Part one: Exhausts
The Siamese port head might be ancient, but it sure makes things rather complicated rather quickly. The normal ”rules” , if there where any in the first place, do not seem to apply. This is odd as the rules mostly do apply, but you’re just looking at it wrong. In this case there are two , or rather three things that set an A-series head apart from a 8 port head. Two Siamese intakes and one Siamese exhaust port.
The nice thing is, when you are trying to model something , is that you are forced to divide the process into small steps and looking at it very closely. If you look very closely at the above cat, the shape of the problem will become clearer as well. Its head looks like a siamese port, the acute problem however it that the Siamese will become annoyed and poke your eye out.
First the exhaust side
The Cylinders 1 and 4 get the benefit of a private exhaust port that could be tuned somewhat conventionally. The centre pair of cylinders have to share space , although it is more like a time share apartment, as the pulses are 360 degrees apart and the exhaust valves of cylinder 2&3 are not open at the same time.
In essence this makes it a 4-2-1 or Tri-Y system, where the plumbing is as follows : 1+4 and 2+3 into 1 collector.
The outer branches (1&4) can be varied to suit , but the hidden centre Y is fixed. Even though there is no sharing of port space as such, the effects of the branch/manifold ( a 3 pipe junction going to the inactive cylinder and the exhaust) on the wave behaviour has to be taken into account none the less.
So all Siamese 5 port exhausts are more or less 4-2-1 systems . Including the LCB (long centre branch), and even the 3-1 while more like a 4-3-1, you could also look at it being a 4-2-1 with super short secondary end branches
Due to the ports being what they are, the end branches are probably more easily tunable. In recent developments (uuhm in a engine where things are measured on a geologic time scale, meaning 6 years ago) Maniflow came up with using a slightly enlarged stepped centre branch that provided some gains when combined with a RCM ( reverse cone megaphone) in an effort to equalise things. Judging from what I have read so far , I suspect it being quite configuration specific. But it is certainly worth a closer look.
So far I have made progress on the simulation front, but it still is not on the level where I’m sufficiently satisfied with the correlation to measured data . Where a simulated BMW K 16v head conversion spits out very decent curves on the first try , the Siamese head is a little , well quite a lot, more involved and takes a lot longer ( a few hours @ 2.8 Ghz vs about 40 minutes for the 16v head) to calculate as well, due to the number of branches and port interaction involved.
The beauty of this simulation is that you can actually look at the wave traces in ”real time” . Down side is that it is not always evident why a certain pattern produces more more torque than another. Furthermore my simulated engine is very prone to having a huge dip at 3000 rpm that I have not seen in real life, and does not produce power like it should. Well I’ll have to soldier on on that front.
I’m doing this all still on a test version of EngMod4T, but I will buy it as you really get an insight in the matter as presented in the Blair 4 st book.