The article concludes in this thread.
We do not want turbulence in the intake tract if we want to deliver a higher volume of air and fuel into the combustion chamber. Air cleaners perform a valuable function here, as the air traveling through the filter is turbulent and disorganized. The air horn on premium air filter backing plates, like Doherty Power PACC and S&S Cycle Super E/G, takes the slower-moving, turbulent, filtered air and organizes it into higher velocity laminar flow.
A carburetor begins mixing fuel with air in the carb’s venturi, way before the intake valve. Therefore, a carbureted hemi head will want an unpolished intake tract. A carbureted Evo, and especially a Twin Cam, will want a slightly smoother intake tract. An EFI Twin Cam will want the smoothest intake tract of the examples given, since the Delphi EFI system squirts fuel toward the back of the intake valve.
This is a great time to discuss atomizing add-on parts, such as a fine mesh screen that covers the mouth of a carburetor or EFI throttle body. The purpose of these devices is to initiate turbulence to mix the air and fuel before it arrives in the combustion chamber. I am skeptical whether these items work at all on an efficient running modern engine, although on the surface it may seem logical. Modern engines have combustion chamber designs and shapes to complement a piston dome profile that efficiently mixes and atomizes fuel. However, there is always the exception to the rule. In this case, I think the atomizers probably work well on hemispherical combustion chamber engines like Shovels, Knucks, Pans, and Ironhead Sportsters, and even flathead engines. These engines have such inefficient combustion chambers that only a low percentage of fuel atomizes with the air in the correct proportions to burn. We sacrifice faster, and thus higher volume, laminar airflow in the induction system on these hemi and flathead engines because what use is more air and fuel if it’s not mixed well enough to burn in the combustion chamber? Any air and gas turbulence is welcome and needed on a single spark plug hemi engine. However, I will bet ownerships that external atomizers do not work on a performanced engine with D-shaped or bathtubbed combustion chambers, utilizing squish bands and mating pistons. In fact, they must surely impede airflow, which is what power is all about. Remember that turbulent air moves much slower through the intake tract than laminar air.
If you want higher volumes of air reaching the combustion chamber, less turbulence is key, not more in this area of the induction system. One of a performance air cleaner’s main functions is to reduce turbulence in the air as it broaches the entryway of the induction throat. An atomizer will take the air that the air cleaner design has calmed into laminar flow and then turbulate it, which will slow its travel and, therefore, volume.
As for an EFI system, what use is an atomizer at the front entryway to the induction throat when the injectors deliver fuel directly to its final destination? Not all will agree with this, but to me, the logic seems inescapable. Atomization devices may work well for the wrong reasons on a carbureted hemi engine, but it is detrimental on its modern Twin Cam EFI bathtubbed counterpart. This is also the reason dual-plugging a hemi head is a great idea, whereas it will not have the same beneficial effects on a bathtubbed one. Hemi heads need all the help they can get. For this reason, at Heavy Duty Cycles we are inclined to smooth the intake port more on an EFI-bathtubbed engine we are porting and polishing, whereas we leave the intake port on a hemi head rougher for turbulation and resultant atomization. Even this will not have much effect as the layer of air/fuel closest to the port walls may have some mixture occurring because of wall roughness, but there will be no effect on the inner layers at all. Those centralized streams of air will be moving in their own plane and at their own velocity.
TC 96 Head Improvements
An improvement of the reworked TC 96 head castings over their TC 88 counterparts is the elimination of a restrictive-to-flow curvature on the inside of the turning exhaust port. This is accomplished with a welcome oversizing of the whole port, but it still radiuses down to the almost same size valve seat. This narrowing down will increase flow, as explained by Bernoulli’s Principle. This pressure difference will result in a net force, which by Newton’s Second Law, will cause an acceleration of the fluid.
Factory representatives allude to a reworking (massaging) of the TC 96 heads. The casting looks to be of better quality and is nicer with smoother edges and more defined areas, such as the oil cavities. However, these aesthetic improvements do not affect anything mechanically; it’s just visually better. The exhaust valve (#18085-05) is also more conducive to improved flow as it approaches a tapered tulip shape where the stem joins the top of the valve as opposed to the 1999-2005 version with a more abrupt, flatter exhaust valve top.
At Heavy Duty Cycles, we measured some 1999-2006 exhaust head ports and found an average diameter of 1.330", which varies slightly from head to head. The new head exhaust port diameter is about 1.510". This increase in sizing is huge, as thousandths of an inch is equivalent to miles inside a motorcycle engine. Intake ports are pretty much the same size, but a noticeable and superior difference is that the seat edges and casting they fit into mate correctly on the heads I removed from my 2007 FLHXI. The improved intake port shape over previous Twin Cams allows for enhanced intake airflow. The port curvature is gentler and not as abrupt when changing course, which allows for smoother flow. The intake valve’s (#18074-05) head is also approaching a higher-flow tulip shape. The valve guide (#16465-06) has a shorter reach into the head, which is less obstructive to airflow. Intake manifold flanges (#26993-06) are new for better angling of the manifold aperture relative to the port it serves. This is an obvious improvement. In keeping with this, the actual flange mounting holes in the head itself are now equidistant from each other, as well as the port’s center. Earlier heads use front and rear flanges (#27009-86A and #27010-86A) that mount to holes in the head that are offset and not equidistant. This improvement, combining with the new injector angle of 25 degrees, from the previous 8 degrees, helps to increase air flow and fuel delivery.
The valve seal improves once again after the 2002 change, which was also superior to what went before. It appears to incorporate an improved, more pliant, rubber-like material into the lower valve collar seal, which will hopefully have a longer service life. The new seal (#18094-02A) is orange while the 2002-06 one is black (#18094-02).
The heads still require porting and polishing to raise the horsepower, which does not increase substantially from the TC 88 to the TC 96, even with 8 more cubic inches and a higher compression ratio of 9.2:1. Nevertheless, the head is a big improvement over what went before. Installing 2007 TC 96 heads on earlier 1999-2006 engines will result in much improved performance. However, it is not as much performance as could be gained by massaging them further. A major Twin Cam engine deficiency has always been insufficient head airflow. The head design is the best since 1903, and quality is excellent. It’s just that the ports and valves do not deliver enough air to allow the engine to be what it could be. H-D addresses airflow to some extent in the TC 96. In my opinion, it is not enough. I hope that improving technologies can deal with emissions dictates. Porting and polishing is much more than removing port obstructions and port resizing, if necessary. Cutting the valve seat is the most important aspect of increasing flow in a head. Valve seat fitment relative to the combustion chamber on one side and its relationship to the port on the inside is also crucial to maximum flow.
The Valve Seat
Valve seats, if cut at one angle only, give demonstrably poor airflow results when compared with a standard three-angle valve cut that produces a satisfactory production motorcycle. Five-angle seat cuts produce far better airflow increases. If a seven-angle cut is attainable, this will give the best results. Normally the base seat cut in the head, where the valve head actually touches and seals the combustion chamber, is at 46 degrees, while the valve face cut is at an interference fit of about 45 degrees. The adjacent cuts to the heads valve seat will be at 30 degrees and 60 degrees, respectively, which results in the three-angle cut.
With these two contiguous cuts, we can determine the width of the actual valve-to-seat area. Cutting the neighboring 30- and 60-degree valve seat cuts deeper will narrow the 45-degree center contact. Cutting the adjacent 30- and 60-degree valve seat cuts lighter will allow a wider 45-degree center contact. Cutting the 45- degree center contact cut deeper will widen it. The seat contact area can move out or in by deepening one adjacent cut relative to the narrowing of the other to place it where desirable on the valve face. Placing the valve contact position to the optimal seat contact location will increase flow.
There is no set placement; as port design, combustion chamber shape, and valve-head size all combine to determine the location of the optimal valve-to-seat contact on the rounding radius of the valve seat. Generally, for best flow, place the seating area close to the rim of the valve. However, do not put the seat right at the rim, as this will lead to a premature compromise of the contact area from the searing combustion heat. This 45-degree center contact cut is important, as a wide contact area may have more longevity before recutting is again required, but will not flow as much air as a narrower cut. An incredibly thin contact area will produce the most flow, but may not have satisfactory longevity before a rebuild is necessary. Very narrow cuts have use on a race bike.
On a streetbike, we need a compromise valve-to-valve seat contact area, both to ensure good airflow and engine longevity, although the choice is up to the rider after consulting with the mechanic. A five-angle valve seat cut adds additional 15- and 75-degree cuts. If we look at the seat from one side to the other, the cuts will be in the following sequence: 15, 30, 45, 60, and 75 degrees. If we look at it from the opposite side, the
figures reverse. The seven-angle cut will add two cuts, one at either end (say, 7 and 82 degrees) to further round out the seat. The thinking mechanic will decide on the angles for these two cuts depending on the contour of the head port approaching the seat and the contour of the combustion chamber moving away from the other side of the valve seat. The key word is rounding. A rounded valve seat promotes the easy flow of air, removing impediments to the volume and velocity of flow.
The valve seat is a separate part, pressed into a cutaway in the head. This is necessary because the seat is made from an extremely hard steel alloy in order to withstand the pounding of the valve as it returns to the seat thousands of times per minute, not to mention the extreme heat that seeks to burn away the seating area. The seating area is subject to searing heat because of its thinness and therefore vulnerability. The head composition is of a much softer aluminum alloy, which could not withstand the pounding or concentrated heat. The seat abutment where it contacts the head in the port, whether it is the intake or the exhaust port, is also crucial to uninterrupted airflow. If there is an overhang, it must be radiused. If an under-hang, the seat abutment requires contouring.
The Screamin’ Eagle heads on the CVO 110" only have a three-angle valve seat cut. Optimally, we want a seven-angle cut, but many times we cannot do this because the seats have already been radiused at the factory. However, a five-angle cut to each valve seat face will further round the entranceway for the incoming fuel charge and initial departure port for the exiting exhaust gases.
The Flow Bench
The flow bench will teach a student the physics of airflow as the valve opens and closes. Hundreds, if not thousands, of hours of work on a flow bench will teach the prospective porter what works and what does not. Measurement of airflow is in cubic feet per minute (cfm). The volume of air passing through the port changes as valve lift or valve opening increases or decreases. This gets complicated quickly, as the reader can now discern a relationship between cam lift and airflow.
Where do we want maximum airflow that leads to maximum power? We begin to design the power curve, representing where power comes on and at what rpm it will begin to taper off. Merrily cutting and grinding away just does not make it! Many reworked heads that I’ve seen actually reduce airflow rather than improve it. This is no area for amateurs, as porting heads with the valuable assist of the flow bench requires skill, patience, and some art. The concept of airflow is a lot like that of electricity for mechanics and riders alike. Both are invisible and what cannot be seen becomes complex. Therefore, many cannot grasp the complexities of both. Electricity powers the engine management system, charging system, and lighting. Air powers the bike by mixing with fuel, allowing it to burn, creating heat, and thus the power to drive the bike forward. We need to measure airflow. We need to know when we make performance and/or efficiency changes if airflow increases or decreases.
When head design engineers and flow bench technicians measure cylinder head port airflow, they do it at differing valve lifts. The exercise becomes a question of how much air will flow when the valve is open at say, 0.510" of lift. Head port design, as well as porting and polishing existing heads, flow air at different rates for differing valve lift openings. Therefore, port design determines optimum lift for the cam. As a simplification, installing .650" lift cams in an engine, whose heads flow the maximum at 0.550" valve lift, may be counterproductive depending on cfm drop-off rate after maximum flow for the remaining 0.100" of valve lift.
Conclusion
Next month, we’ll continue discussing various obstructions to power production on the Twin Cams.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Techline-July09-PERFORMANCE OBSTRUCTIONS - Part I: Airflow through the heads (concl.) 
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