Mass flow in the siamese intake section of the 12G940 Head

top-view-schematic-siamese- There will be lots of wavy bits when my computer stops crunching numbers. First a disclaimer. This is SIMULATION data and should not be confused with truth. The A-series is a funny beast and a lot more complicated than say a 16V K series. The model has been improved a few times mostly to get rid of errors I made. The intake tract is modelled using the casts as a basis to calculate the flow area for each section. As the model is 1D and assumes circular pipe all the CSA (cross sections areas) have to be converted back to circular pipes. In this case it is a single HIF 55 on a RE style large volume manifold with a slight twist that does something interesting . First my personal view ( although I write it like it is fact here) on the whole high speed port idea in the case of an A series engine. Strictly speaking the port is the green part above. The blue part is a plenum and the red part a zip pipe to tune the plenum. Depending on how you add a length of pipe to the red part you can alter the tuning. The plenum is very undersized and a such it can not function as a reservoir. If you look at an NA F1 plenum you will see it’s a huge volume, not without reason. When the air enters the blue section the CSA suddenly increases by quite a bit slowing down the air, which is probably not all that bad considering it has to take a 90 degree turn right after, and a very tight 90 degree turn at that. What allso could/might happen is that there is some modicum of pressure recovery.   A high speed port would probably better referred to as a high speed manifold, in the case of an A series head. A twin SU with a straight short manifold will be like a longer zipe pipe connecting to an infinite plenum (the atmosphere), while a single SU will be more like a dual coupled plenum  especially when it has a very large volume. The question remains what happens in the siamese part. In the simulation you can place traces (basically a virtual pressure sensor) along the tracts. In this case three traces are taken from the beginning of the red part, and border between both green ports and the blue section. The red part ( black in the graph) sees the combined flow, while the other traces show the mass flow of the adjacent ports.


Mass flow traces at 4250 RPM (Peak Torque) Black trace is start of head. Red is cyl 3 ( inner) Green is cyl 4 (outer) (No 3-4 cylinders are easiest to graph).

power curve 1380.RE13 type cam LCB Head is one of mine that where the flowbench data is used to calculate/approximate CD values.

Power curve
1380.big volume manifold .hif 44 with a 150 mm rampipe , 278 degr RE13 type cam, LCB with larger centre branch
Head is one of mine 35/29  vizard pattern where the flowbench data is used to calculate/approximate CD values.

mass flow and pressure traces

mass flow and pressure traces

Next is what to make of all these curves. If you first concentrate on the flow through the shared section and look at the mass flow and the pressure trace you can see very heavy pulsing at max torque.The mass flow that goes into the siamese section more than the flow into the first cylinder and by the look of the pressure trace below it does seem like the blue section is pressurised to some extend when the inner intake valve is can you derive from these curves? (while it is a simulation the values do seem to make sense and the theory behind it is solid thermodynamics).

1. The inner and outer cylinders do not ingest the same mass of air/fuel

2. At peak torque the difference is not huge

3. At peak power the difference is significant ( traces to follow)

4. The adjacent cylinder does not seem to ”rob” anything from the other, at least not with this cam. I will see what happens when you stick in a 320 degree cam

5. The intake has a very heavy pulsing nature, but that seems to be a good thing as at max torque it is very pronounced. I will have to investigate more as to whether this pattern of pulsing is a recurring event at max torque OR maybe it is just rpm dependent phenomenon.


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3 thoughts on “Mass flow in the siamese intake section of the 12G940 Head

  1. looking forward to it.

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