Before doing any porting of an engine part, it is a good idea to move some air through the part and observe how the air influences a thread. This way you can see where the air wants to flow and where it doesn't. This is a very important step prior to removing any material because removing material from an area of low flow will not only reduce velocity in the port, but may also create turbulence in high flow areas. By observing how a thread reacts to air flow in a port, you will be aware of the areas to concentrate on and the areas to use caution.
If you have a shop vac, or even a houshold vacuum cleaner with a hose attachment, you can get enough air through a port to at least get some indication where the air wants to flow. A shop vac with a blower outlet is a big plus, since you can either pull or push the air through a port. And in many cases, it is best to both pull and push the air through a port so that you can observe the thread from each end. If you are thinking that pushing air through an intake port, or pulling air through an exhaust port will give false indications, I can assure you that I can not observe any difference with my observations. Also, the following is from the Superflow website (www.superflow.com)FAQ section:
"A common question is, "Well, donít I always have to suck air through the intake and blow air through the exhaust?" It doesnít really make much difference, as long as you measure the pressure difference across the port. It is the pressure difference that causes flow. Whether you make it higher at the inlet or lower at the outlet to achieve this difference, you will get the same answer from the flow changes."
Anytime you increase the cross section of a port, you reduce the velocity. So the idea is to remove material from the areas of high flow to minimize the velocity losses. Here is a quote from an article in Grassroots Motorsport magazine, by David Vizard called 'Benchless Head Porting':
"Rule #1, Give air room to move where it wants to go - not where you think it should be."
Sounds good to me, but I have to admit, the first time I experimented with a threaded wand, I learned more in just an hour or so than decades of reading magazine articles and books on the subject. My preconceived notions of where air wanted to flow in cylinder head ports, and chambers, were far from accurate.
It is a good idea to make yourself a threaded wand so that your fingers do not disturb the air flow when sailing a thread inside a port. One way is to get a piece of 1/16" brass, or aluminum tubing from you local hardware store or hobby shop and run a brightly colored thread through it. Use a longer thread than needed so that you can adjust the length. Also, since the thread will get dirty and frayed, you will want to pull some fresh thread through and cut off the old. By bending the tubing at the end just enough to place a slight drag on the thread, you can adjust the length and spool out some more when the thread gets worn out.
Getting the thread through the tubing is not too difficult if you hold one end of the tubing inside your vacuum hose for suction as you guide the thread into the end. Just be sure to tie a knot in the end of the thread so that it doesn't pull all the way through.
Before we begin, I just want to apologize for some of the fuzzy shots. I found photographing a thread in air flow to be very difficult. Not only does the thread not want to be still, but I had to hold the shop vac hose to the port, hold the lights, hold the threaded wand, hold the camera still and snap the shot, all at the same time. Even for two people it was an intimidating task.
So now that you have a threaded wand, fire up that old shop vac and go thread sailing!
Let's take a look at a Ford GT40P head and see how the air flows through it. While my shop vac was able to produce about 22" of water at low valve lifts, higher lifts flowed about 12" of water during these observations.
You can't tell from this picture but this thread is hovering
about 1/4" above the floor of this intake port. Notice how
still the thread is indicating that the flow is very stable
and free of turbulence. Also notice how it takes a steep
dive to the short side radius. When sailing the intake port,
you will probably find the most turbulence near the valve
guide. I had to remove the valve for lighting purposes.
By taping a small thread to an intake valve, and blowing air into the intake port you can see how air moves into the combustion chamber.
This is the most dominate direction of flow at .1" valve
lift. You can just see a blur of thread pointing into the
lower right corner of the photo. Notice how the thread hugs
the chamber roof. The small step of chamber wall lifts the
flow off the chamber roof. If you smooth this wall down, the
flow slides straight over to the cylinder wall. This might
provide a good scavenging effect.
As the valve is lifted to about .2", the dominate area of
flow shifts toward the exhaust valve. Again, the thread is
clinging close to the chamber roof as it crosses over the
At about .3" lift, the dominate air flow continues to
migrate clockwise toward the plug boss. It still hugs the
chamber roof as it moves over the top of the plug boss to
the other side.
This shot shows the weakest area of flow that I found, and
it was very difficult to get the thread to move to this area
at all. It is hard to see but the thread is pointing to the
right side of the photo.
As the valve lift exceeds .3" the dominate areas of flow begin to dissipate and flow begins to equalize around the valve. The air begins to flow around the valve face edge, down into the cylinder instead of along the chamber roof. The most unstable areas of flow, indicated by a flapping thread are between 12:00 and 5:00 in respect to the intake valve face, at about 12" of water. The short side radius and inboard side of the valve still remain strong, but the thread is more willing to move to other areas, including the outboard side. The above drawing shows the air flow tendencies at higher lifts.
Working the exhaust side, you can peek down the exhaust
port to see what the thread is doing past the valve. I
was not able to get a camera angle for this type of view.
Here the thread shows the effect that the exhaust valve
shape has on air flow. The air is deflected down the port
by the wide seat surface, and the sharp edge on the
bottom of the seat surface separates the air from the valve.
Here the thread shows how the short turn radius work influences
air flow. The valve was removed for the photo. Using a
threaded wand on the exhaust port showed excessive turbulence
near and around the valve guide.
You can tell a lot about how the exhaust exit is channeling
the air into the headers with the threaded wand. Without a
velocity tube, you can see if the air has a tendency to exit
at an angle. With the velocity tube, and the 'Flowbee', I
found that this port needed about 1/8" clearance from the top
of the exit to the top of the header tube for maximum flow.
If I moved the tube down any closer to the top of the exit,
the 'Flowbee' indicated less air flow.
Testing an intake manifold, I found the air has a tendency
to want to flow upwards out of the port, even though an
attempt was made to channel the air downward in the last
inch or so of port by the manufacturer. Although the
lighting doesn't show this downward angle, the casting mark
indicates the angle of the port. You can test intakes by
placing a piece of duct tape over all the ports except the
test port and blowing air through.
By pulling air through this 4bbl intake carb bore, I could
see how the sharp edge at the bottom of the bore would
create a lot of turbulence.
More good stuff!