 Techline-July09-PERFORMANCE OBSTRUCTIONS - Part I: Airflow through the heads
This article concludes in the next thread.
This month we start an excerpt from Chapter VIII: Eliminating The Performance Obstructions from Donny’s Unauthorized Technical Guide to Harley-Davidson 1936 to present, Volume II. (Some content has been altered to fit AIM’s style and format.)
Originally, my plan was to deal solely with insufficient head flow, the greatest barrier to more power in both the TC 88 and TC 96. However, I’ve decided to broaden the scope of these articles because there are a series of power obstructions, which includes the cams, ignition, fuel delivery, and exhaust system. In short, these articles deal with restrictive emissions controls.
It’s not that these components are a bad design or substandard. They include the most futuristic technologies ever envisaged on a Harley-Davidson, with the exception of the V-Rod. The problem is that the design of these components, which all link together and support each other, is encumbered by tunnel vision. The goal is to eliminate emissions harmful to the environment. Unfortunately, considerations like more power and comfort from less heat are secondary. Harley-Davidson is in a difficult situation. The question for the Motor Company becomes how to comply with EPA and CARB rulings while keeping a tradition alive.
The tradition we all want to protect is dinosaur technology involving a valvetrain versus overhead cams; an awkward cylinder angle with an equally uncomfortable ignition firing order; in-line, air-cooled cylinders; and only two cylinders when more would form a symbiotic relationship. There is nothing here that would not immediately be resolved with overhead cams, liquid cooling, a wider cylinder angularity, and an effective ignition firing order with more cylinders. The rock is tradition, and the hard place is complying with emissions controls.
Key To Power
Air is power! Air must be mixed proportionately with fuel at about a 14.5:1 ratio for cruising and hot idle. For acceleration and cold idle situations, an air/fuel ratio (AFR) of around 13:1, give or take a few points, is required. If there is insufficient air to mix with the fuel in these generalized proportions then combustion and power will suffer. It’s easy to jet (or squirt) more fuel into the cylinder. The process of fuel delivery is not problematic. The delivery of air, on the other hand, requires an unrestrictive air filter (including an air horn), a complementary intake induction tract, properly ported manifold, and head ports with sufficient valve sizing, along with an unrestrictive exhaust with backpressure. Once these conditions are in place, the cams become the masters of flow, allowing many laws of physics to do their thing. Add complementary ignition timing and some compression, and the engine will deliver a level of power correct for its displacement.
No one knew anything about the Twin Cam in 1998-99, except the
H-D staff involved in its development and production, plus the lucky few like myself. Select management kept the secret so well that dealers knew nothing, nor did most other factory employees or management. In fact, dealers became part of the rumour mill, repeating bad information, half-truths, and no-truths. Everyone knew that something extraordinary was coming down the pipe. Then, the introduction of the Twin Cam caught everyone by surprise, utilizing design and technology never seen before on a Harley-Davidson. This necessary secrecy from competitors, and especially the powerful aftermarket, created a huge information void. Many of the new, business-type dealers don’t always have full mechanical knowledge. Dealers were scrambling to answer all the questions put to them by an enthusiastic and demanding customer base.
However, the dealers do know how to up-sell, and up-sell they did. A strange thing began to occur soon after the introduction of the Twin Cam in 1999. Without regard or knowledge for the limited-flow capabilities of the heads, most dealers began pushing the 1550cc (95") upgrade kit for the new engine. Who was to know that the exquisite head design and metallurgy had poor flow characteristics for such a big engine? In addition, who would have suspected that the venerable Harley-Davidson would begin to evolve this type of engine design with a view to marketing upgraded performance parts? In my opinion, it’s not fair to the consumer if some parts are under-engineered for the purpose of creating after-purchase sales, instead of designing and incorporating these improvements into the finished product. However, this may simply be another case of the Motor Company having to meet emissions controls to the detriment of potential reliable performance. To be realistic, this is capitalism at its finest.
I can unashamedly say that I was taken in when first observing the Twin Cam head design in Milwaukee in early 1998 because it was obviously superior at first glance. The heads seem superior to what went before, since their design is superb and the metallurgy is the best. Even the powdercoating, whether it is black wrinkle or blond, is bulletproof. However, in my opinion, the original TC 88 heads are deficient in regards to the flow of air. The newer TC 96 heads are better, but, in my opinion, also deficient for the size engine they serve. The TC 88/96 bathtubbed combustion chamber shape I was familiar with on high-performance aftermarket Evo heads led me to draw wrong conclusions as to head flow. The heads look so good that I made a misassumption as to their flow capabilities. I was not alone.
Squish Bands
The flattop, round piston ascends in its cylinder and squishes up against the bathtub combustion chamber design. Squish forces the air/fuel mixture turbulently into the chamber, mixing the air and fuel better than any previous H-D head design. The proportionately mixed air and gasoline molecules induce superior flame travel and a much higher percentage of burn. The compression ratio is higher, 8.8 to 9:1, compared with the Evolution’s 8.5:1. This increased compression leads to more torque, and thus, more horsepower.
Visually, the TC 88 heads look great. So why do dealers keep selling 95" big-bore kits with no headwork to accommodate the increased cubic inches? Why do they not educate the consumer that sufficient air delivery is essential to the bigger cubic-inch displacement in order for the increased displacement to work to its true potential? Ignorance of the product is the only thing that I can think of. In fairness, everyone was on a steep learning curve with the introduction of the Twin Cam, since it was such a well-kept secret. This is partially due to H-D creating its own aftermarket, for the first time — in necessary secrecy — well in advance of everyone else in the marketplace.
Bathtub-style combustion chambers are the first thing noticeable when the engine is apart. When assembled, the fatter finning, which is for better cooling, is most striking, with a 60-percent increase over the Evolution. With the bathtubbing, there is less room between the valve seat insert and the spark plug hole. Therefore, reducing the traditional Big Twin 14mm spark plug threads to 12mm became necessary. If left at 14mm, the engineers felt that it might cause a weakness in the head casting.
The 242 alloys used in the Twin Cam heads are the same used in Evo heads. The bathtub-shaped chamber is a more rounded, rectangular shape than the bathtub or kidney-shaped configuration that is prevalent in the aftermarket. Previous hemi heads found on the Shovel, Ironhead XL, Pans, and Knuckleheads have rounded, hemispherical, inverted combustion chambers. The piston top or dome is also rounded and upturned in order to fit into the hemi head. This system is not good for uninterrupted flame travel or a high percentage of burn because of the interfering piston dome and lack of squish.
The Evos have a D-shaped combustion chamber. Inside the D, the chamber cuts off instead of continuing in a perfect circle. The rest of the circle is flat and even with the surrounding gasket surface. A flattopp piston squeezes against this flat squish band, forcing the air/fuel mixture turbulently into the smaller D chamber, promoting better flame travel, air and fuel mixing, and a much higher percentage of burn.
The TC 88 bathtub chamber is small, being only 85cc in volume. With bathtubbing, there is a flat squish band all around the chamber so that the big 3-3/4" flattop piston forces the air/fuel mixture even more turbulently into the chamber, promoting the best flame travel and highest percentage of burn of any H-D head configuration. The permanent mold head has the same intake valve size as the Evo, but the exhaust valve is about 0.030" smaller in diameter, though a little thicker. An indent mark in its
center face allows for easy recognition.
Head Ports
Combined with this smaller exhaust valve is a smaller diameter, reworked exhaust port that takes advantage of Bernoulli’s Principle and gets the exhaust gasses exiting faster and more efficiently to help with cooling and emissions considerations. The built-in torque valve is more correctly called an anti-reversion (AR) valve. The AR is not really a traditional valve but a step in the exhaust port to help burnt gases exit the exhaust pipe and stay out. The TC 96 head does not have this anti-reversion in the external mouth of its exhaust port.
Removal of the anti-reversion step at the outside of the TC 96 exhaust port opens up the port considerably, allowing much more flow. This anti-reversion step was a design improvement in 1999 with the advent of the Twin Cam 88, even though it restricts the all-important exhaust flow properties of the head design. Exiting gases partially overcome this by speeding up, since the same amount of gas must pass in the same time according to physics.
Anti-reversion also helps prevent exiting exhaust gases from returning and soiling the fresh intake charge in the combustion chamber. As exhaust pulses are forced out through the exhaust valve, through the port, and into the header system to the mufflers, kinetic wave exhaust pulses return up the pipe many times before finally expelling out the muffler. It’s of the utmost importance that these pulses of burned gases bounce off a closed exhaust valve. These reversion gases also bounce back off anti-reversion steps built into the heads. Installation of anti-reversion torque cones in the exhaust header pipes where they meet the exhaust port on the 2007 models is easily done and inexpensive. The reader can now understand why the oft-ignored exhaust system is complex, as only one of its functions is to control the timing of these power-robbing pulses.
The Twin Cam lungs cannot operate at peak efficiency without increasing breathing capabilities. Porting and polishing stock heads with a little valve work, such as installing the 0.030" larger Evolution exhaust valves, will work wonders for engine efficiency and releasing suppressed torque and horsepower. The Evo valves fit easily and only require recutting the valve seat.
Cutting valve seats, where they interface with the valves, is extremely important, both for sealing the combustion chamber and regarding airflow. Airflow dramatically improves with quality work within 1/2" of the valve seat. This is where the majority of flow improvement will come from. Compare this with about a potential 1 percent improvement by porting and polishing the inside of the intake manifold to a lustrous shine.
Porting is reshaping the intake and exhaust tracts to encourage more flow. Valve guides that stick down into the port also need addressing. We do this in a variety of ways, such as beveling the guide itself so that a pointed, as opposed to a rounded, edge meets the flowing air. In some situations, a rounded guide also works well. The aluminum port material is similarly shaped as it approaches the guide. The reader can see that this is all about easing impediments to the flow of air. This is not work for amateurs, nor is it work for many mechanics.
One of the principal areas to address when porting TC 88 heads is a severe overhang of the valve seat relative to the head port. Valve seats press-fit into the alloy heads. A hard steel alloy withstands the returning slam from a closing valve. For an optimal flow of air, the valve seat should be radiused to blend into the port it serves. Much improved on the TC 96 models, the valve seat is better matched to the head ports than on the 1999-2006 TC 88 Twin Cam heads. But there regrettably still is some overhang of the seat. This needs radius-shaping for better flow.
Many misunderstand the polishing of ports. I polish exhaust ports because I don’t care about the exhaust gas composition flowing out. It is waste and good riddance. I polish the exhaust ports to speed the exit of bastardized gases. If having stock heads ported and polished, some shops will polish the exhaust tract to a mirror shine. This is purely feel-good stuff. It is beautiful, but the lustrous mirror shine is not necessary. A polished smooth surface will suffice to encourage the faster movement of the exhaust gases into the exhaust header pipes.
The intake tract is a different story. I do not polish this port, as all our attention must be directed at porting. I leave the intake tract unpolished to encourage the air and fuel to mix, especially in inefficient hemi heads like those found on the Shovel or Ironhead Sportster. Theoretically, the rough texture of the port walls will cause turbulence and assist in mixing fuel with the air so that it is better able to burn. I will take turbulence when I can get it in older hemi heads as it will assist in the burn. There has always been controversy over how true this is. There are even misguided porters who believe in polishing the hemi head intake, but they are a small minority. I think the confusion may lie in the type of head design and fuel delivery.
Hemi heads do not mix the air and fuel mixture well, so they need all the help they can get in the intake tract. Therefore, turbulence in the intake tract on this style engine assists in mixing the air and fuel. However, turbulent air is a slower moving air. Evo, and certainly Twin Cam, heads with squish bands mix the air/fuel mixture efficiently, so we want to get as much volume into the combustion chambers as possible. The way to move a larger volume of air and gas is through laminar (smooth) flow.
This article concludes in the next thread. Check back issue for pix and extra information. |
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