 Techline-August09-PERFORMANCE OBSTRUCTIONS - Part II: Heads and exhaust systems
This article continues in the next thread.
This month we continue an excerpt from Chapter VIII: Eliminating The Performance Obstructions from Donny’s Unauthorized Technical Guide to Harley-Davidson 1936-Present, Volume II. (Some content has been altered to fit AIM’s style and format.)
There are many variables from one flow bench to another. Operator errors, such as not recalibrating the flow bench for new testing, can result in errors of five or more cubic feet per minute (cfm) in a reading. Leakage from fixtures will also give the wrong cfm flow results. The air inlet on the flow bench should be radiused with all head testing. If the inlet is sharp, airflow is disrupted and can easily fall off by up to 25 percent when compared with the same head on a radiused inlet. Correct radiusing will ensure maximum laminar airflow.
Laminar is defined as a smooth layered flow of a gas, in this case air, in which the neighboring layers flow at different velocities and do not mix. Barometric pressure is a major factor as air density varies with both altitude and weather conditions. Air is denser when cool. Therefore, there is more air in the same amount of space in colder weather than hot. This is why an equalization process of barometric pressure must occur when comparing ratings for head flow or else effective and honest comparisons are impossible.
In my opinion, pinpoint accuracy in comparisons is not possible. However, it is easy to fairly compare not completely accurate flow rates from one bench to the other if all testing parameters are the same. This is where the consumer gets into trouble and may not make the right decision when choosing a particular product. Competing products, like performance heads, are usually tested on different flow benches. These benches may not have the same affixing attachment and may be tested with different pressures. These variations cause wildly different and conflicting results that are not at all comparable. Using other supposedly comparative tests with different parameters skew the results, making one product falsely superior or inferior to another. However, if you are testing the products within the same specific parameters, results will be roughly accurate and thus produce valid comparisons.
Flow Bench Facts
Volumetric efficiency (the highest percentage of cylinder fill) will occur just as the intake valve is closing. In almost all cases, published flow bench tests will advertise the best result. Unfortunately, since the next advertised test might show a result using a different valve lift opening, there’s no way to know which head flows the most air. Therefore, it’s imperative to know that all testing parameters are the same or else the test figures are useless.
As for test pressure, flow benches, like SuperFlow, compare two air pressures; one is a calibrated pressure across a known orifice or opening, while the second is a constant test pressure fed through a breather, carburetor, or, in this case, a head. The air is fed in via an air pump much like that of a household vacuum cleaner. This continuous and constant test pressure drops across the opening. Calculating the ratio of the pressure drop relative to the calibrated known pressure determines volumetric flow, which is fancy lingo for the amount of air measured in cfm. Different testing facilities and manufacturers use different test pressures, making comparisons difficult for the consumer.
A column of water called a manometer indicates and validates the test pressure chosen. The height of the water in the manometer column is a visual marker of the invisible test pressure. The suction of the flow bench drawing air through a test item, such as an air filter, also pulls on the water in the manometer’s column. The height of the water reflects the air test pressure. One atmosphere, which is the gravity weight of air at sea level, is equal to a column of manometer water of 403". Of course, using a manometer of this height is awkward. Using between 1-1/2" and 28" of water is typical. Measurement of actual airflow occurs via a second manometer that measures the pressure drop. If there was a standardized manometer test pressure that different manufacturers and technicians could agree on, the test results would be comparable, everything else being equal. It is imperative to maintain the same test pressure, as indicated in the manometer by inches of water, during the entire test procedure or the results will again be useless. This stuff sounds complicated, but its foundation exists in simple high school physics.
If you need to compare flow numbers that have been taken at different pressures, here’s an easy formula to use. Divide the New Flow Pressure by the Known Flow Pressure. Then divide this answer by 2, and then multiply the answer by the cfm given. Written out, the formula is: Flow = (New Flow Pressure / Known Flow Pressure) / 2 x cfm. For example, say the printed flow numbers are 150 cfm @ 10". To convert this to a cfm number taken at 28", the calculations would be:
Flow = (28 / 10) / 2 x 150
Flow = (2.80) / 2 x 150
Flow = 1.675 x 150
Flow = 251.25 cfm @ 28"
Performance Heads
Twin Cams have been using thin stem valves and matching guides since 2005. Traditionally, Harley-Davidson Big Twin valves have a 3/8" valve stem. This has been reduced to 5/16" on the new Big Twins. This is an important advance since the valves are obviously lighter. The valvetrain in a pushrod-operated engine — like all traditional air-cooled Harleys, — are heavy affairs to move up and down at high rpm. Their weight limits their rpm capabilities. Overhead cams, like those used in the V-Rod, actuate the valves directly, which makes them capable of moving at a much higher rpm. Any time we can lighten the valvetrain, the engine will be capable of slightly higher rpm and, therefore, more power. There will also be the added advantage of increased parts longevity. A lighter valvetrain means that the valve spring seat pressure does not need to be as great to return the valves to their seats as the commanding valve lobe rotates to a lower position. This relieves pressure on the valvetrain and allows parts to last longer.
Edelbrock, a world-renowned automotive performance company, lends its expertise to the Harley-Davidson performance market. Edelbrock uses swirl-polished steel valves along with radius valve seat cutting for superior flow characteristics. CNC-ported Performer heads accommodate up to a 0.650" valve lift and are fitted with 175-pound seat pressure valve springs. Edelbrock claims, and I believe, that its heads have high-velocity airflow that results in quick throttle response. All Edelbrock heads necessitate matching the rectangular port on the manifold with the head’s rectangular intake port. This shape promotes laminar flow, and thus a higher velocity airflow. The floor or bottom of the rectangle slows airflow as it goes around port corners, keeping pace with the faster moving upper flow that must traverse more distance. This design smoothes and reduces the turbulence that slows down an air/fuel charge. The dual-quench combustion chamber promotes more turbulence during the compression stroke to assist with atomization for a higher percentage of burn. As for the D-shaped exhaust port, it is an important anti-reversion feature.
According to the S&S catalog, “On the Superflow 600 flow bench our 89cc S&S Super Stock heads flowed 264 cfm @ 25" water compared to 182 cfm @ 25" water for stock heads from a Harley-Davidson Twin Cam 88 engine… S&S heads allow the use of cams with as much as .640" of total lift.” The 88", 95", or 96" engines can operate without compression releases. However, the way the heads are machined allows the use of S&S compression releases, if needed. If you do choose to use the compression releases, you must also use S&S rocker boxes. The 79cc combustion chamber heads yield 9.1 to 10.3:1 compression ratios. The 1.940" intake valves provide higher velocity fuel and air. The 89cc combustion chamber heads do not perform well until the engine displacement reaches 103". These heads will accommodate up to a 4" cylinder bore. Compression ratios with the 89cc heads on a 100" to 116" engine varies between 9.2:1 and 10.4:1. On both the 79cc and 89cc chamber heads, you can increase compression further by planing the gasket surface up to an additional 0.060". Of course, it is crucial to check valve-to-piston interference if you do this.
The H-D Screamin’ Eagle heads are of the same superb construction as the stock TC 88 and TC 96 heads. Screamin’ Eagle has made many different performance head offerings for Twin Cams. In my opinion, they are all good when combined with the appropriate pistons and other complementing performance parts. However, I believe some are no longer in production because of emissions laws. The old disclaimer that these products are for off-road or race use only is receiving challenges from the EPA.
I will describe some of them, but for me to say which is the best is not fair because they will all work well as part of a coordinated team, depending on the specific type of performance desired. All of the head combinations I will now list require at least an ECM calibration if fuel injected when using only prescribed Screamin’ Eagle components. Fuel modifiers are a better alternative, in my opinion, especially if you are not using the exact components specified for the ECM calibration. Carburetor-equipped bikes will also need to have their fuel adjustments modified.
Harley-Davidson entered the realm of big-inch engines with its offering of Screamin’ Eagle Pro Twin Cam 110+ heads (black highlighted #17071-08; silver highlighted #17072-08) for 110" and bigger engines on 2007 and later models. (Most people I know refer to the silver heads as blond.) These heads can be retrofitted back to 2006 and have a 95cc combustion chamber. They also have a 4"-bore fitment compared to a stock TC 96 bore of 3.750", a 0.250" diameter differential. These heads have fitment for automatic compression release (ACR) to offset starting problems with the 10.5:1 compression ratio. The big 2.120" intake valve and a larger 1.623" exhaust valve can open up to a 0.650" valve lift.
Screamin’ Eagle Pro Twin Cam ACR Performance heads are for 110" and larger engines on 2007 and later models (black #17165-08; silver #17166-08). You can also retrofit back to 2006. Combustion chamber size is 95cc with fitment of a 4.060" bore. If compared to a stock TC 96 or TC 88 bore of 3.750", it’s an 0.310" diameter differential. A 4" bore with 4.375" (4-3/8") stroke equals 110"; increasing the bore to 4.060" with the same stroke will equal 113". This stroke of 4.375" is the same as that for the TC 96 while the TC 88 has a 4" stroke. These heads have fitment for automatic compression release to offset starting problems with the 10.5:1 compression ratio. The 2.080" intake valve and a larger 1.630" exhaust valve can open up to a 0.650" valve lift.
Screamin’ Eagle Pro Twin Cam MCR Performance heads (black #16952-08; silver #16953-08) are for 3.875"-bore (3-7/8") engines. MCR is the acronym for mechanical compression release (#32076-04). Mechanical releases are operated by the rider while automatic compression releases operate automatically. Combustion chamber size is 76cc with a 10.5:1 compression ratio. These heads will accept up to 0.575" valve lift cams and can rev to 6500 rpm. Personally, I think taking a Harley-Davidson pushrod-operated engine to 6500 rpm is flirting with danger. Some Screamin’ Eagle heads have a rpm potential of 7000, but I am never taking my bike to these levels. The powerband will peak before these rpm levels anyway. Fitment is for 1999-2005 Twin Cams that require an additional purchase of two intake flanges (#26993-06) and for 2006 to present models that can use their stock intake seal flanges.
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