Cylinder head porting

Cylinder heads, as manufactured, are usually suboptimal for racing applications due to being designed for maximum durability.

When a modification is decided upon through testing with an air flow bench, the original port wall material can be reshaped by hand with die grinders or by numerically controlled milling machines.

The Ford two-liter F2000 engine in stock trim equipped with the head shown above was capable of delivering 115 horsepower at 5500 rpm for a BMEP of 136 psi.

This aftermarket Pro Stock racing head was used in an engine capable of 1300 horsepower at 9500 rpm with a BMEP of 238 psi.

That is, any time a change occurs in the cylinder – whether positive or negative – such as when the piston reaches maximum speed.

For normal automotive design this point is almost always between 69 and 79 degrees ATDC, with higher rod ratios favoring the later position.

At first glance this wave travel might seem to be blindingly fast and not very significant but a few calculations show the opposite is true.

In an intake runner at room temperature the sonic speed is about 1,100 feet per second (340 m/s) and traverses a 12-inch (300 mm) port/runner in 0.9 milliseconds.

The signals coming from the piston dome, after the initial runner flow has been established, must fight upstream against whatever velocity has been developed at that instant, delaying it further.

Large portions of it bounce off the rest of the combustion chamber and resonate inside the cylinder until an average pressure is reached.

When the valve closes, it causes a pile up of gas giving rise to a strong positive wave that must travel up the runner.

Note how the suction wave during cylinder filling is delayed even more by having to fight upstream against the inrushing air and the fact that the piston is further down the bore, increasing the distance.

When it does work, it is possible to see volumetric efficiencies of 140%, similar to that of a decent supercharger, but it only occurs over a limited RPM range.

In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly.

The complex and sensitive shapes required in porting necessitate a good degree of artistic skill with a hand tool.

5-Axis CNC controls using specialized fixtures like tilting rotary tables allow the cutting tool full access to the entire port.

The combination of CNC and CAM software give the porter full control over the port shape and surface finish.

Successfully optimizing ports requires an air flow bench, a thorough knowledge of the principles involved, and engine simulation software.

A port before and after modifications, exaggerated for illustrative purposes. The general idea of improving port flow is that a straighter pipe and gentler curves provide more peak power. This type of modification is commonly referred to as "increasing the downdraft angle", and is limited by mechanical constraints such as engine bay height, the amount of material in the parent casting, or the relocation of valve gear to accommodate the longer valve stem.

A mold of the suboptimally designed ports of a Ford two-liter head for use in Formula 2000 racing. It is shown as manufactured with the intake port on the right.

Port molds of a highly developed 500 cubic inch aftermarket Pro Stock racing head. Note the height and straightness of the ports, particularly the exhaust port on the left. This design is based on a cylinder head casting purpose-built for racing modifications. The head is supplied with small ports with ample material everywhere for porting specialists to shape to their requirements without having to weld on additional metal.

This highly simplified animation shows how air flows as waves in an intake system. Note the green "valve" opening and closing.

This graph shows the pressure taken from the valve end (blue line) and the runner entrance(red line) of an engine with a 7-inch (180 mm) port/runner and running at 4500 rpm. Highlighted are two waves, a suction wave and a valve closing wave, seen and the valve end and runner entrance showing the signal delay. A lag of about 85 deg for the peak suction wave versus about 32 deg for the peak pressure wave. A difference of some 53 deg due to the movement of the gas and piston position.

A developed velocity profile in a duct that shows why polished surfaces have little effect on flow. The air speed at the wall interface is zero regardless of how smooth it is.