Harley Techline July 2008 - Part 1

[Buzz Kanter]

IT’S ALL ABOUT TORQUE
Part III: Cams and the valvetrain
by Donny Petersen

I’m going to describe the actions and functions of the valvetrain in order to explain some background information about how the demands of the elliptical-shaped cam lobes transfer to their respective valves.

The Valvetrain
The lifter’s roller, which rolls over the egg-shaped cam lobe, spins on an axle located on the lowest part of the solid or hydraulic lifter (also known as a tappet). Solid lifters are just that, a solid connection between the cam lobe and the pushrod it actuates. However, a hydraulic lifter uses pressurized engine oil to pump itself up, so it can reduce lash (gaps) between mating parts. This increases engine efficiency and reduces engine noise. Gaps are created between parts as the engine heats up and expands or cools and contracts.

The balled lower end of the pushrod sits in a mating seat on the top of the hydraulic lifter. The pushrod extends from the top of the gearcase, up into the bottom of the overhanging section of the head, which is on the right side. The pushrod is encased in an chrome pushrod housing (tube) to keep dirt out and lubricating oil in. Stock Twin Cam pushrods are a fixed length, which means you have to remove the top of the rocker box and some of the rocker assembly to remove them. Aftermarket performance pushrods, on the other hand, are adjustable for ease of installation. These adjustable pushrods also accommodate higher lift, performance camshafts. (Interestingly -- and for reasons I cannot fathom, except as a cost saving measure -- the H-D CVO 110” engines use stock Twin Cam fixed-length pushrods. I discuss this at length in the second book in my series Donny’s Unauthorized Technical Guide to Harley-Davidson 1936-2008.) The balled upper end of the pushrod fits into a mating pocket in one end of its rocker arm, which swivels up and down on a rocker arm shaft for a distance equal to the cam lobe lift.

One main mechanical reason for using a rocker arm is to connect the pushrod, which is located on the right side of the engine, to its valve on the left side of the head. The approximately 3-5/8”-long rocker arm transfers the demands of the rotating cam to the top portion of the valve stem, which is encased in a single beehive valve spring on 2004 and later Twin Cams or dual cylindrical springs on the 2003 and earlier engines. The end of the stock rocker arm that rests on the valve stem has a pad machined into it. This pad rubs across and down against the top of the angled valve stem, which compresses the valve spring and forces the valve to open. The function of a valve spring is to pull the valve back onto its valve seat once the elliptical cam lobe rotates past its highest point and allows the lifter and pushrod to drop back down to the starting position.

Rockers & Valve Lift
Now that you understand how the valvetrain works, I can explain why the cam lift is less than the valve lift. If the rocker arm ratio is 1:1, the arm of the rocker arm that cups over the upper end of the pushrod is on the same plane as the rocker arm that actuates the valve. In this instance, cam lift and valve lift are equal. This is the case with 1936-47 Knuckleheads. However, on every overhead valve Big Twin after the Knuckes, starting with the 1948 Panhead and going to present day, one side of the rocker arm is offset to the other. This offset increases the rocker arm ratio to a value higher than 1:1, which increases the amount of valve lift for a given amount of cam lift. The stock Twin Cam’s rocker arm ratio is 1.625:1. Therefore, the cam lift multiplies by 1.625 to achieve the effective valve lift (opening).

Performance rocker arms have a roller on the end of the arm that actuates the valve instead of a metal pad. These rockers may also have altered rocker arm ratios, such as an Ultima roller rocker arm, which has an increase of 0.050” that changes the rocker arm ratio to 1.675 . Another example is a Crane roller rocker arm with a ratio of 1.750, which is an increase of 0.125” over the stock ratio. I use higher-ratio rocker arms to effectively increase lift when a cam has too long of a duration (for my purposes). Increasing a stock cam’s valve lift by using increased-ratio rocker arms is one way of lowering the powerband into more useable territory and gaining extra low-end torque. However, it’s important that the valve springs can handle the increased valve lift without problems. Possible problems are the valve springs coil-binding, the upper valve collar hitting the top of the valve guide, or the valve extending too far into the combustion chamber and hitting the piston’s crown.

As I have said before, I feel many production cams favor touring over city riding. They do this by having longer duration figures relative to lift. In truth, I guess it would be fair to say there are compromise camshafts. But performance is not about compromise. Actually, the last statement is not a fair one either since compromise is part of any decision-making process to coordinate parts for the best, safe, performance gains. To clarify, once the performance mechanic has decided on the type of riding the engine is for, he will choose a specific cam for that purpose, not a compromise one.

Lobe Centerline Angle
The lobe centerline angle (LCA), also known in some tech manuals as lobe centerline (LC), determines two important valve-timing characteristics. First, the LCA dictates the valve overlap around top dead center (TDC). Second, the LCA establishes the amount of intake or exhaust valve closure delay there is beyond the theoretical end of the applicable stroke. High-torque cams have lobe centerlines around 100 degrees. For example, my current Knight Prowler TW408G cams have an intake at 100 degrees centerline and exhaust LCA of 102 degrees.

LCA is not the same as lobe separation angle (LSA). The two have very similar names, but they control different events in the engine. However, LCA and LSA are directly connected. The highest tip on the elliptically shaped cam lobe, also known as the point of maximum lift, is the LCA. Therefore, on a Twin Cam, a two-cylinder engine with four valves actuated by four mating cam lobes, there are four LCAs. In a Harley-Davidson, the front and rear cylinders use the same cam lobe centerlines, so there is one intake cam LCA figure and one exhaust cam LCA.

LSA is simply what it says: the number of camshaft degrees separating the peak lift points, which are the exhaust lobe centerline and the intake lobe centerline. The peak points are also known as the lobe centers of the cam intake and exhaust lobes. Remember that camshaft rotation is based on 360 degrees to complete a full cycle while crankshaft rotation is based on two revolutions of the engine, or 720 degrees of rotation, to complete its cycle.

A major difference between an LCA and an LSA is that a mechanic can adjust LCAs to advance or retard cam timing. However, an LSA is non-adjustable because it’s ground into the cam lobe profiles, fixing the intake lobe and exhaust cam lobe in place. A mechanic cannot degree the cam in the engine to change LSA. I will expound on this subject a little later on.

The intake centerline, on the other hand, is the position of the centerline or peak lift point of the intake lobe in relation to TDC. Degreeing the intake centerline of the cam into the engine, fore or back of the stock centerline position, will change timing to the specific needs of the engine function.

So, how do we figure out lobe centerline angles? Well, now life gets interesting, and I get to show off just how easily Bob Wood simplified the calculations for me when I queried him about the LCA and LSA figures for the BW-408 Knight Prowler advanced silent belt drive system cams in my bike. We will use Bob’s bang-on simplistic calculation methodology, and frame it beside the method mathematicians use. You do not need a calculator to do LCAs and LSAs the Wood Performance way. As Bob succinctly wrote, “This is extremely simple. This will make you a cam timing wizard!” Simply look at the intake duration, which is 248 degrees. Divide that figure by 2 (248 degrees/2 = 124 degrees). Then subtract the intake opening of 24 degrees for the intake LCA (124 degrees – 24 degrees = 100 degrees of Intake LCA). Now take the exhaust duration, which is 248 degrees and also divide this by 2 (248 degrees/2 = 124 degrees). Finally, subtract the exhaust closing measurement of 22 degrees (124 degrees – 22 degrees = 102 degrees of exhaust LCA). Now that we know the lobe centerline angles, we are able to do the LSA calculations. An intake LCA of 100 degrees and exhaust LCA of 102 degrees should be added together (100 degrees of intake LCA + 102 degrees of exhaust LCA = 202 degrees). Then divide the answer by 2 (202 degrees/2), which gives you 101 degrees of LSA.

To calculate LCA) let’s use Bob’s BW-408B cam specifications, which are show in Chart I. To get the intake LCA, use this formula: Intake Duration divided by 2 minus Intake Opening equals Intake LCA. Using BW-408B specs, the equation would look like this: 2480/2 = 1240; 1240 – 240 = 1000 Intake LCA. As for the exhaust LCA: Exhaust Duration divided by 2 minus Exhaust Closing equals Exhaust LCA. The equation is: 2480/2 = 1240; 1240 – 220 = 1020 Exhaust LCA.

Another similarity of measurement and expression of LCA and LSA is that both are counted in degrees of camshaft rotation. Generally, LSAs are between 100 and 114 degrees. However, an LSA is non-adjustable since it’s ground into the cam lobe profiles fixing the intake lobe and exhaust cam lobe in place.

(More to follow in Parts 2 and 3)