More siamese simulation.. 1+4 vs 2+3.. place your bets


In earlier instalments of this quest (if getting fully lost can be considered a quest) I virtually build a 1078cc small bore engine with an unported 202 head.

Now for a 12g940 head with flow numbers I measured on a ported head. A 1380cc with the usual parts : Big LCB, MS intake with  HIF 44, 1.5 rockers and a short exponential stack somewhat akin to what I have on the shelf from Calver ST.

First the bottom end.

takes about 1 minute to set up.. no problems there


Quite a bit trickier. I decided to use only half of the actual port lengths (then add it to the exhaust part) as it was slightly easier when modelling a 3-1 siamese ( which is exactly the same topography as an LCB btw, just with different lengths primairies). Valve sizes are 35.7/29mm nothing odd here. Max flow is 122 cfm @28 inch H20

Cam is a near as a could get it to a Crane VP3c based on what Vizard published. Short duration, high lift cam on a billet. 276/288  0.465/0.489 inch lift ( so quite aggressive). According to Vizard it is the best thing since sliced bread (he does tend to big it up a bit.. or three bits) and on page 302 of the big yellow book its makes 130 bhp at about 7300 rpm .


modelling an LCB makes your head hurt the first few times. Just in case you have EngMod4t.

i’ll give a brief overview:

Its a 4-2-1 type 1+4 2+3 where the primaries of 2+3 are 35mm long (the rest is in the head if you don’t do that the section will be too short to model if you are out of luck). The end pipes are 35mm Plus 11 inch ( 1-3/8 inch diam) and connect to connector pipe (there is a logic to it) 5, the centre branch is a connector pipe  connecting 2+3 to Number 6 and 25 inch x 1.5 inch diameter.. the whole lot then goes into the collector that is 48 mm diameter and in my case 300mm long.

headache yet ?

Muffler ?…. Don’t need one, as my simulated neighbours are deaf, and it adds quite a bit to the computation time.

ImageThe power I get is a lot lower than what Vizard produces but that could have a lot of reasons.

After all this is a simulation and dyno’s are not exactly known to be the most consistent device on the planet ( of course Vizards is reading low.. ( Did anyone ever hear of any dyno that reads high by the admission of the owner..?? ”No mate my dyno reads high and is wildy inconsistent…that i’ll be $500 then  ”)

Unlike a BMW head, where you can just as well just run one cylinder at a time as they are all the same, the Siamese port head has heavy interaction as you have two horses feeding from the same bucket. Coupled to the end cylinders having their private exhaust ports that are a sensible length and the centre pair having to timeshare one port.

The interesting bit are the lower lines. Unlike a engine dynamometer it is easy to view the individual cylinders. For the most part this is pretty uninteresting as a sensibly designed engine makes the pretty much same power on all cylinders and has similar wave behaviour.

If we zoom in on the power of the individual cylinders you get quite an interesting view of things.

It is a simulation (never confuse the map with the terrain)  but I think that all basic ingredients are there for some additional insight into the weirdness of the a-series engine.


The whole charge robbing concept that spurred creation of the scatter pattern cams  is mentioned on page 261 of the yellow book.

With a not too long cam the centre cylinders make more power up to about 5000 rpm , after that the end cylinders clearly have an advantage and the centre ones hit a wall from 5000 to 7000 rpm. Question is of course why!?

Is it the exhaust action ? You need a fair bit of energy/rpm to make a worthwhile suction pulse,  that would explain why the ends with the better  exhaust would fare better at higher rpm. It should show up in the wave traces.

Same for the intake ramming. Compared to an optimised BMW K head you have only a rudimentary intake ramming action to start with. Could the rather large difference be due to this ?

Well , this is something that has to be investigated further…

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6 thoughts on “More siamese simulation.. 1+4 vs 2+3.. place your bets

  1. jooost did I show you this bit I wrote …

    using my tiny pea brain ,iv been conteplating a theory ,and thought i’d stick it on here .. 🙂

    theres been a lot written about charge robbing and the imbalance between inner and outer cylinder mixture ,and the problems that creates .

    general thinking appears to be ,that the inner cylinder pulls in a charge of air fuel mix ,and then just before in closes ,the other cylinder in the pair opens up and asks for its share .the fuel ,being heavy ,wont change direction so easily ,and so the outer cyl gets a bit more air than fuel ,and so runs lean .

    my thought was ,if you run a tube full of gas (eg , inlet tract) at a certain speed ,and close it ,theres a continuing ram of gas compacting on the closure point,for a moment .

    now , imagine if almost concurrently another opening occurs , in this case the outer cylinder inlet valve ., its now opening on a higher pressure gas column ,carrying with it the inertia it built up feeding no3 . so its volumetric efficiency is raised artificially compared to no 3 .its fuel delivery may be similar to no3 , but its air delivery is way higher …in effect , its been supercharged by no 3 initiating the flow speed ,and resulting inertia.

    this would tend to explain the 100 bhp/litre + outputs of some 5 ports ,where on the face of it theoretical modelling such engines with a ve of 85 % simply doesnt replicate reality .

    if this theory holds water ,then its a feature non siamese port engines dont share .and maybe the only type of engine with a built in supercharger ,if only on 2 of the 4 cylinders . 😛

    ps, im sure this must have been written about ages ago .or its total nonsense lol.

    robert .

    that was a while ago on mk1 forum …maybe your sim program has a problem anticipating the increased charge no 1 and 4 .how about stepping up the port flow specs on those cylinders to fool it to bring power up ? or alternatively , experiment with different atmospheric pressures.

    • mowog says:

      Hi Robert,

      I think you may have a valid point there. Larry Widmer mentioned something very much like that when he wrote about MGB’s in the stone age.

      The sim should be able to model the port interaction as it is a wave based system . but I’ll have to see. For normal heads it works very well. For siamese heads at low rpms i get a much lower torque/bhp than you get in reality.

      But ill ask Dr Neels if he has a an idea what might be going on in his sim. It can not really model non pipe like 3D structures so i think you have to model the port even more like a ultra short runner ( essentially halve the valve diameter) directly into a shallow angle collector. But i’ll ask and see what he thinks.

      Vizard mentions that a bigger fuel droplet size made better power. No mention is made which cylinder ran better this way..
      The inertia of the fuel is of course not modelled but i find in interesting that you see a shift in power from the inner cylinders at lower rpm to the higher power on the outers (quite a bit so) at high rpm. this would suggest that the inertia does play a part and the better extraction of the end branches does too at higher rpm.

      I should repeat with dual carbs so that I can at least keep one of the interactions out of the equation.

      Keith Calver mentioned that he had good results using a big bore manifold on a small bore head with a big step down from manifold to head.

      As far as I can tell that might promote above mentioned behaviour as on the way IN the step will blow a new boundary and not be all that bad for airflow. while the wave bouncing back encounters a discontinuity because of this and bounces back towards the valve. there has to be something that pressurizes the micro plenum that is the siamese port but still keeps if filled after the flow has started.

      it is quite interesting this..

      The funny this is that when you look at the sim data the inner cylinder produce more power at lower rpm and if you DECREASE the diameter of the centre branch power goes up quite a bit after 4 K but the outers are affected as well and produce less power because of the change in centre pipe diameter.. go figure.


      btw good work on the Medusa project.. modeled body looked good as well. nicely proportioned.

  2. Aaron Anderson says:

    Hello Joost, I have been looking at the a series head my whole life and it still perplexes me. Until i started to mess with other engines did i realize how an equivalent engine made more power from the same displacement. Maybe that is why we are so amazed when stuff we do works on a mini. Anyway many years ago i had a thought but i never took it further. Here it is
    Since the outer cylinder is robbing the charge from the inner (or the other way around) it would seem if you gave the robbed cylinder another source of fuel air mixture at the port it would take it in before the valve closed.
    Imagine a 5/8 or bigger tube let into the head (and brazed) right at (throat) the intake valve from the front of the head above the spark plug and it was connected to the corresponding cylinder at 360 degrees. So the 1-4 and 2-3 cylinders are connected by a tube.(tube end is near the valve stem)
    So the inlet charge is running down the 1-2 port and across the valve that is open then the valve next to it starts to open and starts to rob the charge but the tube is connected to the 3-4 port and allows the robbed cylinder another source of charge. Hope that is clear enough, it was just a thought. Hydrolastic

    • mowog says:

      I think the problem is in interaction of the (cylinder) pressures, no matter how much you offer , It only wants to go down a pressure gradient. .but it is quite hard to figure out what you are looking at even when you can plot mass flows (which you can)

      if you add yet another conduit … i need aspirin 🙂

  3. Steve says:

    I am not that up on the A series motor, more on the B series. What is the expected usable RPM range of a motor built as you describe? Showing anything much beyond that on your chart is just clouding the issue and not letting you see the detail that really maters.

    • mowog says:

      The useable rpm range for my development work is 4-8k (actually more like 5-8k as the remark was… 4000 no I never go down there) .It is for North American long circuit RACE use. It has absolutely nothing to do with anything you would want to run on the street or even a small circuit. It is also build using some pretty exotic parts. So what is uninteresting to me is basically anything that puts the piston speed over 23 m/s or is physically impossible.

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