Cylinder Heads

Ok, we're starting to get into the juicy stuff here - the head is the key to making power with the little Holdens. There are basically two different head designs used on the six, the 9 port as used on the red motors and the 12 port used on the blue and blacks. It's no exaggeration to say that both types are spectacularly bad from a performance perspective. If the rules and the budget allow using alternative designs such as the Jzed/Duggan I'd certainly think seriously about it - you'd instantly have a massive advantage over others using the Holden designs.

Before we look at the two types of Holden head perhaps we should think about what it is exactly we are after. Obviously we want as much power as possible, but more importantly (and we touched on this earlier) we want the highest possible average horsepower over the usable rev range. In other words we want to maximise the area under the torque curve in order to get the best possible vehicle acceleration, and this is usually a different thing altogether to simply maximising the peak numbers. So what do we want from the cylinder head in order to get the best in-vehicle performance? I think the main points would include (but aren't limited to) these:

  • The first thing that comes to most peoples mind is flow capacity or good old cfm.
  • Of almost equal importance to the flow capacity is the flow efficiency, and this is really just a way of quantifying the flow velocity in relation to flow capacity.
  • A compact combustion chamber would be nice. Combustion efficiency can be just as crucial as flow, maybe even more so. It's no use having a port that can flow 240cfm if only half of those cubes are being burnt..
  • An intake port that induces swirl, to aid quick burning
  • The ability to take advantage of intake manifold designs that enhance cylinder filling

Unfortunately I think too many people get overly hung up on raw flow numbers; after all if a 180cfm port is good then a 200cfm port must be better, right? Part of the problem may stem from the oversimplified way we rate ports with only a flow figure and sometimes a port capacity in cc's. The cc capacity does give some indication of efficiency and is a way to compare ports of the same basic type, but I think an efficiency figure as a percentage would be much more informative.

So how does an efficient port make more power, and how do we calculate efficiency? To answer the last part first, it's simply a comparison of the ports flow and area to a somewhat arbitrary standard, namely the flow of air through a 1 square inch sharp edged orifice. For example, if we had a port with an average CSA of 1 square inch and it flowed 73.5cfm, we would say it was 50% efficient (because it flows half of what the standard orifice flows, about 147cfm). Using the flow figure along with an efficiency figure to rate a port makes a lot of sense, and would give a pretty good indication of how the head would perform on an actual engine as opposed to a flowbench. For example ports could be rated at 160/55 (160cfm,55%) or 180/70 etc.

How then does flow efficiency relate to power output? We already know that output is proportional to the VE or cylinder filling ability of the engine, and a good illustration of an engine with a high VE would be a ProStocker with well over 100% volumetric efficiency (around 130% actually). Now, if we stick a big lazy port on an engine we can probably get it to run at around 100%. The trouble is the port will probably be just as happy to flow backwards as forwards, and over much of the rev range we will lose power to reversion. The flow velocity will be too slow to take advantage of tuned length intake runners. But if we replace that port with a small, high velocity, efficient port we might pick up as much as 10 or 20% in horsepower over a wide rpm range. That could be over 50hp on a healthy 202!

A significant portion of a racing engines output is due to a combination of ram and wave action in the intake tract; by using lazy ports and intake tracts with wildly varying CSAs we are missing out on sizeable chunks of power.

So we've established that an efficient port can increase the in-car performance markedly, but now we need to somehow apply that to our little Holden. And basically that boils down to not automatically ruling out the 12 port head as being too flow-limited, as many people do. The old 9 port head is pretty much the default choice for many competition classes, and there are some good reasons for that. The rules in some historic classes may mean you have no choice anyway, and on top of this the old 9 port has had over 40 years of development. It has a fairly large CSA and the thick port walls mean there is scope to increase the area quite substantially. As a result, ported heads can flow over 220cfm but the efficiencies are terrible, often well under 30%.

The 12 port head by contrast isn't allowed in some classes, and was only in production for a few years. So racing development was almost non-existant in comparison to the old 9 port. In addition the port wall thickness is less, limiting the amount of enlargement possible. Flow from a ported stocker will be limited to somewhere around 160 to 180 cfm, while the thicker walls of the YellaTerra 12 port castings will allow 200cfm or a little more. If we look only at the raw flow numbers it's easy to see why most racers have stuck with the traditional 9 port head. The thing is though, the ports in the 12 port head have an efficiency of roughly double that of the 9 port head. The flow velocity is much higher, and with a correctly proportioned inlet tract they have the potential to make more power than a 9 port with the same or higher flow, and make it over a wider rev range.

The bottom line is this: consider using and developing the 12 port head. It has the potential to perform much better than its flow figures would indicate. For applications where a very narrow rev range is acceptable the old 9 port might still be the best choice, but for anything else the 12 port deserves a very close look. Whatever design you use, any modifications must be verified by flow testing. I've tested several ported and modified heads that have flowed no better (and sometimes even worse) than a stocker. One thing I should make very clear though: the 12 port's primary advantage is its ability to make use of individual manifold runners to boost cylinder filling, whether IR or plenum (tunnel ram) type. If you aren't planning on using one of these manifolds you may as well just stick with a 9 port.

We'll look at combustion chambers later, and at any rate both the 9 and 12 port heads share similar chambers. There are only limited things we can do to change the burn characteristics anyway. Just keep in mind the importance of a quick but smooth burn - there may be times (eg. lumpy domed pistons used to gain CR) when combustion efficiency is traded off for something else. It may well be that retaining a good burn should be the priority.