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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2 thoughts on “7 porter with a difference pt 2 Some data

  1. ref my comment years ago on here about the a series being self supercharging?

  2. mowog says:

    It really does seem to be happening,
    however the charge seems to go to the outsides rather than the inner cylinders. There is still lots to be enlightened.

    I have come up ( after an email conversation with a very good two stroke guy) with a reworked exhaust that gains a lot in the model by better equalising the cylinder outputs . Results have been so good that RP will build one for his race mini to this spec for physical testing. We will see if it does pan out in reality .

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