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A Basic Guide To Two Strokes

I wrote this as just a basic guide for anyone new to two strokes. Hopefully it helps someone.

What is it? How does it work?

A 2 stroke internal combustion engine (also commonly referred to as 2 cycle or 2T, from the German term 2 takt) operates in a different manner than the more common 4 stroke internal combustion engine. A 2 stroke engine does not use a camshaft, mechanical valves, or many other components that a 4 stroke requires to deliver the air/fuel mixture and dispose of exhaust gases. I'll try to expalin the basics of both to you.

In a 4 stroke gasoline engine, a carburetor or fuel injection system delivers a fuel/air mixture into an intake manifold and then to the cylinder head. A camshaft rotates to operate valves inside the cylinder head. The most common 4 strokes have 2 valves, one for incoming fuel and air (intake) and another for allowing the expulsion of gases (exhaust). The intake valve opens to allow an air/fuel mixture to be drawn into the cylinder while the piston moves from top dead center (TDC) to bottom dead center (BDC). This is known as the intake stroke. The intake valve closes and the piston then travels from BDC to TDC compressing the mixture. This is known as the compression stroke. The compressed fuel/air mixture is ignited by a spark plug near TDC in the compression stroke. The combustion and expansion of the mixture accelerates the piston from TDC to BDC while both valves remain closed. This is the power stroke. The exhaust valve now opens allowing the piston moving from BDC to TDC to push out the burned mixture. This is called the exhaust stroke. Now the piston is at TDC again, ready to start the process all over again. Intake, Exhaust, Power, and Compression... 4 strokes. There are variations of 4 stroke engines, including gasoline, diesel, different cam and valve configurations but I've explained the basic principles of their operation. There is only combustion and therefore power created on every 2 complete revolutions of a piston in a 4 stroke engine.

Here's an animated gif showing the 4 stroke engine at work.

You will have to bear with me on the 2 stroke explanation, and understand that multiple events occur at the same time. This is the best way I can explain it with my knowledge. Like the 4 stroke, the 2 stroke engine uses a carburetor or in rare cases fuel injection system to mix and transfer fuel and air into an intake manifold. A lot of similarities end here. There are variations on induction methods, but for the 2 stroke engines commonly found in scooters (case-reed induction) reed valves are used to control the mixture flowing from the intake manifold to the crankcase (that's right, I said the mixture enters the crankcase and not the cylinder or combustion chamber). Reed valves have no mechanical components directly controlling them, they rely on pressure created by the engine to open and close. This makes them amazingly simple, but less efficient than the valvetrain in a 4 stroke engine. The piston traveling towards TDC creates negative pressure (vacuum) inside the crankcase which causes the reed valve to open and allow the fuel/air mixture to enter the crankcase. As the piston travels back down towards BDC positive pressure is created in the crankcase, which closes the reed valve and there is compression of the gases inside the crankcase. At some point before BDC the piston will uncover the transfer port, allowing the fuel/air mixture to travel into the cylinder. The piston will reach BDC and begin to travel back up, closing the transfer port and stopping the flow of fuel/air into the cylinder. As the piston gets closer to TDC it closes the exhaust port and then begins to compress the fuel/air mixture. Just like in the 4
stroke engine, the compressed mixure will be ignited by a spark plug slightly before the piston reaches TDC and the expansion of gases pushed the piston toward BDC. As the piston travels downward it uncovers the exhaust port and the pressurized gases exit the cylinder. The piston will then proceed toward BDC opening the transfer port and allowing fuel in again so the combustion process can continue. All the components (Intake, Compression, Power, Exhaust) are there, like the 4 stroke, but some events are happenening simultaneously. The 2 strokes are called power/exhaust and intake/compression. As the piston travels down it creates power as well as forces out the exhaust gases. As the piston travels up it pulls in the intake charge as well as compresses it. The 2 stroke engine will ignite the fuel/air mixture and create power on every single revolution of the piston.

Here is an animated gif that should help you understand the 2 stroke a little better.



The 2 stroke and 4 stroke engines differ on their methods of lubrication as well. 4 stroke engines have a dedicated system for lubricating essential parts with engine oil. Oil is contained in the crankcase and/or oil pan and an oil pump sends it through passages to essential parts and oil is splashed/slung by the crankshaft. The two stroke engine uses engine oil mixed into the fuel (by premixing or an oil injection system) for lubrication.

Advantages/Disadvatages

Now that you have a better understanding of what 2 stroke and a 4 stroke engines are, you can better understand the advantages and disadvantages of a 2 stroke compared to a 4 stroke.

Advantages

Power - A 2 stroke engine will often produce more power than a 4 stroke engine of the same size (displacement). Since the 2 stroke will produce power on every full rotation of the piston, as opposed to every other rotation like a 4 stroke, it can produce more power. The 2 stroke engine also lacks the need to exert any effort turning an oil pump, camshaft, rocker arms, etc...

Weight - The lack of components in a 2 stroke engine also translates to an engine that weighs less than it's four stroke counterpart. The lower weight of the 2 stroke combined with the often higher output creates a more favorable power to weight ratio.

Simplicity - 2 strokes are usually easier to maintain, rebuild, and modify. 4 strokes require periodic valve adjustments and oil changes that 2 stroke engines don't. If you plan to rebuild or modify a 2 stroke, you'll have less parts to deal with.

Repair / Modification Cost - Less parts to repair and replace can often lower the costs associated with modifiying and rebuilding a 2 stroke engine.

Disadvantages

Longevity - The piston, piston rings, and cylinder walls of 2 stroke engines tend to wear faster than those of 4 stroke engines. This is because 2 strokes don't have a dedicated oiling system like a 4 stroke.

Fuel Efficiency - The timing of valve events in a 4 stroke is much more efficient in a 4 stroke. A 2 stroke can actually expell some unburned fuel for a brief moment while the exhaust port is uncovered at the same time as the transfer port.

Emissions - 2 stroke engine produce more emissions due to the burning of lubricating oil and the afore mentioned lack of efficiency.

Power Band - 2 strokes often have a more narrow power band than a 4 stroke, meaning they make their peak power output in a smaller range of engine RPM. Luckily, proper tuning of a CVT transmission can do wonders for using a smaller power band.

There are other pros and cons, some debatable, but I feel the ones I have provided are the most relavent to the majority of scooter riders.

Performance Modifications / Tuning

Before taking a look at any performance modifications, I would like to make a couple of important points.

Modifying a vehicle often means compromise, though not in all cases. Don't plan on making big power with the typical 49cc 2 stroke without sacrificing something along the way. Very often, reliability and longevity are sacrificed to varying degrees. Not every modification will cause any major issue, but some will. Sometimes the sacrifice is easier to deal with, like building an engine that will scream and pull hard in higher RPM operation often means the engine will lose some of it's low end power and vice-versa. If you have a specific goal in mind, it is very important to choose your modifications wisely. You don't wnat to put together an engine with compression so high that it requires some form of racing fuel if you are trying to build a scooter that is well suited for long trips and trouble free operation. That's kind of an extreme, but you get the picture.

You should also keep in mind that some components compliment each other and some contradict each other. For example, you wouldn't want to add parts to your transmission to keep your engine operating at a lower RPM range if your engine is designed to achieve maximum power in the upper RPM range. Just another reason that all modifications and parts are not suited to all needs, making researching before modding or buying even more essential.

Jetting & Tuning A Carburetor

I am starting out this section with jetting and tuning a carburetor because it will be mentioned in other sections and is essential to properly modifying and maintaing your scooter. I was going to write this section, but this article explains it better than I can and with lots of helpful illustrations.

http://hondanighthawks.net/carb14.htm

I should also note that jetting recomendations are very difficult to make due to variations in altitude, which carburetor and jets are currently in use, and even riding style. For this reason, you will find any reference to jetting for other modifications mentioned here to be vague.

Switching to a larger carburetor is also popular. Using a larger carburetor on a stock engine will often show little to no power gain. In some cases the stock carbretors are pretty tiny, so you may see more of a gain. Don't just run out and buy a new carburetor because you got a mild big bore kit or something like that either, as it is not always needed. If your scooter came with a fairly large carburetor, for instance a 17.5 or 19mm on a 49cc, then you may not need to swap carbs but merely jets. If you do end up using a setup that the stock carburetor isn't ideal for, remember that bigger isn't always better. You will want to find out what size range or specific size carb is recommended for the mods you are planning. Manufacturers may offer advice as well as searching for information on forums and websites from others with your basic setup. If you find that perhaps 19-21mm carbs are optimal for your setup and both should work well, here is something to think about. The larger carburetor would be capable of supplying more fuel/air to support better power in the upper RPM range. The smaller carburetor will offer better low end power and throttle response. Intake manifolds should be matched to carb size.

Air Filter

Installing an aftermarket air filter or modifying the existing air box can give you a small gain in power, but don't expect a big power boost unless your current setup is extremely restrictive. Any major changes are likely to require adjusting and re-jetting the carburetor. Some people like to drill large holes or a series of holes in their stock airbox to allow increased airflow across the filter. Be careful doing this if you do not wish to install larger jets. You can probably get away with adding some small holes or slightly enlarging the current inlet, but major changes are likely to leave your engine running lean. If your scooter was in tune before the filter or mod, you should only need a slightly larger jet.

Exhaust Pipe

Putting an aftermarket pipe on a 2 stroke is different than choosing an exhaust for a 4 stroke. When dealing with 4 strokes, especially of a very small displacement, aftermarket exhaust systems don't always offer a good gain and differences in pipe layouts and backpressure changes (within reason) don't generally have dramatic effects. If your stock exhaust is very restrictive, you may see more of a gain than a scooter with a less restrictive exhaust system. That's the (very simplified) gist of it for a small 4 stroke.

Simply changing to a larger exhaust pipe and a less restrictive muffler will probably offer marginal benefit similar to the effect it would have on a four stroke. However, even small displacement 2 stroke engines can show excellent gains from properly designed exhaust systems featuring expansion chambers. Expansion chamber exhausts (sometimes referred to as tuned pipes) can do suprising things. They are designed to provide the largest power increases within a certain RPM range. Changing parameters such as the length, diameter, or angle of sections of the expansion chamber pipe can change it's operation drastically.

When the engine reaches the RPM range that the expansion chamber pipe has been designed to improve, there is often a very sudden and apparant increase in power. This sudden boost is often called "hitting the pipe." Some designs make the power come in more gradually and other designs can make the pipe hit so hard that it's more of a jolt. On some heavily modified 2 strokes, the rapid onslaught of additional power can be enough to pull the front of the bike off of the ground (don't expect your stock 49cc scooter to do power wheelies with only the addition of a pipe). It's a very impressive thing, especially the first time you witness it.

These are all reasons to choose a pipe that suits the demands of your engine and transmission setup, or modify the engine and transmission to make the best use of the pipes effective range. You may need to change jetting in your carburetor for a pipe, but not necessarily.

Spark Plugs

Some scooterists look for power by switching to spark plugs with different heat ranges, iridium plugs, and spark plug indexing. In most cases, you won't find much power in these areas. Swapping heat ranges is really only necessary if the current plug is showing signs of being cold like excessive fouling or deposits or showing signs of being too hot such as damaged electrodes. The main goal in selecting a spark plug heat range is to have the spark plug operating hot enough to burn off deposits, but cool enough to avoid pre-ignition (when something inside the cylinder is so hot that it ignites the fuel mixture before the spark) and/or other damage. The stock heat range usually works fine, but it never hurts to do an occasional plug read.

Iridium plugs can be a great upgrade for your scooter, but significant power gains are doubtful. Iridium spark plugs do offer benefits over standard plugs however. The properties of iridium allow manufacturer's to use smaller electrodes to better concentrate spark energy and create a stronger spark. This is said to aid in more complete combustion, leading to improved efficiency and power. This may be true, but when you are dealing with very small displacement engines you often don't see much or any noticeable improvement in power output or fuel efficiency. One advantage that you are more likely to notice is that iridium plugs are less prone to fouling, which is an excellent feature when oil is present during combustion as it is in 2 stroke engines. The other great aspect of iridium plugs is that they may actually allow easier starting. Some 2 stroke owners, myself included, have reported much easier cold starting after the addition of an iridium plug.

Spark plug indexing is simply aiming the section of the gap not shielded by the ground electrode where you desire it to be. Where exactly the open spark should be aimed is often debated on 2 strokes. For that reason, I suggest reading more on the subject and coming to your own conclusion. Indexing is done by using special washers called indexing washers. They usually sell in kits including different thicknesses. You mark the spark plug (somewhere that will be outside of the cylinder) where the gap is not blocked by the ground electrode. You then use the appropriate washer to align that mark with the location of your choice when proper torque is applied. Indexing will usually produce between 0 and 2 percent gains in power in 4 stroke multi-cylinder engines. I've never seen numbers for a 2 stroke single cylinder.

Ignition Coil

Installing an aftermarket ignition coil should produce a stronger spark. You really shouldn't need a stronger spark than what your stock ignition coil provides unless you are increasing cylinder pressure or running at very high RPM. It's not a bad upgrade to do, but don't expect any gains if your current ignition system is functioning properly. One great advantage of the aftermarket coils is that they often include a better spark plug boot that is less likely to vibrate off of the spark plug. Don't buy one just for that reason though, because you can purchase better boots made by NGK and install them on most stock plug wires.

CDI

Aftermarket CDIs (Capacitive Discharge Ignition) are frequently claimed to provide a substantial increase in performance by their sellers. This can be true in some cases, but not every scooter will benefit from a "racing" CDI. The two big selling points for an aftermarket CDI are usually the lack of a rev limiter and altered timing or "re-mapping". Many scooters don't have a rev limiter, so the first selling point is null to some riders. If your CDI does have a built in rev limiter that cannot be disabled, then a CDI with no limit may benefit you. This depends what the RPM limit is on the CDI. Some stock CDIs have rev limiters built in that a stock engine would be very unlikely to reach anyway.

The altered timing of aftermarket CDIs may or may not show an increase in power. The effect of an ignition advaned/retarded CDI is also made debatable by variances in different engines and different styles of timing control. Some CDIs provide a standard or fixed timing while others use a variable timing curve which allows differnt timing at different RPM. A lot of tuners find fixed timing CDIs to be pointless, because the timing can often be adjusted in ohter ways for the same effect. I've actually tried the same aftermarket advanced timing CDI on three different bikes belonging to myself and two friends. Two of the bikes showed no noticeable gain, while the third benefited from improved throttle response. Each bike was tried by each owner, and we all agreed. All of the bikes had the same style of engine and similar modifications. Results like that are what make opinions on aftermarket CDIs diverse.

I would advise that you leave the stock CDI in place until one of two things happen. One, you can verify that you have a rev limiter in your stock CDI that cannot be disabled and will limit performance. Two, you have already done a lot of engine work and are trying to squeeze every last ounce of performance out of your scooter. The only other thing that makes a CDI swap compelling, is the thought of increasing performance with such little effort. CDI installation and removal only involves locating the CDI, unplugging it, removing any mounting hardware if so equipped, plugging the new CDI in, and re-attaching mounting hardware.

Cylinder Kits

Cylinder kits can be an excellent choice for anyone trying to make more than a slight improvement in performance. They can also provide a great "bang for the buck" gain, especially with some of the moderately priced kits that are available. Kits may include a cylinder, piston, piston rings, circlips, cylinder head, gaskets, a wrist pin, needle bearing or other acessories. Check the details of specific kits to be sure. You will commonly find two general types of cylinder kits, big bore and standard bore. Big bore kits are much more popular because they increase the displacement of the engine by using a larger bore than stock. Standard bore kits will maintain your stock displacement, but can still increase performance through other means. Standard bore kits are a good choice for someone wishing to compete in a racing class or league that limits displacement, but you can often find more performance for the same price by using a big bore cylinder kit. There is also no need to stick with a standard size bore kit if you think you will remain 100% legal that way, because in most states 49cc scooters and mopeds are only allowed a certain amount of horsepower. Many stock scoots are already rated at the limits of the law.

Big bore cylinder kits will come in varying bore sizes to create different displacements depending what engine you are using. The most common big bore cylinder kits for most 49cc 2 stroke engines will end up around 70cc displacement, but larger bore kits are out there. The added displacement is great, but there are other things to consider with a big bore cylinder kit. Compression ratio, timing, and other factors can make one 70cc kit entirely different than another 70cc kit. One 70cc cylinder kit may be well suited for the average scoot rider looking for a little extra power without sacrificing much reliability or longevity of the engine. A kit on the other end of the spectrum could be intended for racing use only, used for making the absolute maximum power in an engine that will be rebuilt very often. Research any cylinder kits available for your engine carefully, and decide which one will help you fulfill your goals.

Crankshafts

Aftermarket crankshafts can be used to create power in different ways. Better construction with tight tolerances, full circle designs, increased displacement, reduced weight and more. The first thing I mentioned, better construction and tight tolerances, is pretty simple to understand. A better made crankshaft, and bearings for that matter, will relieve as well as handle stresses better and help increase longevity, especially in engines reaching very high RPM. Some cranks are also made lighter to help the engine spin up faster. Another trick that you won't see unless you're purchaising race parts it polishing the crank. Polishing the crank is meant to reduce every bit of surface friction possible, and is more common in 4 strokes to help them cut through oil.

Full circle crankshafts can alter the primary compression by reducing the volume of the crankcase. Reducing crankcase volume leads to higher velocity and attempts to push the fuel/air mixture into the cylinder at an increased rate. This effect can be made greater by "stuffing" the crankcase using a full circle crank that also has chunks of nylon or other material attached to it. More isn't always better. The amount of crankcase volume desired is not universal to all applications.

Crankshafts with a longer stroke (stroker crankshafts) are used to add displacement. This means the piston actually travels farther in the cylinder, enabling it to pump more fuel/air. Your piston can't just travel farther wihtout something being altered. Some stroker crankshafts use a shorter connecting rod, allowing the use of stock components or widely available aftermarket cylinder kits. Other crankshafts rely on the use of special shorter pistons to make up the connecting rod length. Crankshafts need to be a little larger to accomodate a longer stroke, so you will need to know your limitations when stroking your engine. If the crankshaft is too large it can hit parts of the case, so clearancing may be necessary in some applications. It is absolutely essential to know what you are getting or you can have an engine that won't run or trys to destroy itself for lack of proper clearances.

Port Work

Port work can be an excellent way to improve the performance of your 2 stroke engine. Port work can also be a great way to hinder the performance of your 2 stroke. There are a lot of areas that can benefit from some attention inside a two stroke engine, but it is not as simple as it is with 4 strokes. Although not ideal, you can get into a 4 stroke engine and port the intake and cylinder heads with the basic goal of enlarging and smoothing so air has a better path. It is actually a bit more complex than that, but that's the basic idea. If you were to enlarge all the ports on your two stroke with the sole purpose of making them bigger to flow more, you would be likely to affect the engine in a negative manner. Since 2 strokes lack a dedicated valvetrain like a 4 stroke, the piston and ports do the job of the intake and exhaust valves. Making a port larger will increase the time duration that the port remains uncovered by the piston, similar to using a larger camshaft in a 4 stroke engine with longer duration. You should have a very good understanding of 2 stroke operation before you attempt doing your own port work, for that reason I suggest you have it done by a professional or start reading up on more adnvanced theory and operation of 2 strokes. You may alos be able to find guides online detailing what works well for your style of engine and/or cylinder.

Reed Valves

Using different materials, larger ports in the reed block, mounting angles, and the amount of reed petals can change the behavior of your engine. The size of the ports in a reed valve (also called a reed cage) can be sized to allow a greater volume or greater velocity. More volume is beneficial for high RPM power while more velocity is good for low end power and throttle response. Changing the angle that reed valves are mounted also affects the power band. A lesser angle is better for lower RPM and a greater angle is best for high RPM. Selecting the proper reed material is also important. Flimsy fiberglass reads can react faster to pressure changes while stiffer carbon fiber reeds are less likely to flutter in the upper RPM range. Using less reed petals is better for lower RPM power and response and more petals help high RPM power. With all of these variables reed blocks and reed petals can be tailored to suit many needs.

If you would still like to learn more, there is a wealth of knowledge out there. Try searching ScootDawg and the web or pick up a book about two strokes. "The Two Stroke Tuner's Handbook" by Gordon Jennings and "Two Stroke Performance Tuning" by A. Graham Bell are excellent choices for much more technical research.

 

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