Porting a cylinder

In my previous post I showed you pictures of a ported cylinder. In this post I will explain more on how to port a cylinder. What are the basics, what formula's should be used and how do you decide what timings you should use.

First things first. There is no general formula available that tells you which timing corresponds with which rpm. That would be the holy grail of 2-stroke porting. In all these years I have seen quite some cylinders and measured most of them. On that basis I created the tables that you see further down this post. This should give you a idea/guideline on how timing relates to the type of cylinder or purpose.

Most high end racing cylinders (Malossi Team, Polini Evo, Hebo Manston and the like) are somewhere between 13000 - 14500 rpm depending on the stroke (stock, 44/45 millimeter) and the type of engine (Piaggio or Minarelli). The "normal" racing cylinders (Malossi MHR, Team replica, Stage6 Racing) are somewhere between 11500 and 13000 rpm. The so called "mid-race" cylinders (Stage6 Pro, Malossi MHR Replica, Polini Aluminium) are between the 10000 and 12000 rpm. Anything between 8000 - 10000 rpm are the fast cast iron cylinders (Polini Corsa, Top Performance). Anything below 8000 rpm are stock cylinders, maybe with an after market exhaust. The precise rpm depends on the setup used, mostly the exhaust. Do note that we are talking here about cylinders that fit 50cc crankcases. For an overview of which cylinders, exhausts and carburettors work together click here.

For the 125/180cc crankcases rpm and performance levels are different because of the different stroke and bore used and these will be discussed later on.

Lets start with the formula for calculating timing (or port opening). This formula can be used when you measure port distance with a caliper.

The formula for calculating timing is;

D= (180 - Cos(T²+R²-L²)/(2*R*T))*2

T = R + L + C - E

R = Stroke / 2 in millimeter

L = Conrod length

C = Squish (or deck height)

E = Distance from the top of the exhaust port to top of barrel in millimeters.

There are several ways to measure the distance between the top of a port and top of the cylinder deck besides using a degree wheel. You could modify a caliper so it can be used inside the cylinder which works well for measuring distance. If you do not wish to modify a caliper another option is to use piston and piston ring for marking the opening of the port and measuring the distance with a caliper.

Measuring the distance between top port and cylinder deck is done on the cylinder itself and not from a portmap! You should measure precise on a tenth of a millimeter. One millimeter equals about 5 degrees (with a 39,2/39,3 stroke) in timing and will become even larger with a bigger stroke.

Below I will explain the steps for measuring timing with the piston ring and caliper because it is the most straight forward method. Although this is the most straight forward method, it is prone to errors. So be precise with your measurements.

1. Put just the piston ring in the cylinder
2. Press it aligned with the piston
3. Press the piston ring with the piston up to the top of the port that you would like to measure
4. Press the ring up until it almost closes the port. Sit in a room with enough light to see it clearly. Measure each port 3 times and take the average of the 3 measures.
5. Repeat these steps for all ports

Now that you have the millimeters for all of the ports you can easily calculate by hand what the timings are on your ports. Although it is "easier" to put the formulas in an Excel sheet (requires some excel knowledge). Working with an Excel sheet is easier because you would only have to enter the millimeters you measured into the destination cell and get back the degrees  from the output cell. 

Do note that some cylinders have heads that fall into the cylinder! Take this into account with your measurements. 

With these original timings in mind, it is time to decide which timings your cylinder should get. The first thing you should ask yourself is what the purpose is/will be of your scooter. Do you want to drive it everyday with just a little extra punch for those long straights, uphill roads, do you want a fast daily driver that it also able to cruise at 100 kmph (62mph) or are you gonna (drag)race with, where maximum power is needed? For each of these purposes you will need different timings (and usually different cylinders/setup) to achieve.

Sure, having a full race setup for daily driving is fun, but running costs are high (oil, fuel consumption, wear of parts, maintenance etc), require a precise setup of all parts and make a lot of noise that is not always appreciated. It's like taking a F1 car to work. From my own experience it's more fun to have a scooter that works, that you can use everyday. It might be a little slower than having a scooter that is fast, but only when it works. But than again, it is fun...

I made a spreadsheet of exhaust port timings (transfer port timings will come later on). These tables below give you an idea of what timing works good for which purpose and what happens to the timing when stroke changes. I made them for the Minarelli/Piaggio 50cc engine, Big bore engines (44 or 45 mm stroke, still 50cc based crankcase) and the Piaggio 125/180cc engine (Gilera Runner 125/180cc for example). Lets start with the Minarelli/Piaggio 50cc engine.

The dark green values (<180 degrees) are low timed. They don't give much power but are reliable and can be used 20 000 kms without changing rings/piston. The dark green values are most of the time stock cast iron cylinders, which feel best somewhere between 6000 - 8000 rpm. If we move down to the light green area (180 - 185 degrees) we get the more sportive cylinders. Still reliable, but give a little more grunt and will do more rpm (8000 - 10500). From where the cells begin to start yellowish (185 - 190) we get the so call mid-race cylinders. These cylinders are a good basis for fast daily drivers with occasional road racing. Ports in these cylinders are relative small, giving the pistonring good support but are still high enough to get rpm up. These cylinders will run from anywhere between 10500 rpm and 12500. From 190 degrees and up we come in into the area of the real racing cylinders.

The reason why the cells run from 20 to 19 in steps of .1 is because from this point everything starts to listen closer. From this point cylinders will become powerful, but also more peaky. The reason why the table does not go much further than 200 degrees exhaust timing is that there are no exhausts available from factory that handle those high timings, i.e. rpm are higher than the powerband of the exhaust. There is an optimal point between torque and rpm (=HP) and at 200 degrees exhaust port timing, there will be less torque left than with lower timings where factory exhausts work. This loss in torque will not be compensated because a two stroke exhaust will not deliver torque out of its powerband. My opinion is that a cylinder that has an exhaust port timing over 200 degrees is a cylinder that is not usable anymore. The same story goes for the big bore engines.

Because of the larger stroke, cylinders are longer, increasing the distance between top of the port and cylinder deck. Usually these type of cylinders (long stroke) will do a little less rpm because of piston speed. If the stroke increases, the piston speed will increase too.

Anything that moves around 20 meters per second (m/s) is fast. This is where the high end/fast cylinders are. Value above 20 m/s are possible but require the right materials. Piston speeds above 25 m/s will put extreme stress on piston and other parts and require exotic materials. Pistons that travel more than 25 m/s are doing more than 900 kmph (563 mph)!! Most scooter engines will not do more than 15000 rpm. From my own experience, Piaggio engines are (in general 500 - 700 rpm) lower reving than Minarelli engines (70cc racing cylinders). 

Because of this reason I would not advice to go over 190 degrees exhaust port timing with the 125/180cc Piaggio engines. Pistons are relative heavy and with those high piston speeds things might get blown into pieces. Do note that 190 degrees on these 125/180cc engines is nothing suited for daily driving. 

By this time, you might wonder how do (exhaust) port timing and rpm relate? Unfortunately there is no simple answer to that. RPM depends on tons of variables. Carburetor size, crankcase volume and how it is flowed, port layout of the cylinder, compression, combustionhead shape and volume, timing of the cylinder ports, exhaust length, exhaust deflector angles, width of the exhaust etc.

A much used rule of thumb is that 1 millimeter up equals 1000 rpm more. This will work up to a certain point. You simply cannot keep increasing port height in order to get more rpm. In theory this rule might apply, but since the idea of tuning is optimizing the match between parts used, one must tune the cylinder in such a way that it can be used with the exhaust that is used. With the lower timed cylinders (dark green to yellow timings) it is save to say that increasing the exhaust port height with 1 mm equals about 1000 rpm more. The cylinders that are timed in the yellow part, can be brought up to the orange - red spectrum where this rule does not apply.

For example, if you have a cylinder with 185 degree exhaust port timing (21,6 mm with 39,2/39,3 stroke) and increase that with 1 millimeter (20,6mm) you are still in the timing / rpm area where you have exhausts that work well. But if you have a cylinder that already has exhaust port timing of 198 degrees (19,5 mm with 39,2/39,3 stroke) and increase that with 1 millimeter (18,5) you end up with a cylinder that has over 200 degrees exhaust port timing. Good luck finding/building an exhaust that gives power(band) with those timings.

Where in some cases only raising the exhaust port timing can be enough to get the desired performance, you will have to raise the transfer port timing in most other cases in order to keep driveability.

This is because raising the exhaust port will also increase the blowdown period. The blowdown period is the difference between the time that the exhaust port opens and the transfer port opens. It is the period where the exhaust sucks out the burned mixture. The blowdown period can be calculated by:

BD = (exhaust port timing - transfer port timing) / 2

The BD determines the engine characteristics, because it determines where and how the powerband kicks in. The powerband is the distribution of power over a given rpm range. With a low BD (25 degrees) you will have a wide powerband, but there will be no high power outputs. With a high BD (35) you will have very small powerband, but the output will be higher. Engines with a low BD ride smooth, engines with high BD are on/off power switch engines, better suited for racing and sprinting applications. A BD of 30 is a good trade off between the width of the powerband and power output. Most racing cylinders are around a BD of 32 - 33 degrees.

So when you decided to increase exhaust port timing, but you still want driveability/usability you can increase transfer port timing to get a lower BD. What goes for exhaust port timing goes for transfer port timing too. Higher doesn't have to mean better.

The (dark) green area (110 - 120 degrees) is where the stock cylinders are. The orange - light red area (125 - 133 degrees) is where the high end racing cylinders are. The so called mid race cylinders are somewhere in between. The reason why this table doesn't go further than 135 degrees transfer port timing is because it has no use. The larger the transfer port, the lower the flow speed is of the mixture, meaning that you will need an exhaust system that is capable of sucking more fuel into the cylinder or having an inlet system that can push more mixture in the cylinder. Another thing is that with these high transfer port timings, the pulse created by the exploding mixture ends up in the transfer ports, blowing back mixture into the crankcase and thereby disrupting the flow/cylinder filling.

Another factor to take into account is the way ports open. An exhaust port that opens gradually sends a weaker but longer pulse into the exhaust chamber. Giving a wider powerband, but again, less power output. An exhaust port that opens simultaneously over the complete width of the port sends out a shorter pulse, more powerful pulse, giving a smaller powerband, but more power output.

The same goes for the transfer ports. When the transfer ports open after each other (starting with the transfer port closest to the exhaust port), the powerband becomes wider. If all transfer ports open at the same time, the powerband becomes smaller, but power output will be higher. This has to do with how the mixture flows through the cylinder and how it mixes with the burned mixture leaving the cylinder.

Now that you know all these things and you decided what specifications your cylinder should get, it is time to get your marker and mark the distance in your cylinder. You can use the piston ring and piston again for this. Remember that it is easier to remove material than to add. Also take into account the BD period, hence if you don't have the tools for porting transfer ports, don't go crazy with the exhaust port(s).

Take the piston ring and press it up with the piston to the required distance. To mark the right distance I use a black fine liner. But be careful (!): If you use a 1 millimeter liner, you should add 1 millimeter to your distance! Otherwise your port will be 1 millimeter higher than it should be when you are done tuning, which might render the cylinder useless.

Tuning a cylinder is a step by step process and can be time consuming. It is important that you take little steps at the time, work precise and think before you do. Tuning a cylinder is something you learn by experience and testing what works and what doesn't. 

I hope this post gave you some insights on cylinder tuning. As you can see, there is much to it. If you have any questions, leave a reply at the bottom and I will come back to you!

Stay tuned!

Bram

*Disclaimer: I am not responsible for any bad results, you might get from trying your cylinder according this guide, in anyway or in any form.