Porting Cylinder Heads - Port or Abort?The worst head porting mistakes and how to avoid them!
By David Vizard
illustrators: George Trosley
Pro headporters often say the guy who pays their bill without questioning it is
usually one who has already tried porting and has an idea of the work
involved. Success at head porting does not happen overnight. It takes
skill and practice. Most beginners dive into a porting project without
really knowing what they are doing and almost inevitably bite off more
than they can chew.Result--disappointment and a set of
half-finished or half-ruined heads. By following the guidelines here,
that need not be the case. Given the right directions you can get about
70 percent the results of a pro porting job for cents on the dollar, so
let's look at the failure paths leading to the most common pitfalls.
Be CarefulConsider where the worst flow restriction is in the head. It is not, as
most novice porters assume, near the intake manifold face. Check the
basic small-block Chevy port in Figure 1 which is divided into three
sections. Between the two arrows of the first part is a high flow zone
of some 95-percent efficiency. The turn into and the bowl (valve
pocket) is a medium flow zone of typically 75 percent efficiency while
the zone into and immediately from the valve seat only averages about
50 percent. The efficiencies of these individual sections indicate your
port reworking priorities. Typically two-thirds of the airflow gains to
be made from an 'as cast' head are found from a half-inch into the
combustion chamber to about a half-inch past the port's guide boss.
Basically this can be described as "deep pocket porting." It finds the
fastest, easiest gains and, other than cleaning up, should represent
the limit of your initial porting efforts.For valve lift values
up to about 20 percent of the valve's diameter, the valve seat, and the
approach to and from it is more important than the port itself. The
rule here is that poor valve seat jobs produce poor results almost
regardless of how good the port might otherwise be. Parallel two-valve
combustion chambers, typical of American V-8s, suffer from a problem
called valve shrouding (Figure 3). This is more common on OE heads than
aftermarket heads. On production closed-chamber SB Chevy heads where
valve shrouding is prominent, be aware that if larger valves are
installed shrouding will get worse, as will flow. The installation of
larger valves must be accompanied by a like amount of valve
de-shrouding.
Go Easy
About as ambitious as you should get for your first porting job is to blend seats into the port, skinny down guide bosses and de-shroud the intake valve by about a sixteenth of an inch. After this, just clean up the rest of the ports and chambers with course emery rolls.
Take Your TimePorting takes time and aluminum cuts faster than iron. Figure (with the
recommended tools) spending about 8-12 hours on the actual porting job.
To avoid wasting time on half a job ask yourself the following: are you
good for completion and are the heads crack-free.
Shape It, Don't Shine ItPro-ported heads often have a high shine finish, but understand that this is done to add perceived value for those with limited porting knowledge. It is
shape not shine that makes horsepower.
Use Correct ToolsThe right porting tools must be used or you will be defeated before ever
starting. A small eighth-inch collet and an electric die-grinder is
totally inadequate because they lack power. If you have an air
compressor, an inexpensive air die grinder is the way to go for fast
metal removal. Since shine is not on the agenda, only 60 and 80 grit
emery rolls and flat pads for finish work will be needed. If no air
access exists then (though more cumbersome and expensive) an electric
die grinder is the answer.Although carbides are often best spun at
15,000 -22,000 rpm, the same cannot be said for emery rolls and the
like. For either air or electric, some form of speed reduction is
essential. For an air-powered grinder, a cheap pressure regulator gets
the job done. Although not intended for such, a light dimmer switch is
a cheap and effective answer to speed control for an electric die
grinder.Cheap high-speed steel cutters are available from many
cut-price tool stores. Unfortunately they are totally inadequate for
the job. In this area it is true to say that anything cheap will not
work. Your cutters must be carbide and, while not expensive, they are
certainly not cheap. Two places that have excellent prices and cutters
explicitly for cutting heads are CHA (Cylinder Head Abrasives) and
Roger Helgesen. The porting mods described here need only two or three
cutters to get the job done. These are a three-eighths and half-inch
olive and a three-eighths ball. For cast iron, you will need a
fine-tooth form and for aluminum a course tooth. To reduce cutters
loading up when working aluminum, a kerosene/WD-40 mix can be used, but
to completely eliminate loading and to speed cutting, the special fluid
sold by Roger Helgesen is the choice of pros.Chamber reworking calls
for seat protection. This is best done utilizing valves of the
appropriate size having the seats ground with only a few thousands
margin left for a near flush fit in the chamber.
MachiningUnless you intend to rework a pair of new heads, do not overlook the fact they may need machine work done. If the CR is to be raised while retaining
the pistons currently in the motor then, even new heads, will need at
least a milling operation. The most likely job that will have to be
done on an OE head is a new valve seat job, but there are exceptions.
Some factory existing seat jobs are sufficiently wide (an eighth inch)
to allow the inside edge to be radiused into the port body. If so then
a new seat job is not essential. If the heads being reworked are used,
then they may require face milling to true them up. Your machine shop
can check this out for you with a machinist's straight edge. If you are
looking for more compression then the head can usually be faced 40-60
thousandths-if the casting is good for it. However, doing so will mean
the intake manifold will have to be machined to realign the ports.
Guides Are CriticalIf the guides are worn or the seat design is far removed from a 3-angle
pattern or is worn, then a guide job and seat re-cutting will be
mandatory. Be aware that worn guides can cost as much as 10-15 hp and
can also lead to the head coming off the valve. Do not use guide
knurling as a fix for worn guides as this does not last and compromises
exhaust valve stem heat transfer. Just bite the bullet on this one and
go for new cast-iron or bronze guides as it will pay off.
Safety FirstA carbide cutter driven at high speed means high-speed chips of metal
flying around. If you do not protect your eyes with safety glasses, a
hospital visit is an absolute certainty. When it comes time to use
emery on your head casting, be sure to wear a mask to filter out the
dust and metal particles. Even short exposure times can cause
respiratory problems. To assist keeping the air you breath and that
around your workspace clean, use a shop vac to s*c* the work debris
directly out of the ports as you work.
Keep It CleanAfter grinding a couple of sets of heads in a work area, getting rid of the
resulting grit and metal dust is a major job. Use an enclosed workspace
that is completely separate from wherever you may subsequently build
your engine.
Testing Your WorkThe porting moves described so far are almost 100 percent certain to work, so a flow bench is not needed to get results, only to quantify them. If the
intent is airflow optimization, the situation changes and a method of
measuring flow becomes essential. A big 'main frame' flow bench such as
the Super Flow 600 used here may ultimately be what you want. However,
it is possible to get into a surprisingly accurate, entry-level,
shop-vac powered flow bench for as little as $800.
ResultsBy porting your heads and observing our 10 "don'ts," how much extra
flow and power can you expect? You've seen what was achieved when the
techniques described were applied to a set of World Products Chevy
Sportsman II heads. The result, on motors of 350 inches or more is 45
(or more) extra hp. Similar porting exercises on 5.0 small-block Ford
can show an additional 25-30hp.
- Spoiler:
Hits the spoiler for some PORTING picture to share
Subject: kelebihan dan keburukan port & polish Head 
4/6/2008, 2:23 pm
