Gas flowing, or porting of cylinder heads, is viewed by most laymen as being more mysterious than the US foreign policy. What does it actually mean? How is it done? Does it even work? Of course, there’s no short answer. But hopefully, this article will serve to clear the murky waters somewhat.
The function of the cylinder head, and the inlet ports in particular, is to introduce air from the inlet manifold into the combustion chamber with as little restriction as possible, and hopefully take advantage of the velocity of the fast moving air in some way. The exhaust ports must do the same as they get rid of the burnt products of combustion.
Now, a mass produced cylinder head will have ports which are left “as cast”, so they have a rough finish and various casting flashes which will all cause a disturbance to airflow. They will also often be of a less than optimal shape. This is particularly true of the older designs like the Ford Kent Crossflow or cast iron Chev V8 heads, which have all sorts of undesirable lumps of metal in the wrong places. And you only need a brief glance at an MGB head to see that it’s about as free flowing as a blocked drain. So, with these older heads, it’s a relatively simple matter to remove the offending lumps, and gain a heap of airflow. This is normally done with a pneumatic handheld porting tool or die grinder turning at 5 or 6 thousand RPM, and is extremely easy to make a complete mess of, which would result in the cylinder head being chucked in the bin. For this reason alone, it’s worth leaving it to a professional cylinder head shop. Outfits which habitually do large numbers of the same heads might CNC mill the ports, but in this country, we never see these volumes, so it’s all done by hand.
Generally, after removing metal with an abrasive mounted point or tungsten carbide burr, the ports are polished with fine grit sandpaper, again mounted in a die grinder. The ideal finish of the ports is the subject of much debate – some tuners prefer to leave the intake ports slightly rough, and some prefer a smooth, almost mirror finish. Bench testing seems to indicate there is not much difference to airflow either way.
But what about the size of the ports? Does it make sense just to make them as big as possible before one goes right through the port into the water jacket? No. There is definitely an optimal size, based on the intended speed range of the engine, the capacity of the engine, the valve head size, the camshaft, carburation, etc. In fact, some standard ports are already too big, and more adventurous tuners have filled in these ports with special epoxies. The problem with very large ports is that gas speed drops as a result of the increased port area, and bottom-end power suffers. This may not be an issue in a racing engine which never goes below 4000rpm, but it certainly is in a road car.
The shape of the port is equally important. Material may need to be removed in some places and not in others. Generally one would try to “straighten out” the port as much possible, removing material from the roof of the port and leaving the floor more or less untouched. The area just below the valve seat, called the throat, is also important as it needs to accelerate the gases as they enter the chamber, taking advantage of the venturi effect caused by slightly reducing the cross sectional area of the port just before the exit into the chamber.
The valve seats are then cut using a specialised machine (usually known as a Serdi), giving it a smoother profile and a narrower valve seat which will aid gas flow at small valve openings.
The valve itself is also reshaped to aid gas flow – a particularly critical area, as all the gas must pass over the back of the valve. If one compares a typical valve from 20 years ago to a valve from any modern high-performance car, it is obvious how much development there has been in this regard.
So if all this works on older cars, does that mean it doesn’t work on newer engines? No, of course, it works; it’s just a bit harder, and the gains may not be as big. And if the application is totally different, for example, building a full race engine from a normal road car engine, then bigger valves and bigger ports will be needed to allow the engine to breathe at the higher RPM it will be operating at. There are some exceptions, like an E90 M3 V8, for example, which has big, beautifully-shaped CNC-machined ports, which most sane tuners wouldn’t dare touch.
Measuring the changes in airflow can be done using a device called a flow bench, which measures the flow rate of air through a port at a constant vacuum. It provides a useful yardstick, but is by no means a sure way of predicting the power increase, as the actual conditions in the engine are quite different to what is happening on the flow bench. For example, the vacuum in a combustion chamber is nowhere near constant. It’s also a very time-consuming exercise. And there’s always the infamous story, no doubt greatly exaggerated by now, of a 1950s works Jaguar Le Mans team, which measured each gas-flowed six cylinder head on the flow bench and obviously kept the higher-flowing heads for themselves, selling the others to the customer teams. Imagine their surprise when the customer cars turned out to be faster than the works cars down the Mulsanne straight! Back to the drawing board…
There are other aspects to gas flowing or modifying a cylinder head; one may want to impart a circular motion to the gas as it enters the combustion chamber which will aid cylinder filling. This is known as “swirl”, and is most often associated with 2 valves per cylinder heads.
Obviously, this subject could have several postgraduate theses written about it, so it’s only possible to lightly scratch the surface on these pages, but hopefully it’s enough to enlighten the average armchair enthusiast. Just don’t be tempted to get out your Dremel and start hacking away at your ports on the weekend…
074 106 0260
Gas Flowing Explained By Gerald From Camtech SA