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 ported.

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 air.”

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 technique

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 Hog!)

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 types.

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 custom-built.

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.

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