a couple times when we were running it, always on the fastest runs of the day, I'm assuming because I had the motor loaded down well on those runs is why it would burn down then, the other slow runs I'm guessing I wasn't loading the motor like I wanted to because my clutching was in the way wrong direction, and my log book says the same thingTwo stroke engines fascinate me.

They are so simple and fun that I've always enjoyed tinkering with them. There are many ways to boost the power, one of which is by installing a "tuned" pipe. The way a two stroke motor works causes them to be fairly noisy and a bit inefficient.

2 stroke porting, part 3 - Converting Exhaust Port Map to Area.

A tuned pipe has a set of cones- Divergent meaning the cone gets bigger and Convergent meaning the cone gets smaller that are built to cause "echoes" or pressure waves to reflect back, which if done properly can increase the power of the engine.

Check out this link, it will make a little more sense. That's a very loose explanation, but there you go. I've always wanted to build an expansion chamber This is where P. This Piece Of Junk was and is my first motorcycle Somehow, this one snuck home. I dug it out of storage a few years ago and began this process, so follow along as I make mistakes and learn a thing or two about building your own expansion chamber!

2 stroke exhaust port shape

Please note, this is NOT the only way to do this. I think there is an easier way- done by cutting out two sheets in the correct shape, welding the edges, and the pumping ultra high pressure water in to "expand" the pipe, but I didn't have the tools at the time to do it like that.

This is just how I did it Did you use this instructable in your classroom? Add a Teacher Note to share how you incorporated it into your lesson. This is NOT an easy quick project. It requires a lot of big, expensive tools. I'm lucky enough to have access to a shop that has everything I needed. There are alternate ways to make pipes, which I will discuss at the end of this instructable.

Exhaust port size...

You never know! Tools: -Safety equipment- safety glasses, ear plugs, gloves, etc. Great program. I tried a few free ones, but the first attempt at making a pipe from those didnt work so well In my case, that was advrider.

I'm not going to go into great detail, but you can't just build a pipe and throw it on there expecting it to work well. You have to know a lot of things about your engine- port size and location, port timing, desired application, etc.

You will have to figure out all of this information. Here is a VERY basic rundown of what you need to do: Before you can start at all, you have to know your port timing. Port timing is measured in degrees. Remove the engine side cover over the flywheel. Set the engine at Top Dead Center TDC - meaning the piston is all the way to the top of the cylinder as far as it will go. Attach the Degree Wheel to the flywheel, and align it with something on the engine or use a laser like I did. See Picture 1.

Make a note of which port is opening and the degree at which it opens. Continue rotating, taking note of when the ports open and close.

I did my port timing on Sketchup, which allowed me to draw lines at the degrees I measured, then use the protractor tool to measure the duration.

Exhaust starts escaping the cylinder, then fresh fuel and air is being pushed in at the same time which also helps push the exhaust out.This section describes the basic function and the most important processes in the 2-stroke cycle a tuner need to consider in general and with the Bimotion tuning software in particular. The high performance 2-stroke engine is a pulse resonance engine which means that the operation in general and the scavenging in particular is not only dependent upon the pulses created from piston pumping but also from combustion properties.

This is an important process to understand and is the reason why not only the cylinder decides the tuning degree but also the exhaust pipe, cylinder head and ignition. The charging efficiency is dependent on the pulse energy created from the cylinder pressure at exhaust port opening. At this point the fuel property will decide the heat release, at which time the fuel will produce work. A well designed fuel is an important factor to race engines.

During the combustion, the molecules break down step by step with different heat release at each step. The picture shows the relationship to crank angle and crank moment. As a schedule example, if fuelblend 1 releases most of the energy in a very short time after TDC with a high pressure peak and blend 2 in a longer period of time with a moderate pressure peak then the moment on the crank will be different over the time for the two cases.

The resulting power output will be an integral of crank moment over time. If the pressure on the piston varies as in the picture at a certain rpm, the area below the curve could represent the work. A1 and A2 Note that A1 could be equal to A2!

A high pressure peak for a short time might produce less work than a low peak for a long time at a certain rpm. This means that a low rpm engine will produce less power with the fast burning fuel and visa versa. The engine stress and detonation risk would also increase with the higher peak. The different heat release properties will suite different engine characteristics and should be orchestred with the actual exhaust port height, which decides the opening pressure, i.

Fuel blend 1 will produce a lower pipe charge pressure than fuel blend 2 with the same exhaust port. This could mean that the first choice is less sensitive to exhaust pipe changes and more sensitive to cylinder head geometry at a certain rpm. The two heat release curves from above could also represent 2 different types of cylinder head geometries with the same fuel. The first curve could use a high efficient squish band, which speeds up the combustion by adding more kinetic velocity dependent energy to the gas mixture.

That will also transfer more heat into the head wall surface, which will be a measured as a power loss in the end through the cooling system. If you move your hand quickly in warm water or blow on your skin in a hot sauna you will feel that the skin get warmer due to heat transfer from velocity.

If a geometry change would give a higher factor with the same squish velocity, then we could expect more heat loss due to the increased area. The actual value don't need to be focused, but the changes to different head shapes are interesting to observe, especially if cooling is a critical factor in the original configuration.

With high performance engines kinetic energy is needed and with short compression times due to high rpm and short connecting rods the heat transfer will not necessarily be too high to the cooling system. Adiabatic compression. A head designed to a high tuning degree will work best with a high speed engine, and a moderate tuned engine will feel a great improvement with a moderate tuned head instead of an head without a tuned squish band at all.It enters via the intake port as the piston rises into the crankcase and then is transferred to the combustion area via the transfer ports as the piston descends and the piston top uncovers the port opening to the cylinder.

After it is burned the exhaust gas exits via the exhaust port. For higher rpm there needs to be more degrees of opening for each port because with more revolutions per minute there are more "cycles" dividing up the same minute which means each cycle of crank rotation takes a smaller amount of time so that more degrees opening are needed for the same amount of minimum time needed for movement of gases from one area to another. Higher rpm needs more port duration. For a reed valve intake there needs to be holes in the piston that allows passage of fuel mixture from the carb to the crankcase starting at BDC bottom dead center.

More crankcase compression ratio means there will be more pressure when the transfers open and so the entrance of fuel mixture will be more rapid, needing less degrees to make the complete transfer. Most common is around a 1. More ratio means more pressure which is advantageous for high rpm power but a disadvantage to low rpm power. At higher pressure the mixture enters the cylinder too fast at lower rpm and loops around to exit partially through the exhaust port before the ascending piston closes it off.

Another factor is the angle of the transfer ports roofs. That affects the angle of mixture entry. A steeper angle gets the mixture up to the spark plug faster which is good for high rpm power but bad for low rpm power. A good example is two cylinders I have for the same engine. One had transfers with only degrees and 45 degree roofs. The other had degrees and 15 degree roofs.

Cylinder Porting: Basic Principles By Eric Gorr

The one with the lesser duration achieved rpm, whereas the other only achieved rpm. But the one with 15 degrees had more power at low and mid range rpm. One way to cheat the system is design for high rpm power long duration and steep roofs but allow a bleed off of crank pressure by a narrow boost port that opens much earlier than the main part of the transfers. This does not ruin high rpm power because anything other than the main opening of the transfers is hardly noticed at top rpm.

So its like not even there at high rpm, but at low rpm it effectively releases much crankcase pressure before the main part of the transfers open so that the mixture enters at a slower speed to be less likely to loop around and exit the exhaust port.


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