Do you want more power from your
Harley-Davidson® engine? If the answer is yes, you should know that porting
is essential for achieving your engine’s full potential. Any add-ons
you install to increase performance (different pipes, carb, etc.) will
not give you as much power as they would if your engine had first been
Why all stock engines should be ported
Think if your Harley engine as a pump. It
is physically capable of “flowing” a given volume of air (cubic feet
per minute, or CFM). The more air if flows, the more power it makes.
Imperfections in the passages into and
out of the combustion chambers—the intake and exhaust ports—will
restrict your engine’s air flow, or volumetric efficiency.
Volumetric efficiency is the ratio
between actual and theoretical flow of air through your engine’s
combustion chambers. All mass-produced engines have less than optimal
volumetric efficiency, because the manufacturers cannot afford the
skilled labor required to clean up the ports of each individual engine.
This cleaning-up process, porting,
is the enlarging and refinishing of an engine’s intake and exhaust
ports to maximize its volumetric efficiency: its ability to “pump
The black art of engine tuning
Because of its complexity and the
physical dexterity required by the operator, porting is the black art of
engine tuning. It can not be done by a machine, only by a person. If
your engine’s porter is skilled, he can improve its volumetric
efficiency—and power— by 20 percent or more. If the porter is
inexperienced or incompetent, he can decrease your engine’s
volumetric efficiency by at least the same amount, and bring on
overheating and rough-running to boot.
Proper porting gets “free” horsepower
out of any engine, from dead stock to fully race prepared. The
improvement in “breathing” increases power all the way across the
rev range, and adds smoother running and greater reliability, all
without adding parts or using more fuel. Apart from the cost of the
service, there is no downside. It’s beautiful!
How porting is done
So what exactly does an engine porter do?
Think of a porter as a sculptor, who
incorporates specialized tools instead of hammer and chisel, and whose
marble is your Harley’s cylinder heads.
The porter clamps your cylinder head to
his porting bench, and proceeds to sand, shave, grind and polish in all
the right places, to remove the inconsistencies of mass production, as
well as known deficiencies in your engine’s design.
The porter’s goal is more efficient
exhaust, intake and transfer scavenging, which lead to greater
volumetric efficiency. He works to optimize the radius around the valve
seat, the width of the valve seating area, transition into the
combustion chamber, and radius on the back side of the valve. The goal
is smooth flow, not sheer volume. A common misconception is that bigger
ports are better—on street engines the opposite is more often true!
Harley engines are very sensitive about
their exhaust-to-intake ratio. They run best when that ratio is around
90 percent, but factory levels are more like 78 percent. A good porting
job can give you the optimal exhaust-to-intake ratio for your Hog, but a
bad one can make it worse.
Better atomization via the Tooled Finish
Expert porters know that efficient atomization—the
transition of fuel from liquid to vapor—gives the best Dyno numbers,
and the secret efficient atomization is the finish of the ported
surfaces: rough in some areas and mirror-smooth in others.
A common beginning porter’s mistake is to
apply a nice, shiny finish to all ported surfaces. If your heads come
back this way from a tuner, expect less, not more power!
The author of this article uses a process
he devised called Tooled Finish to apply a coarse surface to the
manifold and port throat, graduating to a finer finish in the seat
pocket area, and ending in a high polish starting at the 45-degree
portion of the valve seat and down around the radius into the port,
approximately 3/4 of an inch. This process has consistently given the
best Dyno figures for Harley engines of all types. But bear in mind that
the process is slightly different, depending on the generation of Harley
engine the porter is working on.
Flow bench results—and fibs
The porter uses a flow bench to measure
his progress. Before starting work on your cylinder heads, the porter
will place your cylinder head in his flow bench and run air through
it, measuring its throughput in cubic feet per minute (CFM). As the
porting process goes on, the porter measures again and again. The porter
should be aware of the optimal CMF for your engine’s desired state of
tune, and when he has reached it knows his work is done.
Flow numbers are achieved through common
flow bench calibration standards. Unfortunately, some tuners use
different calibrations to produce “superior” numbers—numbers that
don’t really tell the truth about their porting quality. Like all
statistics, be suspicious about overly-dramatic flow bench numbers, and
take their source into account.
Porting first, add-ons later
Porting should be the starting place in
your Harley engine upgrade program. Do it before you add pipes, a new
carb, or anything else. It’s the most fundamental modification you can
make to your engine, and everything else follows. (A sure way to look
like a beginner is to hang a bunch of power mods off your non-ported
Choosing your porting service
Choosing your porter is one of the
biggest decisions you’ll make in the ownership of your
Harley-Davidson. Imagine you are commissioning a sculpture, and have in
your possession a valuable block of Carrara marble, the same type
Michelangelo used to create his statue of David.
Like selecting a sculptor, you choose a
porter by examining his work, reviewing his career, and talking with his
previous clients. There is no substitute for experience. Ask, and
verify, how long the porter as been working on Harley engines, and which
Price should be near the bottom of your
priorities, especially since porting isn’t all that expensive even
when done by an expert. Expect to pay from $300 - $700, depending on your
Harley engine type—a little more for the new Twin Cams (which, by the
way, benefit greatly from porting).
Your porter should be a specialist, not a
general-purpose mechanic. Like other master craftsmen, porters tend to
be individualists—they often set up their own shops rather than work
for dealerships or general-repair operations. Don’t look down on a
porter who’s a one-man operation working out of a small shop. Guys
like him are often the very best at their craft, and if you look
closely, you’ll see he’s accumulated hundreds of thousands of
dollars worth of equipment in that tiny workroom, much of it
Take your time to seek out a good porter.
It’s better to send your cylinder heads across the country to a top
professional than have them done by the beginner across the street.
Be prepared to wait a couple of weeks,
and you will be rewarded with an engine that, for its lifetime, will run
noticeably smoother and make better power across its rev range. You’ll
have the satisfaction of outdragging your pals who have the same add-on
speed equipment as you but no porting work—or porting carried out by a
less experienced craftsman.
When they ask the secret of your bike’s
greater performance, you can give them that old line, “It isn’t the
bike, it’s the rider!” Only you, and your porter, will know better.