Porting Cylinder Heads - Port or Abort?

The worst head porting mistakes and how to avoid them!

By David Vizard

Pro head porters 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 Careful
Consider 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 Time
Porting 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 It
Pro-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 Tools
The 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.

Machining
Unless 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 Critical
If 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 First
A 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 suck the work debris directly out of the ports as you work.

Keep It Clean
After 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 Work
The 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.

Results
By 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.