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.
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.
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.
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.
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.
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.
Fig. 1 From this drawing you...
Fig. 1From this drawing you can see that the real impediment to flow is not in the main body of the port but at and around the valve. Making improvements to the least efficient regions of the port pays the greatest dividends.
By determining the average...
By determining the average velocity within the main body of the port and comparing it with the average velocity at the seat, we can establish at what point the seat priority swaps over to port priority. On this near-stock SB Chevy head we see that until 390 thousandths lift, the seat was actually more important than the port.
In this drawing anything within...
In this drawing anything within the circle around each valve has the potential to shroud the valve. Just installing big valves can put the edge of the valve nearer the chamber wall, thus shrouding it further than it may have been stock.
In as-cast form, many heads...
In as-cast form, many heads have guide bosses as seen here. These are an impediment to flow above about 200 thousandths lift.
If they are cut and narrowed...
If they are cut and narrowed as seen here the difference in flow can be quite substantial.
This is about all the hardware...
This is about all the hardware you need. Two or three carbide cutters and an arbor for the emery rolls will cover your consumable needs. If an air source is available then an air die grinder from Sears, Lowe's or Home Depot can be had for $20.
Standard Abrasives is the...
Standard Abrasives is the major player for consumable porting supplies. Visit their web site for an online catalog of just about everything you might need.
If no air supply is available...
If no air supply is available then an electric die grinder like this one will do fine. Suitable ones are made by Bosch, DeWalt and Makita.
If no air supply is available...
If no air supply is available then an electric die grinder like this one will do fine. Suitable ones are made by Bosch, DeWalt and Makita.
Seats are an important element...
Seats are an important element of a porting job. Be sure your efforts are accompanied by good seat work.
Milling the head for compression...
Milling the head for compression is always a good move but be sure to budget for manifold machining as well so as to restore port alignment.
To improve the approach to...
To improve the approach to the seat it is important to back cut the valves so as to leave the seats only marginally wider than those in the head.
This is the flow measuring...
This is the flow measuring part of the Audie Technologies Flow Quik. This unit, which reads out directly in CFM, allows a shop vac to be used as an air flow source.
When testing the Audie Flow...
When testing the Audie Flow Quik, we used this $39, 2.5hp Sears shop vac and flow tested successfully to about 230 cfm. A 4 or 5 hp unit would certainly deal with the requirements of a 300-cfm head and this would cover most needs.
How heavy would you guess...
How heavy would you guess this block of air at standard temperature and pressure is? To give you an idea of the size, we have parked a 20ft long GM/Chevy extended cab truck on top.
Port Volume -- bigger is not always better!
For port volumes most racers think 'bigger is better.' This is due to not realizing how heavy air is or knowing what an idealized port should look like and how it works.First, let's deal with the question of 'how heavy' air is. Take a look at the 100ft cube of air in Figure 1 below and see how closely you can guess its weight. Having made your guess, we'll tell you it's 38 tons. Chances are, you completely underestimated how heavy air is and as a result underestimated its velocity-induced cylinder ramming potential.
In Figure 2, we see an idealized port form. On the right is the manifold plenum and at the other end is the bowl just prior to the valve seat. Bringing the air up to speed in the high-speed section and slowing it down in the expansion section achieves two important things. First, allowing the air to slow in the expansion section allows it, in a real port, to make the turn to the back of the valve easier. Second, the expansion turns some of the kinetic energy of the high-speed section into pressure energy. This in turn raises the air's pressure at the valve thus helping to push it into the cylinder. In a race engine, bringing this hig-speed air to rest at the end of the intake stroke can result in as much as 7lbs. of velocity-induced port pressure, which all goes toward supercharging the cylinder. Your street motor can benefit in exactly the same way, so don't assume that bigger is always better.
If we could straighten out...
If we could straighten out and idealize the form of a port, this is what it would look like. The key to its success would not only be the fact that it is a very efficient shape but also a form that maximizes the effects of energy contained in the velocity that is built up in it.
Port length is a factor toward...
Port length is a factor toward maximizing horsepower but the reality is that port velocity due to the port's area is even more important, especially if a wide power band and high specific torque is a goal.
Swirl = Torque
Swirl is very important for low- and mid-range torque and, with an automatic trans, it's torque that launches the car. Small-block Chevy heads have a port that has a natural tendency to generate good swirl and this can be enhanced by biasing most of your port reworking to the cylinder-wall side of the port. Many Ford heads are much poorer in terms of swirl and often have little or none. To help offset this, make a conscious effort to smooth out both the finish and the curves of the cylinder-wall side of the port. By helping the flow stick better to this wall some additional swirl can be generated. In a 5.0 this feels just like extra cubes.
A SB Chevy's intake port has...
A SB Chevy's intake port has a natural tendency to swirl because most of the flow takes place on the cylinder wall side of the port. As a result, most of the air enters the cylinder on the 'A' side of the valve and continues in the direction indicated by the arrows.
SB Ford heads typically lack...
SB Ford heads typically lack swirl because most of the air tends to flow on the cylinder center side of the port and dumps straight into the cylinder. To boost the ports swirl capability you need to get the air to stick to the outside wall better so more flow occurs along the path indicated by the arrows. Making as smooth a radius of the dog leg as shown here helps.
Compression -- The budget racer's friend
Increasing compression increases torque throughout the rev range. At low speed it has just the same effect as more cubes and at high rpm the same as more cam. It makes all your other mods--especially a bigger cam--work that much better. It improves drivability and gas mileage. If milling the heads and manifold is in your budget and you are sure piston to valve clearance is not an issue go for all the compression you can consistent with the fuel octane to be used.
Five Golden Rules for Successful Porting
1:Give the air room to move where it wants to go, not where you think it should go.
2:Identify the points of flow restriction and fix them (as far as they can be fixed) in the order of severity.
3:Develop charge motion (swirl) as this increases burn rate and reduces octane requirement.
4:High-speed ports with minimal redundant volume ram cylinders better and result in better mixture quality, higher torque and a wider power band.
5:Go for as much compression as the situation will allow.
The author has not ported every single head on the market, but of the many he has, here is his pick of the easiest to work on.
Iron Chevy SR Torquers. Cheap to buy and easy to do for excellent results for truck or real street application. Chevy Sportsman II and Roush 200 Ford. Reasonably priced and can be easily reworked for semi-serious race applications.Aluminum Ford Windsor Lites and Junior Lites. With a hot street cam (240-245 @ 50) and 12:1 CR can be easily ported for over 1.5hp-per-cube.
Iron Eagle. Slim guide bosses and emery roll the rest of the cast surfaces and you are done with a set of heads ready to make some semi-serious race power.
Chevy and Ford Track 1. These heads are good out of the box but with a basic port and chamber tidy -up become very effective entry-level race heads.
SB Ford and Chevy. All the Canfield heads from their smallest port model to the largest deliver good flow out of the box but can be significantly improved upon with little more than basic guide boss and port cleanup. The price is also good.
Airflow Research (AFR):
Have used a lot of AFR products over the last 25 years- especially their Chevy castings. Whether dyno'ed in 'out of the box' or ported form, they have always delivered the goods. Example: a race hydraulic flat tappet cammed 12:1 350 Chevy equipped with cleaned-up AFRs topped 550 hp. AFR's SB Ford heads also score big.
SB Ford Performer and Performer RPM. Edelbrock made some changes a year and a half or so ago without announcing it. These changes made a good head design even better. High intake swirl right off the seat gives these Ford heads Chevy low-speed torque. Along with good swirl are good flow numbers for power. Basic porting can be accomplished with little more than a few emery rolls.
SB Chevy Very good flow and swirl out of the box and can be quickly ported for pro looking/functioning results with almost zero cutter work.
SB Chevy Vortec. Blending out chamber plug boss with a cutter is the longest job you will need to do on these heads. After that the ports need only be smoothed out with 60 or 80 grit emery rolls to produce a top-notch pair of street heads that will deliver both ends of the rpm scale.
1:2:Don't start an extensive porting exercise until you have completed some simpler projects.
3:Don't start your intended porting job unless you are prepared to see it through to the end.
4:Don't worry about a shine on the finished surfaces.
5:Don't, under any circumstances, start without appropriate tools, cutters, die grinders and some old valves for seat protection.
6:Don't overlook budgeting for a valve seat job and possibly head/manifold milling.
7:Don't ignore loose valve guide fits as this costs more power than you might at first suspect.
8:Don't skip safety measures such as a mask and glasses.
9:Don't port in the same space used to assemble engines.
10:Don't sweat not having a flow bench - yet!