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).
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)