Techline-August09-PERFORMANCE OBSTRUCTIONS - Part II: Heads & exhaust systems (cont.)

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This article concludes in the next thread.

The intake flanges for Twin Cam heads changed circa 2006. The Screamin’ Eagle Pro Twin Cam 103+ heads (black #17071-03B; silver #17072-03B) allow use of the new flanges. Therefore, they will accept the 50mm electronic fuel injection induction module, which is bigger than any stock unit, including the CVO 110s, by at least 4mm (depending on year). The combustion chamber is 98cc with a big 2.080" intake valve and a 1.630" exhaust valve. These heads will allow a valve lift of up to 0.650". Head design is for 103"-plus inches on 3.875"-bore cylinders or larger. This bore is 3-7/8" for both the 95" version of the TC 88 and the 103" version of the TC 96. In my opinion, these heads are too big for the 95" engine. The flywheel assembly needs to be upgraded to 4-3/8" stroke to achieve 103" with the 3.875" bore. Another way is to increase the bore yet again to 4" or 4.060" to achieve the cubic inches that these heads need. These heads approximate a hemi head design as opposed to a bathtub one. However, there is still a squish band, which may elude an unpracticed eye. Piston valve pocket cutouts interrupt the hemi/squish band that looks like the beginning of a piston dome that reverses itself in the piston dome center. This squish is an angle at the beginning of the combustion chamber, and there’s a similar angularity on the piston dome. I suspect this design combats valve shrouding whereby the valve gets in the way of incoming and exiting gases. The reverse dome lowers the compression ratio to 10.5:1. The heads have a 7000-rpm capability. The head machining accepts compression releases (#32076-04). Fitment is for 1999 and later Twin Cams with 3-7/8" big-bore TC 95 or TC 103 kits; 1999-2005 Twin Cams require intake flanges (#26993-06).

Screamin’ Eagle Pro Twin Cam Ported HTCC heads are currently the highest flowing head from the Motor Company. Therefore, this is one of my favorite head choices. They’re on my 2007 Street Glide with the 50mm EFI induction module and late-style intake mounting flanges. HTCC is the acronym for high torque combustion chamber. The piston and head design use the hemi squish band. I would rather have the bathtub head design (black #16933-99B; silver #16926-99B) with the 95"-style HTCC pistons (#22439-00A), but my mama says that I cannot have everything in life. These 10.5:1 compression heads have an 84cc combustion chamber with a 2" intake valve and a mating 1.623" exhaust valve. These heads accept up to 0.650" valve lift cams and come in black (#16925-02B), or silver (#16934-02B). Their design is for 103"-plus inches on a 3.875" minimum bore cylinder. The heads have a 7000-rpm capability. The head machining accepts compression releases (#32076-04). Use the Screamin’ Eagle manifold (#29635-99) in conjunction with these heads. The fitment is for 1999 and later Twin Cams with 3-7/8" big-bore TC 95 or TC 103 kits; 1999-2005 Twin Cams require intake flanges (#26993-06).

Screamin’ Eagle Pro Twin Cam HTCC heads have a 10.5:1 compression ratio with an 84cc combustion chamber. A 2" intake valve, which accentuates low-rpm torque, has a mating 1.550" exhaust valve and accepts up to 0.575" valve lift cams. These heads come in black (#16933-99B) or silver (#16926-99B). They are designed for 95" on a 3.875" minimum bore. These are my favorite heads matched with HTCC pistons (#22439-00A). The heads have a 6500-rpm capability, and the head’s machining accepts compression releases (#32076-04). Use the Screamin’ Eagle HTCC manifold (#29634-00) in conjunction with these heads for correct head port matching. The 95" HTCC heads have a hemi-multilayer squish band arrangement. The intake port is a D shape as opposed to round, thereby promoting a high-velocity fuel charge. The intake port has a step in the bowl area to minimize intake reversion while the intake valve is still open when the piston begins ascension on the compression stroke. The exhaust port has an oval shape to assist in exhaust gas scavenging with a smaller exhaust valve that takes advantage of Bernoulli’s Principle for velocity of exiting gases. The fitment is for 1999-2006 carbureted models plus 2001-2005 EFI Softail, 2002-05 FLT models, and 2004-05 EFI Dyna models with 1550cc big-bore kits.

Screamin’ Eagle Pro Twin Cam Performance heads (black #16952-99C; silver #16953-99C) are for 1999-2005 EFI TC 88s and 1999-2006 carbureted models, and 2006 to present TC 88 and TC 96 EFI Twin Cams (black #16952-06, silver #16953-06). These heads utilize Evolution-style port shapes with Twin Cam 72cc bathtub combustion chambers. The heads accept up to 0.575" valve lift cams. Therefore, they use a TC 88 or TC 96 flattop piston. Do not use the higher-compression cast pistons that normally increase TC 88 compression to 10:1. A compression ratio of 9.5:1 on a TC 88, or 10:1 on a TC 95, or high compression forged pistons will bring both configurations to 10.5:1. The fitment is for 1999-2006 carbureted models (black #16952-99C; silver #16953-99C) plus 2001-05 EFI models. The 1999-2005 Twin Cams require intake flanges (#26993-06). The fitment (#black 16952-06; silver #16953-06) is for 2006 and later EFI models. The head machining accepts compression releases (#32076-04).

The accompanying chart gives you a full rundown of each head’s specs. What is not listed is that the compression ratio is 9.5:1 on a TC 88 and 10.0:1 on a TC 95, although high-compression pistons will bring both configurations to 10.5:1. The compression ratio for 79cc heads is 9:1 on a TC 88, 9.6:1 to 10.3:1 (depending on piston) on the TC 95, 10.1:1 on a 96" and 100", 10.4:1 on a 106", 10.7:1 on a 107", and 11.5:1 on a 116". The compression ratio for 89cc heads is 9:1 on a TC 88, 8.3:1 to 8.7:1 on a TC 95, and 9.2:1 on a 96-incher. Therefore, we see that the 89cc heads are not appropriate for these smaller engines. The compression ratio for 89cc heads on a 100" engine is 9.2:1, 9.8 on a 106", 9.7:1 on a 107", and 10.4:1 on a 116-incher. With Edelbrock heads, the compression ratio depends on the piston used with a 10.25:1 ratio utilized as a power package.

Exhaust Systems
Many exhaust pipes manufactured today are too skinny, too fat, too long, or too whatever to produce good gains in horsepower. There is even the odd exhaust system touted for performance gains that will cause a decrease in torque and horsepower compared to the restrictive stock ones. There is no single production exhaust that I or anyone else can say is the best overall. Many exhaust pipes are the best in certain specific areas. Some pipes are versatile because they are tunable.
However, we can say that there is a best production exhaust system for a specific engine and weight-to-power ratio, based on the total weight of the bike and rider compared to engine performance in a particular riding style. In general, I can probably narrow the search quite effectively to a 2-into-1 collector system with a bigger muffler and restrictor pipe. Systems designed purely for looks or sound are suspect in the performance world.
In most instances, noise does not equate with power. Actually, the only situation that comes to mind where extreme noise and mega-power coexist is with top-fuel Harley dragsters. There will be some noise with excellent power gains, but not always as much as one might suspect. Some exhaust systems are light years ahead of others because their design adheres to principles of sound physics. It has been common knowledge for almost 100 years that muffler volume should be at least 10 times that of the cylinder that it serves, with a restrictor pipe the same size as the header pipe exiting the muffler. This will quiet the exhaust noise with no loss of power. Skinny mufflers had better have some happening new technology going on if they want to compete with larger expansion chambers.
Of course, today we know that we can increase power with correct header pipe diameter size, length, and muffling. However, style traditionally interferes with performance. Some manufacturers today will achieve a stylish fat appearance by building a cosmetic shell around a narrower performance header pipe. The header pipes join to the heads and continue down to the muffler(s). For some inexplicable reason, fatter is prettier on a Harley-Davidson. Perhaps it’s another rebellious trait that draws so many to the freedom machine. This type of design, where cosmetics shroud hard-working efficiency, combines form and function in true wonder. Technology and design improvements abound.
The Power Chamber is an innovation for Harley-Davidson aftermarket exhausts brought to us by Vance & Hines in 2001. The Power Chamber connects both staggered dual mufflers internally to increase volume to improve the scavenging of each cylinder for more efficient combustion. This system develops more torque and thus horsepower while keeping a formerly less efficient exhaust design. This cool originality delivers good looks and power at the same time.
It’s strange that we like a fat header pipe that, in most cases, will reduce power, and yet we hate a fat muffler that usually increases power. Modern exhaust designers are dealing with this ugly issue by making the collectors sleek and stylish as opposed to the tin can look that you see on Japanese racing bikes. Check out the exhausts used on some Buells; they’re all function and power production. They may also hide under the bike for aesthetic reasons, as well as centralizing weight. Buell exhausts are state of the art. They meet all performance, noise, and emissions requirements. However, while they may be good looking to a sportbike rider, they sure fail with the average H-D rider.
Screamin’ Eagle exhaust slip-on mufflers maintain the stock headers and emissions fittings like the crossover pipe, and O2 sensors and bungs, as do some other aftermarket slip-ons. In my opinion, the older Screamin’ Eagle mufflers were discontinued a few years ago because the off-road disclaimer began to wear a little thin with the EPA. In fairness, I think the EPA and CARB will go after the major manufacturers first before they look at dealers, aftermarket shops, and, finally, the consumer. I also think H-D, realizing future constraints, voluntarily developed a new performance exhaust line in advance of enforcement, which complies with existing laws. This is both intelligent and progressive.
The new Screamin’ Eagle exhausts meet mandated decibel (dB-A) noise ratings and current emissions controls. The new slip-on mufflers are for specific models. They have never been interchangeable primarily because of muffler bracketing. Pleasant muffler sound is accomplished by changing the diameter of the muffler itself with differing types of baffles that filter out higher frequencies. The TC 96 Softail, Dyna, and Sportster mufflers are 3.5" in diameter, while FLT mufflers are either 3.5" or 4" in diameter. It is no mistake that stock FLT baggers make more power than other stock model Twin Cams, since their mufflers have more internal volume. The EPA is not destroying muffler performance functionality so much as causing R&D investments to control mutually harmful emissions, noise pollution, and, thus, push the development of superior performance exhausts.
Harley-Davidson mufflers will give a few extra ft-lbs. of torque and perhaps
4-5 hp extra without ECM recalibration. Adding its air filter, which helps complete the breathing equation, will almost double the muffler performance increases. The Stage I recalibration may or may not be necessary since many TC 96 Twin Cams already come with a slightly rich AFR from the factory. If not doing the recalibration, make sure you check that the AFR is not too lean via intermediate and full throttle spark plug color readings.
Python’s staggered duals have always been excellent for performance when compared to the two separate pipes also known as staggered duals. Python is the first production exhaust that I’m aware of that incorporated torque cones, also known as torque valves, and more properly known as anti-reversion valves or cones, in the top of the header pipes. Anti-reversion is descriptive in that these reverse-shape cones deflect returning kinetic wave pulses out of the exhaust instead of allowing them to reenter the combustion chamber to pollute the virgin charge of air and fuel. The word torque valve also describes these cones, which increase engine torque. Moreover, Python staggered duals came in different performance versions for stock or modified engines and for bigger displacement engines. The newer Python staggered duals have standard 1-3/4" headers feeding into 2-1/8" muffler bodies with a hidden crossover pipe that increases performance throughout the rpm range. They are now also available with O2 sensor bungs. The baffles are removable, or you can order quieter baffles that coincidentally increase low-end torque. A 2-into-1 exhaust system will produce the most performance gains. The Python 2-into-1 exhaust has a big 3-1/2" muffler and is available with O2 sensors. Riders may not like the look of big mufflers, but they should remember that the muffler should have 10 times the volume of the cylinder(s) it serves. Once in possession of this little-known fact, the reader can immediately begin eliminating many exhaust systems from his performance lists.
One of the many great design changes in the Twin Cam 88 engine is the cast-in anti-reversion step in the head where the exhaust port meets the exhaust pipe. In fairness, the credit should go to Edelbrock, followed by Patrick Racing. They were the first, to my knowledge, to use this anti-reversion design. They originally did so on the exhaust ports of their Evo aftermarket heads. S&S Cycles and Ultima also incorporate anti-reversion steps on all their heads, many of them designed well before the advent of the Twin Cam. The exhaust port exit in stock Shovel and Evo heads is normally round, as it now is on TC 96 heads. The modified shape of the exit port found in stock TC 88s, and the aftermarket Evo and Twin Cam heads, assists the exhaust valve in reflecting returning exhaust pulses.
Thunderheader is a division of Rich Products. One only has to see the big collector or mufflers to realize that they can produce some of the best torque and horsepower results given certain engines and combinations of support high-performance parts. Thunderheader, with its unusual sound, has become a street-performance enthusiast favorite. Its distinct look and length oozes performance.

This article concludes in the next thread. Check back issue for pix and extra information.