Port Mapping

Most of the people who have gotten hooked on mopeds have come into this hobby as their first foray into mechanics. As amateurs, their approach to tuning takes various routes, but sooner or later the inquiring tuner discovers the power of cylinder porting. The port map, or the placement and size of the ports where they contact the face of the cylinder, serves as the 'brain' of the engine, and a little grinding can have tremendous impact on the characteristics of the engine. My first investigation into porting took an artistic approach, borrowing more skills from my jewelry training than my engineering training. Grinding a half millimeter at a time can yield impressive results, and provides excellent practical experience in what affects what, but as the budding tuner's skill and sophistication improve, a higher level of accuracy is necessary to push the ragged edge of performance. Planning a port map and transferring it accurately to the cylinder, so the physical ports come out the same as the theoretical model you designed, is key to pushing a cylinder to its limits.

For this article, I'll be demonstrating the process I use to create and transfer a port map. I'll be using a stock vespa cylinder which i'm planning drastic changes to. I'll have the full write up on this project done in a couple weeks.

Creating the stock (initial) port map:

To modify the porting accurately, its important to start by determining the initial port map. The easiest way to do this is by 'tracing' the ports, the same as you would take a rubbing of a texture or relieved letters. I start by cutting a rectangular piece of paper approximately the size of the internal surface of the cylinder. Apply a piece of masking tape along one of the sides and roll the paper into a tube. When you put it inside the cylinder, make sure the top edge is perfectly aligned with the top of the cylinder and the paper is fitting tight to the cylinder. You want the paper to perfectly mimic the surface of the cylinder wall.

From cylinder blueprint

If you want to secure it in place with a piece of tape reaching up onto the head mating surface, it might help it from sliding around as you trace. You will want the seam to be away from any of the ports. Now take a pencil, and sketch over the edges of the ports.

From cylinder blueprint

Sketch the border of the paper also, to give you an idea of the actual dimensions, then remove the paper.

Now that you have your port map sketch, go along the port edges with pen or pencil and flush them out. Take your calipers and dimension the ports. The bottom edge of the exhaust is typically 'bottom dead center' and is more important than the top edge. Many cylinders, especially on stock bikes, are taller than the stroke of the piston. If this is significant, it might be helpful to measure up from the exhaust port however far your stroke is, and draw a 'top dead center' line. On this vespa, TDC is a full 3 mm below the deck of the cylinder. You can always change the base gasket spacing to adjust the exhaust port to perfectly match the top of the piston at BDC. When you finish up you should have a drawing that looks like this:

From cylinder blueprint

In the case of this vespa, i've marked up the TDC line, the cylinder stud holes which i have to avoid cutting into, and made some notes about deck and stroke.

'The Math'

Now that you know your original port map, you can start to plan the modifications you want. I'm not going to go in depth on this because its a whole different article, or series of articles and there is lots of good info out there already (Jennings, Bell, et al), go to the library, check out a book, and read it.

Once you've calculated your desired port map, take your calipers to the tracing you made and lay out your new 'ideal' port map. You can see on my map where i've laid out the 'horizontal' axis along the bottom, and measured up from the 'BDC' line along the bottom of the exhaust port. In this map, i'm also rotating the porting slightly so i can get more exhaust port in along side that stupid cylinder stud, so you can see where i've laid out a new 'exhaust centerline' and spaced everything off that point. You can also do this all in CAD if you want to take the time to draw it all in cad. Either way you should end up with something like this:

From cylinder blueprint

Now that you've got your port map, take an exacto and cut out the ports. If you're doing 'finger porting' or fully open transfers, you'll want to cut long strips. When its all cut up it will look like this:

From cylinder blueprint

To get the porting traced back into the cylinder now, take your port map, and tape it into the cylinder the same way you did the first time. Alignment again is critical. Align the top edge of the paper with the top of the cylinder, open the tube of paper up tight, and tape it in place. You'll have to either eyeball the centering around the exhaust port, or in my case, the edge of the port near the cylinder stud is going to remain the same so im lining it up with that edge, and level to the bottom edge of the port. Unless you're getting tricky with raising the ports by the base gasket, the bottom edge of the exhaust port is your BDC level. If everything is aligned well and its taped securely in place, the port sizing should work out perfectly.

From cylinder blueprint

Now, we see why we cut the holes out with the exacto. You can either glue the paper in place and use that as your guide, or, simply trace the edges of the paper with a marker. Paint marker is the best, also those 'Pilot gold sparkle' markers work great, because they stick to metal better and are opaque and easy to see with a fine point.

From cylinder blueprint

Now you can pull out the paper and grind away what you've drawn in. The sizes you calculated come out perfectly, and your porting should do what you want.

good luck!