The article concludes in this thread.
Timing, duration, and overlap are measured in degrees of flywheel rotation. The flywheel assembly (crank) needs two complete revolutions, totaling 720 degrees, to complete the four basic 180-degree cycles of a four-cycle engine: intake, compression, power, and exhaust. The camshaft rotates at half the speed (time) of the crank.
Valve lift (or simply lift) and cam lift are measured in thousandths of an inch. In practice, the specification measurement of cam timing, lift, and duration begin at 0.053" off the baseline heel of the cam lobe. Purists feel that the effect of this will compromise true overlap figures and analysis. The rocker arm ratio in a stock Twin Cam is 1.625. Stroked and/or big-bore engines with more cubic inches can absorb more duration. The powerband will also lower on a bigger cube engine using longer duration cams. We want more compression in tandem with a longer duration cam in big-inch engines. Though these are generalizations, the factors mentioned are proportional and interrelated.
If this were my bike, I would have put in much larger cams, but Chris is a high-miler. I’m being conservative with his engine because of his bad experiences. Make no mistake though; after I revamp his engine, it will be much faster than a stock CVO 110 — reliably fast. The pistons we put in will increase the compression marginally from 9.3:1 to 9.65:1.
Interestingly, the H-D CVO 110" engines use stock Twin Cam fixed-length pushrods. One would think that Harley-Davidson would incorporate its Screamin’ Eagle adjustable pushrods into its Screamin’ Eagle engine. We installed the appropriate Screamin’ Eagle pushrods and pushrod tube kit (#17997-99A) into Chris’ engine. There was an improper installation of one of the middle pushrod O-rings during the previous attempt to repair this engine. The O-ring did not seat properly, thus oil leaked down and blew onto the head/cylinder fins and dripped off the back. Therefore, Chris had two oil leaks baking onto the CVO 110 cylinder fins and messing with the components underneath.
This Screamin’ Eagle engine also did not come with Screamin’ Eagle roller rocker arms. The stock Twin Cam rubbing rocker arms with pads were used. Performance 101 dictates the use of roller rocker arms with performance cams, both for preventing frictional loss of horsepower and for valvetrain longevity. The major mechanical reason for using a rocker arm design is that it connects the pushrod on the right side of the head to its valve located on the left side of the engine.
The upper end of the pushrod, which has a ball on it, fits into a mating pocket in the right end of the rocker arm. The rocker arms swivel up and down a distance equal to the cam lobe lift on a rocker arm shaft. The approximately 3-5/8"-long rocker arm transfers the demands of the rotating cam lobe to the top portion of the valve stem, which is encased in the single beehive valve spring on 2004 and later TC 88s and TC 96s. (There are dual cylindrical springs on 2003 and earlier TC 88s, which is what was used on all earlier overhead valve Harleys.) The stock rocker arm pad, which is on the opposite end of the rocker in relation to the pushrods, sits over the valve stem. This pad rubs along and down on the top of the angled valve stem, forcing it to open when the cam lobe tells it to. This obviously creates friction on top of the stem and on the sides of the pad as it pushes the valve stem against the angled valve guide. This causes a frictional loss of horsepower and wear issues, as you would expect. A roller rocker rolls effortlessly over the top of the valve stem as it pushes the valve open with minimal side force against the valve guide. The function of a valve spring is to set the valve back on its valve seat after the elliptical cam lobe rotates past its highest point (TDC). The springs push the valve closed as the valvetrain follows the descending cam lobe.
Now that this is explained, I can talk about why cam lift is less than the valve lift. If the extension of the rocker arm that cups over the upper balled pushrod is on the same plane as the rocker arm extension on the other side that actuates the valve, then cam lift and valve lift are the same figure and at a 1:1 ratio. This is the case with the 1936-47 Knuckleheads.
However, on every Big Twin from 1948 to present, one side of the rocker arm is offset to the other side, so that a multiplication of the cam lift transfers to the valve stem, thus increasing valve-opening lift. The stock Twin Cam’s rocker arm ratio is 1.625:1. Since we had to replace the stock rocker arms with roller ones, I took the opportunity to use a set with a higher rocker arm ratio (1.675) to increase the valve lift (torque), relative to cam duration (horsepower). Torque gets the beast going and horsepower keeps her going.
Compression & Cam Duration
A stroked and/or big-bore engine with a large-cubic-inch displacement can absorb more cam duration. Cylinder pressure affects power. More cylinder pressure will usually increase horsepower. The three main factors that determine cylinder pressure are compression ratio as it is modified by camshaft duration, and altitude. Higher altitudes have a lower atmospheric pressure because there is not as much air up there as there is at sea level.
Therefore, less air results in less compression. As a general rule of thumb, the higher the altitude, the less camshaft duration is necessary. This does not apply to riders that only visit a higher altitude, as cam duration reduction is not possible without replacing the camshaft. Less duration is necessary when living and riding at higher altitudes most of the time.
Increasing cylinder pressure is determined by more compression and less cam duration. Conversely, decreased cylinder pressure is the result of less compression and more cam duration. Therefore, in performance applications more cam duration is necessary to bring in more air to burn more fuel, which creates more pressure to drive the piston down on the power stroke with increased force to turn the rear wheel, with the following caveat: more cam duration will necessitate a higher compression ratio to maintain a higher cylinder pressure to maintain increased power.
Conclusion
I’m going to leave you thinking about compression and duration for the next few weeks. We’ll pick up in the same spot next month.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Techline-Jan.09-H-D 110" CVO -Part I: The head gasket oil leak,etc. (conclusion) 
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