Nine Port Heads
While engines from other manufacturers of the same era had head porting that was at least adequate or even too big (eg. Cleveland or square port BB Chev), the 9 port Holden head was barely able to feed stock 149 motors. The bigger, later motors were equally asthmatic despite having bigger valves. Where other engines responded well to intake, exhaust or especially cam upgrades, the old Holdens never really woke up until the head was modified. The intake ports in particular were abysmal, but on the positive side even the most godawful butchery of the ports nearly always produced an increase in power. Perfectune recognized an opportunity to provide an exchange head with improved porting and bigger valves, and sold squillions of their YellaTerra heads. The mods were basic and mainly carried out on automatic machines, keeping the prices low. Power and fuel economy could be substantially improved with nothing more than a head upgrade. There are still a lot of these YT heads around and on a mild performance engine they do a reasonable job, with the so called "Bathurst" style heads capable of making 200 odd hp.
Lets look more closely at the 9 port heads. The most obvious feature is the siamese inlet ports, with the six cylinders grouped into three pairs and each pair sharing an inlet port. The valves are arranged like this: EI IE EI IE EI IE. Cylinders 1 and 2 share an inlet port, as do 3,4 and 5,6. Traditionally siamese ports have been considered unsuitable for high performance engines and in many cases (eg. BMC 4's) there is good reason for this. However, in the case of the Holden motor port sharing can hardly be blamed for the heads poor performance. If we look at the centre two cylinders (3&4) for example, we see they are 360deg apart in the firing order, and even with the longest duration cam there is never a time that both cylinders have their intake valves open at the same time. So obviously there is no chance for one cylinder to rob it's neighbour. The end pairs of cylinders are slightly different, and there is a short period during each cycle where one intake is closing while the other is starting to open. But this period is so short (and occurs at a time when there is so little flow) that any inter-cylinder influence will be negligible. It's not the fact that the ports are siamesed that hurts the flow, it's the basic design of the port along with that head bolt that passes through it.
The valves are all inline and only slightly canted and this, combined with the fact that the ports are quite low, makes for a sharp, almost right angled bend in the valve pocket area. Add to this a cast iron pillar that runs up the centre of the port near the gasket face and things are looking even worse. This pillar is where the head retaining bolt passes through, and is quite thick, almost a third of the port width. Over the years there have been several approaches to solving the head bolt problem, the most common being to cut the thick pillar out, replacing it with a thinwall steel tube. This is what YellaTerra did, and it's quite effective in increasing flow. Some people have cut the pillar out and installed a socket head cap screw in the floor of the port to clamp the head down, then screwed a flush fitting plug into the hole in the port roof. I doubt that there is much difference in flow either way, but the conventional steel tube approach is the most convenient.
Nine port head from a red 202. Note the thick bolt boss in the intake port.
Fortunately there is a lot of meat in the port walls to work with, and it's easy to get big increases in flow and power output. If you're serious about making power, you should leave the port work to someone with the experience and equipment to get good results, and these people can get a 9 port head to flow enough to make over 310hp. In fact, in terms of sheer bulk flow you'll probably get more from a 9 port head than a 12 port, though of course bulk flow is only part of the story.
The standard valve sizes are too small, and you should aim to use at least XU1 or VH/VK size valves. The centres are fairly widely spaced, so there is plenty of room for bigger valves and seats. The downside to this is that the valves are very badly shrouded at the sides of the chamber, and it's pointless to try to widen the chamber because the side walls already overhang the cylinder walls. And anyway, there just isn't enough material between the adjacent chambers to lay the walls back much and still have sufficient thickness in between to support the head gasket. Of course the shrouding becomes worse as the valve size is increased, partially negating the benefits of using those big valves. Not only is the gas flow restricted by the chamber wall, it has to negotiate the ledge at the top of the cylinder bore. If you lay a head gasket on a cylinder head you will see that the openings aren't perfectly round, and match the shape of the chamber. Now lay the gasket on the block deck, and you can visualise the step or ledge under the chamber. Obviously the smaller the cylinder bore, the bigger the ledge, and it's a good reason to use the biggest available bore size. It's not uncommon to see these ledges on each side of the top of the bore chamfered or radiused back with a grinder to match the chamber, but if you decide to do this I'd be careful not to go too deep. The chamfer will definitely expose the part of the piston above the top ring to a lot of heat so I'd be wary of going more than about 3mm deep. You would expect that replacing this sharp ledge with a chamfer or radius would help flow - and nearly everyone does it - but to be honest I haven't been able to measure much improvement in flow. For a street engine I wouldn't bother.
We'll talk about combustion chambers more after we look at the 12 port heads as they are pretty similar with both types of head. If youre doing the head work yourself, all I can suggest is that you resist the temptation to make the ports huge and concentrate on slightly raising the roof of the ports, tapering them back from the port face to the valve bowl, so in effect the angle under the valve is less severe. Of course, you need to be able to match your intake manifold. Larger valve seats will have to be blended in and the bowl area can be opened up. The Holden ports are a bit unusual in that they seem to flow best when the bowl area is pretty much straight sided, and almost as big in diameter as the inside diameter of the valve seat.Don't grind the port floor at all, except to clean up any dags. There is no need for significant widening on any reasonable street engine. The biggest gains will come from fitting oversized valves, reducing the shrouding and from reducing the width of the head bolt boss. It isn't strictly necessary to cut it out and fit a steel tube, just narrow it and streamline it.
The earlier engines had intake valves of about 1.49" in diameter, and these are hopelessly undersized for nearly any application. Later red 202s and 173s had 1.625" valves, but these are still a bit small for anything but the smallest or mildest of engines. For a high output application you really need an intake valve of around 1.7" to 1.74" diameter. There's not much point going beyond this because the shrouding just becomes too tight and especially with the small chambers excessively big valves may actually flow less. The YellaTerra heads generally used valves 3/16" oversize.
Enlarged nine port intake with bolt boss and port divider removed. Either that or it's a two-car garage... Good flow; poor velocity.
The exhaust ports flow quite well by comparison, and again there is plenty of meat to work with. There is a thick wall dividing the centre four exhaust ports, and these also have a head bolt passing through them. Unfortunately on some heads the wall doesn't quite extend all the way to the gasket face so these ports are at least partly interconnected, and I assume this would reduce the benefits of using tube headers or extractors. As with the intake, resist the temptation to go overboard with the grinder. Work out what size primary pipe size you will be using on the exhaust and match the port to this (keep it slightly smaller actually), trying to keep the cross sectional area fairly constant. Ideally there will be a step up of about 1 to 1.5mm all around the port into the exhaust manifold flange. You will probably find you will remove little if any wall material apart from a cleanup and some streamlining of the guide boss. The 1.275" exhaust valves of the earlier engines will be much too small, but you could get away with using stock 1.37" valves in a mild engine, and 1.48" blue/black valves will be big enough even in fairly highly tuned engines.
We can summarise the 9 port heads like this:
The standard head is extremely restrictive and won't make much power no matter what other engine components you have
Siamese ports might be less than ideal, but on the Holden 6 the firing order makes flow robbing from port to port a non-issue.
An expert head porter can achieve massive flow increases, up to 300 odd hp, and even an amateur can get good results with care.
Oversize valves are necessary, but it's no use going overboard because of the chamber shrouding.
There are many different types of manifold available for the 9 port, more than for the 12 port.
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