Porting a block

It has been a while. So I figured I would update my blog with a new post. In this post I want to elaborate on how to port the cases, on what you should pay attention to and how you can easily port your own cases.

When is porting your block a good idea? When you change to a cylinder that benefit from block porting...This sounds a bit obvious, but not all cylinder do need a ported block. When should you start thinking about porting your block? When transfer tunnels between cylinder and block don't match anymore. With cylinder transfer tunnels we mean this:

This is where the mixture flows from the block into the cylinder. 

If you move up from a stock 50cc to a low reving 70cc cylinder (Malossi/Polini sport, DR etc), porting your block will not be useful. This is because the transfers tunnels are the same size as the stock cylinder transfer tunnels. You could say, I will just adjust the transfer tunnels myself and match the block to that. That is an option, but I am not a big fan of doing so.

The reason why I am not a big fan of doing so, is that the transfer tunnels are important for the flow/direction of the mixture. Transfer ports in the cylinder are shaped/directed in a particular direction (middle of the cylinder). This is most important with loop scavenging cylinders! This is very much where power is won or lost!

It would be better to fill the engine with some epoxy filler to match it to the cylinder. Again, don't bother with the low reving cylinders. Only the real high-end cylinders (TEAM, EVO, MANSTON, 2FAST4RACE, etc) will benefit from this.

Some professional tuning companies have different "stages" of tuning. In my opinion the most important "stage" is the first stage where block transfers are matched to transfer tunnels of the faster cylinder. Further stages mostly consist out of "flowing" the case. It comes down to removing material and trying to "aim" flow towards the transfer tunnels of the cylinder.

I have tried and tested different ways of carter porting and came to the conclusion that flowing, by only removing material, doesn't matter (dyno/street testing). My opinion is that if you want to do flowing to right way, you will have to do more than just remove material. That means filling up dead spots. There are several dead spots above the crankshaft and several in front of the crankshaft (behind the piston). A good example is to take a look at a racing go-cart engines where most of this is stock.

Take a good look at the space under the boostport.

This case is probably casted in this form, but with some good epoxy filler you can achieve the same effect.
I once tried this on a my own engine, unfortunately, it was never tested properly because the epoxy did not hold. It was removed before it could do any damage to internal parts. It looked like this:

Yes, I know this doesn't look nice, but it was to put my idea to practice.

Off course, carter tuning isn't only about matching the cases to the cylinder and make sure that the flow has a smooth transition to the cylinder tunnels/ducts. Although these mods do make a very noticeable difference, there is also something like carter volume.

To be short; more volume is better. This is because of the suction of the exhaust.

The divergent cones (the first part of the exhaust) creates suction and the convergent cones (parts after the straight part = belly) create pressure. In the old days they would relay on the piston pressing the mixture into the cylinder channels when it came down. Cases used to be stuffed, with very high crankcase compression and exhausts did not have any convergent cones. These two stroke engines were pretty inefficient.

A lot of changed since then.

Exhausts became better and better, creating more suction. We are now at the point that is possible to have over 100% carter volume suction; having the exhaust and the belly mouth of the carb in connection with each other (=exhaust sucking the mixture from the carb). Do note that this requires skills and perfect match and setup of parts. This is not as easy of just slapping on the "best" exhaust you can buy.

Because exhaust do create much more suction, it isn't necessary anymore to have stuffed crankcases and depending on the piston pressing the mixture into the cylinder. The exhaust will simply suck into the cylinder when the piston goes down.

Now that you know this it's also important to know where you want extra volume and where not.
You want extra volume behind the transfers. This can be achieved by having a longer cylinder and a short block or a short cylinder and a "longer" block. Most scooters have a somewhat shorter cylinder and a longer block. The places where you want volume is behind the cylinder transfers. Another tip is to increase the room around the crank. Polini is the only manufacturer I know that recommend doing this mod to their high end cylinder (yes, 70cc too).

The reason why this is done is that when the room between the crank webs and the carter are to small, the crank starts to act as a hydraulic pump and not pumping the mixture in the right direction. Another option is to increase the space between the crank webs, but this is more tricky to realize.

Usually, when you are tuning a carter, the volume behind the transfers get bigger, because the transfer tunnels of the cylinder are wider. Another way to increase volume behind the transfers is mounting a longer conrod. These days you can buy crankshafts with 85-90-95mm conrods. A longer conrod will also give less stress to the piston. The longer the conrod, the less the piston will tilt, the less tilt means less stress on the piston (ring).

It does also not hold, that the longer the better. Using a longer conrod will give the engine more "dwell"; a wider powerband. For some applications, this might be something that you do not want for sprinting/drag racing. Another thing is that you can have to much volume and that it is key to find the right balance (like always).

Most of the tuning to a case can be done with a Dremel tool (or the like) and some sharp cutters to remove material. To can use a base gasket to mark the difference between and start cutting the raw shape. But don't remove all the material, since gaskets and cylinders do not match 100%. Just slide the cylinder over the studs and check with the naked eye.

You can also use a marking fluid to mark the form and shape of the transfer tunnels on the block.

When you use a gasket or marking fluid, make sure you mount the cylinder the way it would sit when it would be frame mounted...


I hope you found this informative and fun to read.

Stay tuned,

Bram