This is probably the most argued and discussed section of a performance built engine right next to the camshaft. I feel the best answer for a novice is to work with a company you know and trust and that has a good reputation for making cars / trucks respond the way you would expect and then do exactly as they did. However, this is a technical writing and you came here to read my thoughts and hopefully gain a better understanding of who, what, why, and all the particulars of head-porting and how that relates to Frank Racing Inc.

 

Lets start at the valve and work our way back into the head and discuss each section and what we do and why we do it that way. We'll also stop from time to time and explain what "other" people are doing today and why we at Frank Racing Inc. have chosen not to do certain things.

 

A valve job is worth horsepower all by itself, and lays the foundation for the porting to follow. 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. Historically, many head-shops used enlarged valves to make up for deficiencies in the head design and shape, yet for the third generation, small block HEMI, this is of no concern to us. The HEMI's 13* head allows a user perfect alignment of the flow path from the intake, straight to the backside of the valve. This runner shape allows for extreme port velocity yet with good, gross flow numbers in a tight 2.00" valve which can be kept very light for high RPM use. Typical flow numbers in stock form (5.7 liter) are 282 to 288 CFM @ .600 lift.

The next step is a simple bowl blend. The port is cast while the seat and throat are machined and pressed in. Where the two meet there's 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, and followed by a stone will shape the job nicely. Cartridge rolls and / 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 and is a major part of the Frank Racing Inc. design regardless of the extent of work you are requesting.

The reality of any conventional port is that the flow path has to make a turn from the runner down toward the valve into the cylinder. Moving air 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 short side turn, while the opposite side  is the long-side (floor vs roof). The long-side, as the name implies, has a longer flow path, and naturally has a larger radius of curvature. The short side 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 short side will reach a velocity at which it will miss the turn, or separate. We also see speed differences between the mass hugging the floor as apposed to hugging the roof.

The velocity of air moving through the port is primarily a function of how much air it is moving vs. the port's volume ( cross-sectional area). As the port moves more air, the velocity goes up; eventually getting so fast it blows the short side 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. Typically speaking, using 28" of water on the SuperFlo flow bench, we see the 5.7 HEMI stall around .750 lift and the 6.1 HEMI stall around .820 lift. This tells us the line-of-sight is far superior than most any head ever produced in mass quantity.

The gross volume of air that will get through before the short side separates and the port stalls will depend on the shape of this critical part of the port. The short side form is a key limitation on the ultimate flow potential of a head. All ports aren't created equal, and some have a decent short side / alongside 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.

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 engine has its major limitation in the port area at the point were the runner is pinched-in to clear the pushrods or to allow water passages. 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 yet still with regards to port velocity with a given gross volume. 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. However, when you consider total engine displacement, RPM, cam selection, and weight of mass, you might find that bigger isn't better and in many cases is actually worse.

Using Frank Racing HEMI heads is money well spent. The time spent studding flow characteristics and how those numbers relate to the cam selection which relate to your RPM and gross weight ensure that you end up with maximum power across the entire power curve as apposed to wasting your money on exotic materials that will show no benefit on the street given typical conditions. Our typical street / strip head flows more than enough for everything other than a dedicated race car and is affordable enough to be competitive with anything on the market. Frank Racing Inc's experience and ability to hand-port, allows the customer to work on projects not included in our regular catalog. Frank Racing can custom fabricate a set of heads for any race team, extreme performing street cars, and exotic stroker motors / power adders.