To understand the question at large, it is first good to know what the application is and how it came about.
The engine in question is the older type of 4-cylinder engine used in Series-A Minis (not the modern BMW manufactured one, but the one produced by the British Motor Corporation). This Mini used a specific head design which has a Siamese port configuration. The shared port work is where:
- Cylinders 1 & 2 share an inlet port
- Cylinders 3 & 4 share an inlet port
- Cylinders 2 & 3 share an exhaust port
This image of a highly modified Series-A head shows what I'm talking about.
You'll notice there are five ports. All of the ports are on the same side of the head. The square ports are exhaust ports and the round ports are intake ports. This somewhat strange configuration, while compact, has disadvantages. The two main problems being cross-port interference and flow potential.
Cross-port interference stems from the shared aspect of some of the ports. For instance, depending on where the stroke is occurring in the Otto-cycle, the engine could be trying to perform an intake function in one port (at the end of the intake cycle) and one next to it could be starting its intake cycle. Both cylinders could be trying to intake at the same time, which would mean both might be getting starved from the potential air flow if they were using the port all by itself.
Flow potential refers to how much air a single port is able to move. Since cyls 1 & 4 have their own exclusive exhaust port, they have better flow potential than does the shared exhaust port of cyls 2 & 3. Since there is no competition for the exhaust port, cyls 1 & 4 have better flow potential than does the single port shared by cyls 2 & 3.
This brings us to the question: What is a scatter cam?
There are single pattern camshafts where both the intake/exhaust lobes share the same profiles. The cam lobes open the same amount. They open and close for the same duration. A dual pattern camshaft is where there are two different cam lobe profiles on the intake and exhaust sides. A proper scatter cam as used in the Series-A engine, would utilize four different lobe profiles.
The reason for the scatter cam profile is to move the intake/exhaust events which interfere with one another away from each other, so as to lessen the impact of the events. This means less scavenging would occur between intake events on the shared intake runner. To better understand, here is a list of the different lobe profiles needed (as specified by the MiniMania forum):
- One set of timing figures for inlets 1 & 4
- A different set of timing figures for inlets 2 & 3
- A specific set of timing figures for exhausts 1 & 4
- A specific set of timing figures for exhausts 2 & 3
The article on that forum goes on to say:
When considering the problems in hand, it is easier to cut the engine down into two two-cylinder engines rather than looking at it as one four cylinder engine, treating cylinders 3 & 4 as mirror images of 1 & 2. The basic principle then is to arrange the cam lobes to maximise performance by minimising cross-port interference and maximising port flow performance caused by the port sharing and mismatched ports problem. Told you the principle was easy. I suppose the simplest explanation of what a scatter pattern cam does is to say it reduces the shared time of the siamesed inlet ports, thereby reducing the negative interaction of the inner cylinders by the outer ones.
However, before any cam timing figures and phasing can be considered you need to appreciate a few things. The inner cylinder of each pair draws fresh charge first, followed 180 degrees later by the outer one - as observed by the little experiment outlined above, the valves are open together for a great deal of time. Hence the outer cylinder is robbing fresh charge from the inner one, further exaggerated by the scavenging action at the end of the induction stroke of the outer cylinder over-powering that of the inner one. And the inner exhaust port is fundamentally less effective in terms of airflow capability compared to the outer one; one side effect being different scavenging periods.
The simpler form of the scatter cam just adjusts for cyls 2 & 3, as you were suggesting. As you can tell above, there are far more complicated cams which can be designed which helps the entire engine breath better. The simple scatter cam is a trade off between performance and cost of research/development. (This goes back to the old saying, "It costs money to make horsepower.")
The main purpose of the scatter cam is to get the most out of the engine. It is mainly designed with performance in mind. As you can tell, with a normally aspirated Series-A motor, there is only so much you can get out of one of these engines. They were originally meant to push a small car down the road with minimal amount of horsepower needed to do so. Due to the layout of the vehicle (low center of gravity; wide stance), it has become a favorite of European Motorsports enthusiasts. As with all enthusiasts, there is always a goal of better performance. The scatter cam helps squeak out just that little bit more. For the average person, these cams do not make much sense. To the performance minded, it can be that little bit needed to get them to the finish line first.
As to why is it called a scatter cam, the only thing I can figure out is this is due to the scattered cam profiles. As described, there can be four different cam lobes present out of eight different lobes ... the term "scattered" seems to fit quite nicely here. This is an assumption on my part, but seems to work.