Why does adding manifold length does something different than adding more trumpet ?


The sensible choice for more Trumpet

In the  big yellow book David Vizard remarks that adding more bell to the induction system of a mini was no substitute for manifold length, but it did not go into much detail as to why. I always found that a bit odd and thought it had to do something with the difference in cross sectional area of the venturi in the carburettor causing a reflection . Then recently Dr Vannik commented on a thread somewhere on Speedtalk which explained it ( further)  . While the constriction of the venturi might still have some effect for how it reflects the finite pressure waves, the other reason is actually quite simple.

The propagation speed of waves in a gas (or a solid for that matter)  are dependent on the density. That is why when you stir a fizzy drink with a spoon it changes tone: the amount of co2 will go down and the density will go up hence the speed will increase and the tone will go up.

The density of a gas is dependent on temperature, that is why a hot air balloon does what it does. Hot air has a lower density than colder air and hence it will want to float upward. Maybe more correctly the denser air has to go down due to gravity. Anyway, we like cold air as it is more dense and pack more oxygen met volume unit, thus you mix it with more fuel and set it alight, releasing more energy.

If you wash your hands with alcohol ( or any volatile liquid, but they tend to be really nasty)  it will feel very cold as it extracts energy in the form of heat during the evaporation process. That is why foggers on an inter-cooler work or why a fire extinguisher can cool your beer quite quickly.`

Now put this all in one bowl and shake a bit and there is the answer:


The point where the fuel is added is the starting point of the evaporative cooling process..

The cold pipe will tune to a lower frequency for the same length.It is not a huge difference but a few hundred rpm’s is entirely possible

Adding more bell length will add more length to the intake as well. But if you have a limited amount of room and you need the maximum effective tuned length, you would be better off using a very short bell and all manifold.

Of course too cold is not good as well as you will get frozen carbs. I’m not sure if it is a myth but it seems that the idle gear was needed as they turned the engine around to have the carburettor at the back as they had problems with icing. Then again how about the 7 and 8 port engines ?

For a port injected car it does not matter much as the injectors should not cause much of a CSA change and the position of the fuel insertion will be the independent of the intake length

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Feel the power of the smurf !


Congratulations to Richard ,and  Jim and Jan and further crew with the stellar performance in the VARAC Canadian Historic Grand Prix  16-06-17. One win and a few 2nd 3rd and 4th places the last one just the odd second behind a Ford Cosworth powered Ginetta which unlike a mini has Aerodynamics.

Ginetta G4

a found on the net Ginetta, really light, slippery shaped and with a Ford “Kent” Cosworth    x- flow under the bonnet




A very wet slightly unhappy Smurf during a shakedown/race earlier this year. The unhappiness was due to dying ignition parts it later turned out. Still running mid pack with 2,75 cylinders working

The Cooper S is smurf blue and equipped with an actual Smurf to make it go faster. I have been trying to do my small part to further the engine “development program” (the team being a few blokes in a shed with wild ideas with a fixation for 5 ports a series). After a few tries ( well 5) the pin now seems to stay firmly put in the A series grenade and it did it’s best to sound like a proper race car while revving to quite scary speeds for the whole weekend. It also managed to beat all other a series powered cars a few times so I’m quite chuffed to have played a part.

Plans to squeeze a bit more power out of it are in motion, so hopefully we can bridge the aero gap with slippery cars like a Ginetta, although looking at the  approximately  350 isch bhp (from a 997 cc !)  of the Project 64Kiwi LSR mini which did  146 mph, the long back straight will always be a challenge when you are shaped like a brick 🙂




Not much stirring..


As I’m currently doing actual science  and I am a workshop less ex-patriat for half the year, things are a bit slow.  Also since I moved I now have directly attached neighbours and flow testing is a very noisy business at times.

There are some very exiting things brewing, but alas you won’t be able read about it here as I would have to kill the lot of you (and that is a hard job as people actually read this blog halfway across the globe).

What is the frequency Kenneth ? A rethink on the LCB theme part-2 and some enormous segways.

cuckoo clock

Well that took a while but I finally managed to make a picture of  a few simulations. Never mind the ridiculous names (RP , fear not, it is not). Power figures are halfway realistic but should not be taken too seriously as I have been fiddling with burn models and it is based on an actual race engine and we don’t want to give too much away, do we . If you want it to say 200 bhp it can be arranged..


Two power curves, the only change is the exhaust. The black line has a purpose build 3-1 race header. It looks more impressive that it is due to the scaling of the graph, but it helps readability.

LOB vs 3in1

Before anyone starts emailing angry : yes it starts at 4000 rpm and goes to 9000 rpm.

Is 9000 rpm a good idea with a 3.2 inch stroke? Basically yes if you are interested in only power, but there is a large BUT, and that is that the piston speed is 24.4m/s. At 9250 it is 25 m/s and at 9800 27 m/s and that means that it then has a higher piston speed than a world super-bike, 20.000 rpm old skool v10 F1 … Basically the only things that have an even higher piston speed are top fuel and pro stock dragsters (ca 30m/s) but they only have to last for a few seconds.

Is >9000+ rpm possible for standard stroke an A-series engine, principally yes, but it will not last long (as in road engine long) . It will also need some very fancy parts to actually make more power. Starting with a Killer 5 port head or an 8 port or better a KAD or BMW-K 4v with porting. Then a nice crank,rods, light pistons so it can stay together and in the end it will  be very very expensive. They do tend to go Co Coo on you as well, and it involves oil, blood, sweat and tears and an overheating credit card.

But any-ways,  spinning a bit faster in a race setting is not such a bad idea if you wallet can stand the stress and it actually produces a faster car ( I’m not even saying more power, because you can have more peak power and go slower). However the 5 port head does not breathe all that well at higher rpm’s, so it does make sense to not rev it where it has trouble breathing and try to make as much power as possible is the rpm band where it does breathe ok.

A more fundamental question is why does it not want to do it. The head flows about 130 CFM and the often used rule of thumb says 2 bhp per cfm (which is BS I know, but still), hmm maybe try 130 x1.67 ( which a more conservative rule of thumb number coming from Superflow) is still 217 bhp..

So why do we only get 123 ish simulated BHP? Well apparently the rule of thumb does not work for an A series, but it does for a lot of other engines otherwise it would not be a rule of thumb ( a classic case of wrong but not unfounded).

Breathing is not CFM flow capacity on a bench then. Well that is bad because I know exactly how to get a lot of flow.. use a very big hole. What makes power are not high cfm numbers but high trapped mass numbers. It is chemical power (there is a very good NASCAR 101 lecture on this on youtube) in the end that makes cylinder pressure, which turns the crank, which makes torque.


And KiloWatts are a measure of power per second (1000 j/sec)

So to make lot of KW you need TRQ and RPM.

For a high trapped mass you need a efficiently working induction tract , meaning intake and exhaust working in unison with the cam timing to cram the absolute maximum amount of mass into the cylinder, then close the door quickly and hopefully the mix is in a state that will burn in a manner that produces the right amount of cylinder pressure at the right part of the crank rotation.

Ok via this enormous segway we come back to the root problem : half of the engine has a very bad bmep at high rpm. The other half is actually not all that bad

As I have stated before : In an A series the inner cylinders do have the problem that the exhaust is shared, but to make matters worse it is basically tuned wrong for the rpm the outers are happy at. This results in a lower trapped mass and less stuff to burn, causing less cylinder pressure



Screenshot 2015-11-04 23.43.01

The P/V Loops that illustrate the difference in power (here surface area of the upper loop) for individual cylinders.

The red and green lines shown in the figure show that the 2 & 3 cylinders basically keel over at 6500 on the engine with the needlessly complicated name, but the” rerun “lines do flatten out but they don’t fall off. The outer cylinders are basically unaffected (keep in mind that this sim is based on a race engine that pretty well sorted and it could be a lot worse). In reality the power will fall off a bit at the very top but as I said before I am still futzing  with the burn model.

So there you have it, loose a little pipe, add a little pipe. Below power per cylinder compared between a 3-1 ( which is better than a LCB in this application) and a improved tuning version. Note that the green and orange lines gain quite a bit (30 vs 26 isch bhp ergo a gain of ~8 bhp on the inner cylinders).

Screenshot 2015-11-04 19.07.59

This idea has had a fair few iterations before it landed here. The original idea I started out with was suggested by a guy who has a long and prolific background in killer 2 stroke engines btw. His suggestion initially was to lengthen the outer until they were just right then force down the frequency using basically a very long LCB

The beauty of simulations is not that they can tell you what to do, but that you can go trough a lot of ideas relatively fast without cutting and fabricating a zillion exhaust manifolds. You do have to have a fairly reasonable grasp of the matter at hand though, otherwise you will iterate yourself into oblivion.

If all is well you end up with a thing worth building and testing in real life on a dyno.There is still no guarantee that it works , but if the model is good enough, it should be quite close.

We will build it (Meaning  I wont but it will be build).

If you really want to build one drop me a line.

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What is the frequency Kenneth ? A rethink on the LCB theme part1

I have written a bit about my effort in simulating a 5 porter and a reverse 7 porter ( with proper exhausts and shared intakes)  and the effects of pipes on the power delivery.

The 5 porter has a lot of problems, not the least of which are the shared exhaust ports. I know that there has has been a lot talk about charge robbing and such, but the exhaust has a much larger impact on mass flow than the intake, however well tuned it may be.

Wave Tuning is frequency matching and if you look at the sketch below things should become slightly clearer.

Note that the pipe coming from 2&3 aka Centre Branch is a secondary where as the pipes from numbers 1 and 4 are primary pipes.

The centre Branch has twice the amount of pulses as the nr 1 and 4 branch. Not exactly rocket science but still IMHO a salient point.  The LCB has a LONG centre branch and it works quite well. Why does it work ? Well that is a bit of a mystery. Well we all sort of know that it works quite well, and that the values in the big yellow book are pretty hard to improve upon, even though they are by now 40 plus years old. But why does adding that bit of pipe work ?

If you look at just the centre branch and forget about the other cylinders what would be the sensible length compared to the outer branches ? The two outer branches are single cylinder engines ( forget about the intakes for now because that will make your head hurt) and the middle pair is either a twin with primaries that are tuned to 5.000.000 rpm ( at 12mm long) or it is an angry single cylinder running at twice the speed of the two outer singles.

If you look at it like that the CB of the LCB should if anything be shorter, not longer as it has twice the pulses !? So while to overall power might be helped with going LCB, the inner cylinders are paying for it by not  having an ‘optimal’ (hey it is an A-series) tuned pipe at higher rpm. The simulations strongly suggest that the inner cylinders are indeed not pulling their weight at higher rpm’s.

Before someone comments on the rpm’s and usable power. Yes you are right, normal road engine speeds are from 1000 to say 4500 99% of the time and maybe 4500 to 6500 for 1% if you are a very sporty driver. To be honest if you want a fast road engine there are plenty of proven ways to do that. It is basically a 240 to 270 cam (sw5/sw7/RE13/piper255HR or what Graham Russell has come up with lately)

Add a decent head and LCB and a hif 44 or twin Hs4’s and you’re set, the hard part is not to build out 15 bhp at assembly . It has all been done, but lets face it you are not going to break any records power wise (without a turbo) when you do not rev past 6000. For circuit racers in a mini or a sprite/midget, especially those racing on long North American circuits, the rev counters will only sporadically go below 4000 , and that is probably a when you bodged an up-shift. So for this use the equalising of power between cylinders is getting quite useful. Now, how could one try and achieve this ?

Provide each cylinder with what it likes. There we already run into a few problems. What it really likes is a intake and exhaust port all to its self, but that is not going to happen unless you use an X flow head . The intake situation is basically what it is and adding a lot of manifold length seems to work well there.

On the exhaust side, the ends have all the luck, and at least the centre cylinders have two mirrored cylinders sharing the same exhaust port quite far apart pulse wise. As a consequence it is probably ok to treat the centre branch as a single cylinder but at double the rpm. Hence the length of the centre branch will be not longer but shorter as the frequency will be higher.

In the next part the effect of this idea will be illustrated by simulated data.

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Cd maps : beyond simple flow bench numbers. ( still being edited)


A flow bench is nothing more than a Hoover with a measurement device attached coupled to whatever object with a hole in it you want to measure.

Whatever value you measure is dependent on how hard you pull at the straw. There is a perpetual debate at what depression (negative pressure) one should test heads at. First you had the small Superflow SF100 desktop benches that measured at 10 inch water depression. That is a pressure ratio of 1,025 ( or 0.025 bar pressure difference). Then it moved to 25 inch H20 for the popular SF600 benches (There are a lot of other ones as well but I’m not about to write a short history of flow-benches). Then you have the 28 inch apparently propagated by Smokey Yunick ( A Nascar legend with a cowboy hat that few people at my side of the pond knew about before the internets) of which a lot of people think is THE magic number. The magic being that is was supposed to be close to the depression as produced by a running engine (I don’t know if smokey actually thought that but ”people say”) . That is total codswallop it turns out.

Do you manic depression ?

Do you need full on manic depression ?

How much depression does a person or a flow-bench need ? The answer is a little more complex than you think at first. The actual running pressure on an engine varies with rpm and valve lift.

What a standard flow bench test does normally, is test at one fixed pressure and a lot of valve positions. Then it measures the amount of flow in CFM , CMM or whatever you happen to think is convenient. There are formulas that are somewhat usable when you want to convert from one depression to another, as long as you do not try to convert from 10 inch depression to 200 inch depression.  200 inch! I hear you say . Well it turns out that actually the running pressures at lower valve lift are 200 inch H20 and then some, at full valve lift you are looking at about 60 odd inch H20 depending on what engine you have.

A very nice write up can be found in PDF here ( by Vannik developments)

Back to the conversion and why it only works when you convert from a value quite near to the one you have. The reason is that the efficiency of a poppet valve is dependent of the pressure differential. So 25 to 28 probably no problem, 10 to 150 probably not so good, but that depends of course of the head in question.

So how do the people who actually know what they are doing ( i.e. not me), make sense of this? Well they use a sort of flow bench, but not a blue one with a bunch of Hoover motors. It is basically the same concept but build using sturdy metal pipe and it does not use a bunch of vacuum cleaner motors but a cellar.. Yup a big 150.000 Liter bunker that they suck all the air out until they have a 700mm Hg vacuum. That is 375 odd inch H20. Open the tap and hey presto you have a pressure ratio of 2.

Now instead of one pressure and many lift points, you plot many pressure ratios vs valve lift. The end result is a Cd map that looks a bit like a contour height map, but instead of drawing lines between point with equal height, they are drawn between points with equal Cd.

GP Blair : SAE R186

GP Blair : SAE R186 . These are maps for an intake valve but if you swap flow directions then it is an exhaust

Ok Looks pretty but how can it be useful ?

Cd is basically the Discharge Coefficient which is a metric for the efficiency of a valve. the Cd(actual).

                              Effective throat area

            Cda=  ————————————

                          Geometrical throat area

Read as follows : How much of the available area is actually used. If you have a very efficiently shaped valve aperture you will use more of the available area at higher pressure differentials, moving more mass. If you have a less efficient aperture it will/could be more non linear .

The figure above tells you that up to a quite high lift point the Cda of the valve at inflow is pretty much independent of the pressure ratio. In this case for a 2000cc BL engine ( I suspect it is a BL-O series, not the pinnacle of British engine design I must say) with 2 valves at 90 degrees with a cam lifting L/D 0.66 ( or 9.6mm) . You could come to the conclusion that testing at super high depression is not of much benefit for most of the lift curve, just on the very last bit it might make a difference.

This is not the case for the outflow and exhausts in general  especially  for higher lifts and lower pressure ratios (the right bottom corner of the map) the Cda suffers quite badly. Slightly counter-intuitive, but by lifting the valve less is gets quite a bit more efficient.. Of course you still have to have enough mass flow capability, A 100% effective, but tiny valve aperture is still not going to flow the amount of mass you need. At a certain point the potential gain of the extra lift will be offset by its increased inefficiency.

This is corroborated by people running identical 1/4 mile drag race times with exhaust cams that have been wiped (that is lingo for ruined and missing a large part of the nose of the cam reducing lift substantial) and the common practice of running less lift on the exhausts in American V8’s, as does Porsche with some 911’s (you can’t really say ‘the 911’ can you.).

Of course it depends on the port and such, but the plots are uncannily similar across designs including pent roof 4v’s so it seems to be a general trait.

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This is not a pipe


I’ve been beavering away at this simulation lark for quite a while now. I think  everyone who is interested in understanding an internal combustion engine  ( Be quick ! they are bound to be replaced by supercap powered electromotors in the next decade I reckon) and does not have an allergy for computers and maths should get EngMod4T and a copy of publication SAE R-161. As far as engineering books go, it is quite readable, but a novella it is not. I have read the first few hundred pages, but let face it, it is quite hard to read it like a story. The Fun part (for me at least) is to fiddle with a model, then stumble upon one of many oddities that arise then refer back to the book and re read it. Sometimes you are just as confused as before, but every once in a while you start to get it.

What bugs me is people telling me that you need to spend $5000000 to get a simulation that works. The argument mostly goes like this : OEM’s spend millions on software, how can a $99 or $500 piece of software be any good.

OEM’s will spend more on the design of a doorknob than a mid level race team on engine development.

What OEM’s want, is to cut development time on a 800 million euro platform development, they are not really bothered by a 10K a month licence fee, if it does what they want it to do, fast. Something like AVL Fire or GT power can and has to, cost a lot because the market can bear it and because it costs a lot to develop software that can tell you what happens when you move a panel a bit like this, the pipe xyz gets more like that and how is that going to affect the emissions and the temperature of the glove compartment. Then only sell a few thousand seats at best.

But still it is a simulation, not reality. Just as the picture of the pipe above is not actually a pipe, the simulation is not actually an engine and in the case of a 1D model is it not even a mathematical 3d model of 3d physical model as you seen in many of colourful CFD picture and animations, but a more simple linear mathematical model. The maths and the ”laws” of physics are also a model of reality. Newtons laws of motion give a very good description of what happens when you chuck an anvil from your window. So for that situation the model ( f=m*a) is very accurate even though it does not take into account a whole lot of variables (air resistance, micro fluctuations in the earths gravity, eddies in the space time continuum, etc.).  However if you chuck the anvil down a black hole it probably is not accurate at all.  This is a round about way of saying is,  that you can have a very capable model in common conditions that omits a fair bit of variables.

Gripe number two. People tend to think that when using simulations you all end up in the same place. ‘CFD is not the answer otherwise all F1 cars would look the same’ and more in this vein.

This is utter tripe. Just as a file/hammer/violin does primarily produce worthwhile work in the hands of a skilled operator. It is a tool.

IBM Pollyanna Principle: Machines should work. People should think.

There is no program that will design something from a blank sheet of paper.

Ergo you still have to come up with a starting point that is near some kind of optimum. All the program does is solve the ever so entertaining maths, that would not be practical to do by hand ( if would take you a 100 years and you would not have any friends left). There are systems that allow parametric optimization of for instance a intake port using CFD. That  is clever stuff and can find the ”optimum” port shape but only within the constraints you gave beforehand.

It is a really good method to weed out the bad options from the probably quite good options so that you are in turn closer to an optimum point when you actually do the physical testing.

This are two plots I nicked/borrowed  from the GT power/ Gamma technology website . It is promotional material  for top level software so I’d expect it to be , well not the worst data you could expect.

PowerTorque GT power PowerVolumeEfficiency GT powerYou will notice that the plot matches very well but not exactly, although it is a torque plot and the BHP plot will be very near indeed.

How well does EngMod4T fare ( with my model ! If I make a mistake in the input , it of course gives a wrong answer)

One of the simulations I did.

the engine is an a-series for lower ‘budget’ race class

  • stock bottom end 998cc
  • A stock head big bore head (actual flow data of the head was used but the data was pretty generic for a good but bone stock 12g940 head).
  • An LCB based on measurements on a Minispares Evolution Medium bore LCB I have on the shelf.
  • The prescribed cam for the class (S96 files where used that can be used in a cam grinder so they are pretty good) valve motion was with 1.3 ratio rockers.
  • Intake is a Weber on a long manifold.
  • Burn model is not tweaked/forced to make it follow the dips and rises of the curve. You would get a better match but it would no longer produce accurate predictions for different set-ups.

dynomatchThe power values are left blank as it is a race engine and it is not public domain (although this is just the baseline before it gets some trick new parts) . The torque and bhp values do no cross over at 5250rpm btw as it is in KW and NM (It’s that new  thing called The SI system)

All in all not half bad for a program that probably does cost less that half of the monthly licence fee of GTpower ( granted you get a few other bits and bobs).

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7 porter with a difference pt 2 Some data

Ok, on to some :


I have been sidetracked a bit and seem to have made significant headway .. yeah me.

Then the reverse 7 port head made a reappearance, And I did some runs for a head using the normal Siamese intakes of a well developed race head ( 135cfm+ @28”) but using a well flowing exhaust for each cylinder. Then I used pipemax to spit out a 4in1 exhaust using the rest of the engine data.  It is a 34x650mm primary into a merge collector and a straight collector of 60mm and 360 mm long. Experience tells us that for a conventional engine what Pipemax says the engine needs is pretty close to optimal already you can sort of use it for a 5 porter but you need a kink in your brain.


1: A single exhaust per cylinder is a significant improvement over a standard LCB for the centre cylinders.

2: The use of a dedicated exhaust port will make the charge robbing worse.


Always the same engine:

1310 cc race build with a fairly long custom cam, a Weber DCOE on a long intake, power output is irrelevant for the argument and not public anyway. I changed from part 1 this as the F11 cam is not realy wild and you want to see all the nasty stuff that happens with a long cam. I forgot about the stub exhausts (they are not practical anyway) and went for a proper header design with a 7800 rpm tuning point.

So forget the numbers and never mind the silly rpm’s, and yes , it does rev to 8500 or so in real life as well.

The basic premise is that it is the same engine, but there are two types of head (5 and reverse 7 port) and 2 types of exhaust : a 4 in 1 for the 7 porter, a optimised LCB based on the data I got from Paul S of port injected turbo mini fame.

First the Rev 7 port vs the optimised lcb.


Black is reverse 7 port, Green is 5 port LCB which makes approx 120bhp. The reason is that the DR (delivery ratio similar to VE but for mass ) of the inner cylinders takes a nose dive after 7000 rpm.


Power of individual cylinders. The thing to note is that for the Rev7 there is a notable improvement in power output after 7000. Also note that no sane human would drive a car like this on the road , it is made for long north American circuits . If you look at it the other way : a well optimised lcb is pretty much just a good as individual ports with an equal length merge collector system right until where the normal rev counter in a mini is deep red.

If you look at other data like the trapping efficiency, the charge efficiency and the cylinder purity the above picture gets a bit more complex. As it turns out that even with a private pipe the number 2 and 3 cylinders are actually a lot worse than the lcb at 5000 rpm.

So yes it does make more power, but if you look at the BMEP plots for nr 2&3 cylinders , the 7 porter dips down  from a very fair 11 to a decidedly lowly 9 in 500 rpm while the LCB engine bravely chugs on . In contrast, the good cylinders have a BMEP of about 13.7 up to about 7000 rpm which is really remarkable.

To be honest a port this bad should not be able to do this at all, however it looks like the Siamese intakes ( as it does this even with the 4 exhaust ports) enables the outer cylinders be very efficient (even though the inners spoil the party).

more to follow in part 3.

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Seven Port heads with a difference. Pt 1


A conventional 7 port head using a short and rather big integrated intake for a pair of DCOE Weber carburettors.

As the five port head has the ”evil” Siamese intake people have thought about ways to get rid of them. This can take all kinds of forms, ranging from fitting an Arden 8 port head, Splitting a 5 port head, A KAD 16 valve head, A BMW K 8v or 16v series conversion , Elder 7 or  8 port or a slightly mad person FIAT conversion on LPG ( you know who you are).

AKM from Denmark did a 7 porter somewhere in the 70’s where the intakes are ”normal”  but the exhausts where like the 5 porter (So 4-in 3-out). As the centre exhausts are paired but 180 degrees out,  the thinking is that it is not much of a problem. Since then there have been a few 7 port variants (most recently a aluminium billet effort from Specialist Components that do a lot of stuff for K conversions as well) and including a remake of the AKM head available from Mini Sport (supposedly with better (smaller) ports). One of the attractions of the 7 port head is that you can still use almost all readily available parts ( LCB, intakes, cams etc etc) , contrary to an Arden-8 porter which pretty much needs half a new engine to work making it quite costly.

I’m not going to say much anything about all of these versions. If you want to do an 8 port and you are not running some FIA historic racing class where you are limited to period items, the BMW K-100/1100/RS/K1 head conversion is probably the least expensive, if not slightly more involved conversion using a head that is light years ahead of a 5 porter as far as design goes.

But what happens when you make a reverse 7 port (2 siamese in/4 proper exhausts)? Intake tuning effects are comparatively minor to exhaust tuning effects. Will it cause huge ”charge robbing”effects ? Will it be just like a normal 5 porter as the exhausts are 180 degrees out.

I will be trying to get some idea by making a reverse 7 port in EngMod4T.

The model is a 1310cc using a Weber on a long manifold using a very good intake port flowing about 138 cfm and a Kelford F11 cam as used for mini7 racing in the southern hemisphere . The reason is that I use these parts is that I have the files handy and that saves a lot of work.

Setup is the following.

First a run using 200mm stub exhausts that are ”tuning neutral” according to Prof. Blair. Long enough not to mess things up but short enough to not be specifically tuned.

Think of it, most WW2 aeroplane engines used something like this



Second a run using what are called zoomies which are basically the same as above, but using a tuned lenght (680mm).


Mr Max’ ride sporting ridiculous zoomies.. just imagine the under car routing and a 3 mile length

Then compare the 3 exhaust port head to individual exhaust ports and see what it shows up in the traces. Both on the exhaust side (using one pipe per cylinder keeps things nicely manageable for the brain) and the intakes.

There is some progress and i have some data (now what to make of it..) but I have been sidetracked on some other development exhaust weird idea and sometimes I have to work as well (it really interferes with hobbies that work malarkey)

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Hurdeberdirburhde bork bork, bork!

I have been quite busy with running simulations,but it’s all for a competition project

so I can’t show and tell what has been cooking. The main ingredient seems to be Naga Viper.