There is little dispute that cylinder head airflow equals power; we dare say the relationship has been fairly well established. If you are doing everything right to make the most of a set of cylinder heads from a given engine combination, each cfm of intake port airflow is enough to make just over 2 hp in a V-8 engine. If cylinder head airflow isn't up to the task, no amount of cam or carb is going to coax big power from the engine. Okay, so flow is good and desirable, but where do you find it? These days, there are plenty of cylinder heads on the market that offer outstanding flow right out of the box, and more than a few companies equipped to CNC-port a variety of popular head castings. Still, when looking to up the ante on a set of existing cylinder heads, porting is the answer. That said, porting takes much more than just making the passages bigger. By knowing the rules and applying them with skill, the volume of air passed through a head's ports can be significantly improved.
Porting is one of those areas of engine building that for most of us is shrouded in mystery. We've all heard of the gurus who can take a mundane chunk of factory iron and make magic. We've also heard all the warnings about overly enthusiastic but misguided efforts actually ruining a head's flow. There's truth on both sides here. What does it take to pull out some grinders and cutters and actually make a head better? Porting isn't a pursuit that's for everyone; it comes down to the left brain/right brain combination of a scientist's mind and a sculptor's touch. The science side is grounded in the physics of how air behaves, fundamentally based on fluid dynamics. Results are usually quantified through experimentation and flowbench testing. The sculptor's touch is the inevitable reality of having to carve a passage by hand with a high-speed grinder.
Even if you are willing to have at it, and are armed with a battery of carbides and "tootsie rolls", without knowing how to approach porting the chances of really improving the flow is pretty slim. It takes years of experience and know-how to be a top-notch cylinder head expert - able to study a port, and carve it to amazing flow levels. In fact, airflow is an elusive goal, and sometimes the pros make the wrong moves. Even so, by understanding the basics of porting techniques - techniques that apply to practically any brand of cylinder head, even a novice can achieve worthy results. What are those basic techniques? Read on as we reveal the secrets in the following pages.
Porting is best done with a fixture or stand to allow easy access to the areas being cut,
TOOLS OF THE TRADE
Porting takes a variety of specialized tools, though just a die grinder and a few carbide bits are enough to find meaningful gains. There really is no end to the variety of cutting, smoothing, and measuring devices used by those in the porting trade, but there are some universal tools that should be in any porter's toolbox. Many odd, homemade tools can be handy, from gauging dog-bones, to old valves for protecting a machined seat while working in the combustion chamber.A good die grinder should be the first item on the list. Carving cylinder heads requires a high-speed (15,000 rpm-25,000 rpm) die grinder, with a 1/4-inch collet. Both air and electric die grinders are popular, but electric is definitely the way to go for the casual porter, since air die grinders require an unbelievable volume of compressed air for porting. A compact high-speed Makita electric is hard to beat; though some prefer the air grinder for its ability to be throttled on the fly.
It doesn't take a fortune to get started in porting, a grinder and a few carbides and abra
To accomplish the required metal removal, carbide burrs and stones are used. A grinding point (stone) is of some value for minor contouring of cast iron, but worthless on aluminum. Carbide cutters are the workhorses for metal removal in both iron and aluminum and come in different pitches depending upon the material being worked. Cutters with long shanks allow reaching areas of the port inaccessible with standard short-shank cutters. Once the heavy cutting is done, there are a number of items used for fine smoothing and polishing. Cartridge rolls mounted on mandrels are most commonly used, while the flap wheels are very good for general port blending and shaping the short turn. Other useful tools are flat rotoloc discs and cross buffs.
Measuring tools are a vital guide. Layout die and a scribe are used to mark guidelines, while various dividers and calipers help gauge the port size, as well as metal thickness in some areas. The Helgesen "E-tool" is used to measure the casting thickness in the pushrod pinch point. It's important to set the head up for easy work access, and a comfortable work position. There are a variety of bench-top headstands available, including simple "V" stands. The best porting stand we've seen is the Helgesen ProBench shown here.
A machined valve job should be the start of every porting job. These Mopar Large Port Comm
THE VALVE JOB
Port work should begin with the valvejob. A performance valve job is worth horsepower all by itself, and the machine work lays the foundation for the porting to follow. Though some machine shops use seat-grinding equipment, a seat-cutting machine, such as the Serdi shown here, uses carbide cutting tools to remove and shape metal. Naturally, a decision needs to be made about valve size before the valve job is done. Larger valves do not guarantee an increase in flow or performance, but with complementary machining and porting will usually do both. For most engines there are established valve sizes which are readily available, and have been found to work in typical performance applications, such as 2.02/1.60s in Mopar and Chevy small-blocks, 2.14/1.81 in big-block Mopars, or 1.94/1.54 in 302 Fords. Since they move the seat out further radially, bigger valves will locate the seat on virgin metal, often saving the day if the seats are worn or sunk, or if the previous valve seat machining was a disaster.
The other half of the valvejob is the valves themselves. Flow is nearly always improved wi
Often several machining operations are required for the basic full prep, which includes a sweeping cut to de-shroud the combustion chamber adjacent to the valve; machining the valve seat with either a radius or multi-angle cutter; and a throat cut, which removes bulk material under the valve seat. The throat cut accurately clears much of the metal in the port bowl, meaning less hand metal removal when porting. We won't get into the specifics of how to machine the valve job, since this story isn't aimed at the professional machinist. However we would advise anyone serious about modifying a set of heads for performance to seek a shop with the equipment and expertise to perform a machined valve job to performance specs, including the de-shrouding cut in the chamber, and the throat cut beneath the seat.
Typically, the valve job alone will add to low lift flow, but will not lead to substantial flow at high lifts by itself. The bottom-cut angle below the seat will usually form a very pronounced sharp edge, disrupting flow. What the machining does do, however, is set the stage for large improvements with the porting to follow.
THE BOWL BLEND
The most basic level of porting is a simple bowl blend. The port is cast, the seat and throat are machined, and where the two meet there's almost universally a sharp edge, mismatch, or step. This is particularly true after enlarging the seat for a bigger valve or machining the throat. Bowl blending is just smoothing the transition of machined to cast surfaces with a hand grinder. A carbide bit gets the metal removed quickly, though a stone will also do the job. Cartridge rolls or a flap wheel can be used afterwards to provide a smooth surface. Generally, a nice machined valve job combined with a minor bowl blend will be enough to really improve the flow of any head.
Here we have a big-block Mopar head after seat machining. Notice the rough edge and step w
When it comes to a basic bowl blend, the exhaust is done in much the same way as the intak
It's just the reality of any conventional port that the flow path has to make a turn from the runner down toward the valve into the cylinder. Moving air, like it or not, wants to travel in a straight line. The form at the turn is critical to how much air will ultimately make it through the runner, around the bend, and past the valves into the engine. The turn-in from the floor of the port down toward the valve seat is referred to as the shortside turn, while the opposite side (going down from the roof) is the long-side. The long-side, as the name implies, has a longer flow path, and naturally has a larger radius of curvature. The shortside is by definition a much sharper turn, and just like making a sharp turn around a corner in a car, going too fast will make it lose the turn and hit the wall. Similarly, air going around the shortside will reach a velocity at which it will miss the turn, or separate.
The velocity of air moving through the port is primarily a function of how much air it is moving vs. the port's volume (more accurately, its cross-sectional area). As the port moves more air, the velocity goes up; eventually getting so fast it blows the shortside turn and separates. When that happens, the port will typically stall, flowing no more or even less air no matter how much further the valve is opened. How much air will get through before the shortside separates and the port stalls will depend on the shape of this critical part of the port. The shortside form is a key limitation on the ultimate flow potential of a head. All ports aren't created equal, and some have a decent shortside form while others are handicapped in this area. A large radius turn represents a better flow path than a tight one. Some heads are compromised in how much material there is to work with, while others are generously endowed. Race heads are often designed with raised intake ports and a large meaty radius to help get the air around the bend without separating.
Reworking the shortside when porting at its most basic level begins with removing any sharp edges. An edge or ridge will trip the air prematurely, causing the air to separate at the short turn, costing flow. More involved modifications attempt to rework the turn to provide as large and gentle a radius as possible, given the amount of material there is to work with. Generally, a rounded concave radius is the best form. For high-lift flow, the short turn should not project in toward the valve stem, but rather needs to be laid back toward the floor. Water lurks under the shortside of most heads, so beware.
The short turn is critical to peak flow. It's not uncommon for a port to hardly gain anyth
What works for the intake side also works at the exhaust: a large-radius, rounded form. A
The Vortec Chevy head has one of the best basic short-turn forms of any production iron he
POCKET O' POWER
It's no secret that a great deal of flow typically lives in the port bowl, hiding under the valves and around the valve guides, just waiting to be harvested by the carbide. Reworking this area, accessible from the chamber side of the port, is referred to as pocket porting. There's a broad range of what one guy or the next will call a pocket-ported set of heads, but the idea here is to smooth-out or enlarge the bowl area and long side of the port adjacent to the valve guide boss, and even re-profile the guideboss itself. Don't go overboard when enlarging the bowl, however, or a flow separation at the seat will be the likely outcome.
Here we have a good example of a worthy pocket-porting job, defined as blending the "pocke
Sometimes in the pursuit of airflow, greed can get the best of any porter, and the tendenc
Here is a similar before and after of the exhaust port. The idea is to make the transition
The port runner is the "straight" part of the port before it turns into the bowl towards the valve. The runner is usually the best flowing part of the port, however virtually any pushrod V-8 has its major limitation in the port area at the point were the runner is pinched-in to clear the pushrods. This is particularly true of intake ports on inline-valve engines. The runners in most heads start off relatively tall and narrow, and then broaden out and get shorter as the runner approaches the bowl. The runner cross-sectional area is tied to some major aspects of how the engine will perform, with peak torque rpm at a given displacement closely tied to minimal cross-sectional area. If the port is too small, the engine will tend to run out of steam prematurely. In contrast, a runner that is too large will soften the bottom end.
Very few head porters will go through the effort of making a study of cross-sectional area, even though there are major power gains to be had by fully exploring these characteristics. Most ports are cast with fairly dramatic changes in cross-sectional area along the length of the runner. When runner cross section changes, the air is forced to speed up and slow down, and that costs energy and flow. Some simple measurements with a set of dividers and calipers to determine the height and width of the runner at various points of constriction can "map" the port, giving a good indication of "tight" areas needing attention.
Whether or not opening up the runner will gain flow depends upon how well the rest of the port is doing. If the shortside and bowl are putting a cap on peak flow, working the runner will gain nothing. A flow system is only as good as the weakest link, and as the rest of the port starts to move some air, the pinch point becomes a limiting factor. In the upper leagues of high-performance engine building, getting enough cross-sectional area in the pinch becomes a major factor in getting high rpm output out of an inline valve engine. A cross sectional limitation here will drive up port velocities to the upper limits of port flow, putting a cap on peak torque rpm, and ultimately high-rpm horsepower.
How far you can cut back the pushrod pinch before breaking through? Here's where the Helge
It pays to take some time to study the runner before digging in with the carbide. The typi
Metal thickness in many areas of the head cannot be gauged without a sonic tester, so it's
INTO THE CHAMBER
Often there is considerable flow to be gained in the shape of the combustion chamber. The chamber de-shrouding cut made as part of the initial valve job machining is a big help in most instances. Hand blending these cuts into the chamber often yields a flow gain. Further work often involves recessing the chamber adjacent to the intake valve leading to the spark plug boss and often in working the plug boss itself.
Here is a nice example of a reworked production cast iron chamber, fully detailed for flow
POLISHING FOR POWER
Once the heavy cutting is done with the carbides, the ports and chambers can be polished, using a combination of sanding attachments, flap wheels, or cartridge rolls. Often, the polishing process will uncover minor surface irregularities, bumps, and dips that are hard to see but typically present in the rough carbide-cut surface. Boundary layer airflow physics tells us there is little flow to be gained from surface finish, but in practice, significant flow gains are sometimes seen from polishing the ports. The polishing process does remove metal, making the port slightly bigger, and the removal of minor irregularities probably accounts for the balance of any improvements.
Beyond the potential for flow gains, commercially ported heads need to have a nice uniform
With the stock small intake valve smog heads, and nothing more than a good valvejob to hel
SHOW ME THE NUMBERS
Just what kind of gain can be had from practicing the porting tips we're preaching here? We ported a set of lowly 360 Mopar factory smog heads to put theory to the test. We did not take these heads all the way to polished gems, but simply had the machining done as discussed, and carved on them with an assortment of carbides to dramatically improve the port shape. With basic porting we achieved a gain of about 30 percent in intake flow and a 45 percent improvement on the exhaust side.
For our next trick, we heaved on our freshly ported heads. The 360-cube test engine seemed
The flow numbers themselves were impressive enough, but we wanted to see if it would stack up to more power on the dyno. We had a fresh 360 Mopar short-block that had been rebuilt with stock internals, and fitted with .030-inch-over Federal Mogul 10:1 hypereutectic pistons. A custom Comp solid lifter cam with FL272/FL276 lobes on a 108 degree lobe separation provided around .550 lift at the valves with our 1.6:1 Probe roller-tipped rockers.
Our "stock" heads were a set of production smog #587s, same as the ported versions, with the original 1.88-inch intake/1.60-inch exhaust valves. The "stock" heads did have nice Serdi-cut performance valve seats, but no additional massaging. Running this much lift meant that the tops of the guides had to be cut down to clear the retainers, and we needed more spring than will fit at the stock 1.65 inches installed height. A set of Engle dished retainers and #993 single springs took care of that. We were ready to run with an Edelbrock Torker II intake and 750 Speed Demon bolted on top, and open 1 5/8-inch Hooker headers mounted to the other side.
|INTAKE PORT FLOW |
|.050" ||27||31||31||31||31||32 |
|EXHAUST PORT FLOW |
ALL TEST WITH CLAY RADIUSED ENTRY
Test 1: Stock port with fresh Serdi valvejob
Test 2: Bowl blend
Test 3: Blended/profiled shortside turn
Test 4: Deep pocket porting/profile guideboss
Test 5: Intake pushrod pinch opened; Exhaust roof kink blended
Test 6: Chamber mods
*PEAK OBSERVED FLOW WITH 2-INCH 90DEG FLOW PIPE