Sometimes in the pursuit of airflow, greed can get the best of any porter, and the tendency is to go too big in some
places. Nowhere is the price to pay higher than going too big in the port throat, the point of constriction just below the
valve seat. Make the throat too big, and the venturi effect is ruined, and usually the flow will be too. Keep the intake port
throat no larger than 90percent of the valve diameter, and the exhaust throat down around 85percent.
The bowl area and the rest of the length of the port have important functions in controlling some of the dynamic behavior
of the waves that traverse the system as well as setting up the air for a good entry to the throat. Shape, cross section,
volume, cylinder swirl or tumble and surface finish are factors which must be considered in concert with the overall design
of the rest of the engine and vehicle to achieve good results.
THE " PORTING AND POLISHING " MYTH
It is popularly held that enlarging the ports to the maximum possible size and
applying a mirror finish is what porting is. However that is not so. Some ports may be enlarged to their maximum possible
size (in keeping with the highest level of aerodynamic efficiency) but those engines are highly developed very high speed
units where the actual size of the ports has become a restriction. Often the size of the port is reduced to increase power.
A mirror finish of the port does not provide the increase that intuition would suggest. 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. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which
can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do. Flow
bench testing shows that the difference between a mirror finished port and a rough textured port is typically less than 1%.
The difference between a smooth to the touch port and an optically mirrored surface is not measurable by ordinary means. Exhaust
ports may be smooth finished because of the dry gas flow but an optical finish is wasted effort and money.
The reason that polished ports are not advantageous from a flow standpoint is that at the interface between the metal
wall and the air, the air speed is ZERO. This is due to the wetting action of the air and indeed all fluids. The first layer
of molecules adheres to the wall and does not move significantly. The rest of the flow field must shear past which develops
a velocity profile (or gradient) across the duct. In order for surface roughness to impact flow appreciably, the high spots
must be high enough to protrude into the faster moving air toward the center. Only a very rough surface does this.
The cylinder head is the part of an engine that is most responsible for its performance characteristics. Once the basic
geometry of an engine is established, there is no other part that has as much influence on the amount of power developed,
and the shape of the power curve. All the other parts are merely supporting cast.
So, what determines the worth of one head over another? First, you must understand that any design is a compromise between
what is desirable and what is possible. Engineers who initially design an engine rarely have free rein to make it the most
powerful piece possible - and they may not want to either. Even in Formula 1 racing, where engines are designed from scratch
to make as much power as possible, there are compromises that are determined by the rules of the sanctioning body and the
necessity to install the engine in the car. Like other vehicles, aerodynamics and handling requirements require compromises
in size, shape and weight of the engine.
The vast majority of today's popular aftermarket cylinder heads are compromised because they adhere to standard OEM port
geometry. This is done so the supporting components designed to that geometry can be used on the new head. As engine builders,
most of us have to work with parts that already exist. They may be production parts or aftermarket parts, but they all have
compromises, and it's up to us as porters, to minimize the compromise.
Expert Advice from Joe Mondello:
Joe Mondello, who’s name has long been synonymous with high-performance cylinder heads, said a lot of people who
don’t really know what they’re doing jump into head porting and make big mistakes.
"They take out metal where they shouldn’t be taking out metal and end up with ports that are too big and don’t
flow as well as they should. The shape of the port is far more critical than the overall size of the port," stated Mondello.
Mondello, who teaches the secrets of building, porting and flow testing high-performance cylinder heads at his Mondello
Technical School in Paso Robles, CA, said he also sells special porting tools that are designed for every part of the cylinder
"When you’re doing the short-side radius of a port, you don’t want to take out too much metal. You just want
it to be nice and smooth," instructed Mondello. "Trying to get around the short-side radius bend is difficult unless you use
a cutter that’s designed for that purpose.
"When cleaning up the bowl area, blending alone won’t improve flow unless you also remove some metal to increase
volume. Many people don’t do valve bowls properly. You have to blend everything from the base of the valve guide to
the base of the primary valve seat, and then do a 3-angle valve job. Otherwise you’re just scratching the valve bowl
and ports, and aren’t really gaining anything."
"We teach port matching, not gasket matching. I pick the largest port, match all the others to it, then do all the work
inside the port to maximize air flow around the pushrode tube turn because that’s where the biggest restriction is in
the port," said Mondello.
"The largest gains in horsepower are found on the intake side by raising the roof of the port (the side closest to the
valve cover) by .100" to .175". The amount of metal in the top of the intake manifold runner will determine how high you can
raise the roof.
"On late-model Chevy Vortec heads, you don’t want to change the shape of the port much. The best advice here is
to clean up and equalize the ports so they have the same height and width. On small-block heads, there’s a large pocket
right below the rocker arm stud in the roof of the port. This should be filled in with epoxy to improve air flow. Doing that
will give you an extra 15 cfm.
"On exhaust ports, if you tried to match the port to a header gasket you’d probably destroy the port. The secret
of exhaust porting today is not how big the port is, but the shape of the port and the velocity of the exhaust flowing through
it. We don’t even flow test exhaust ports anymore because most heads have plenty of flow capacity as is. All we care
about is velocity and pressure.
"Nearly every single exhaust port today, except for Ford 302, 5.0L and 351 heads, are big enough. The only thing we do
to enhance air flow is raise the roof of the port about 0.100", depending on the headers used. We don’t touch the floor
of the exhaust port or the sides unless we have to get rid of a hook, seam or rough area in the casting," said Mondello. "Any
time you start making the ports bigger on the exhaust side, you usually end up killing air flow in the head. I’m talking
a reduction of 25 to 30 cfm. All you need to do is clean up the valve bowl, blend the short-side radius, and raise the roof
slightly. Don’t touch the floor or walls."
Mondello explained that CNC machining and hand grinding are two different techniques for porting heads. "Everybody says
CNC is the way to go. But you first need someone who can take a raw casting and rework it so it has good air velocity and
flows well. Then you can digitize it and reproduce it with CNC tooling on other heads. There are a lot of CNC profiles being
sold today, but I think most have some room for improvement. Additional hand grinding can usually pick up another 10 to 12
or more cfm."
As for polishing, Mondello said a smooth finish is great for exhaust ports, but a rougher finish flows better on the
intake side. He recommends using 300- or 400-grit paper followed by a Cross Buff for polishing exhaust ports, and 50- or 60-grit
paper for the intake ports. A slightly rough surface texture in the intake ports and intake manifold runners creates a boundary
layer of air that keeps the rest of the air column flowing smoothly and quickly through the port.