TECHLINE Part 3
Degreeing Cams
The mechanic installing the cams can move or adjust LCAs by rotating the cam gear a few degrees in either direction. The positioning of the gear attached to the camshaft will affect cam timing. Some cams, notably 1999-2006 Twin Cam cams (excepting the 2006 Dyna series) have their cam sprockets cast onto the camshafts so that they are non-adjustable -- well, sort of. We can adjust cam timing via an advanced rear cam-drive gear. Although the 2006 and all 2007 to present Twin Cam cams have pressed on cam gears, it is much easier to change the rear-cam drive gear timing when using stock factory camshafts. Another way is to position the cam forward or backward of its timing mark indent with its mating drive sprocket. We can adjust cam lobe centerlines by advancing or retarding cam timing. First, the mechanic uses a degree wheel to ensure the cam installs at the manufacturer’s suggested intake LCA, which is the maximum point of lift of the intake lobe. For example, if the cam manufacturer suggests the cam-intake LCA is 100 degrees, the degree wheel will ensure the intake lobe’s highest point is set at 100 degrees. This is the stock setting. The mechanic can advance cam timing by 4 degrees by lowering the install point below 100, or he can retard the cam timing by increasing the install point a few degrees above 100. How many degrees becomes the question. However, there are usually up to four degrees of variation either way. Advancing or retarding cam timing will shift the powerband up or down based on the particular needs of a bike or riding style.
Although I said earlier that an LSA is non-adjustable because it’s ground into the cam lobe profiles, nevertheless there is a way to do adjustments at the manufacturing level. The manufacturer may grind advanced timing or retarded timing into the intake cam lobe as part of the cam profile to raise or lower the powerband a little. The mechanic cannot reverse what the manufacturer does with LSAs whether stock, advanced, or retarded. However, we can adjust LCA equally by up to 4 degrees either way by advancing or retarding cam gear timing during installation. As usual, the aftermarket comes up with innovative performance options.
Bob Wood’s Knight Prowler optional gear drive setups are available with 2- and 4-degree advance and retard keys. The Wood Performance 2006 Dyna and all 2007 to present Twin Cam cam gear (#216020+4) advances both cams 4-crankshaft rotational degrees. The #288015+4 gear fits all Twin Cams from 1999 to 2006, except the 2006 Dyna. Either cam gear fits on any stock or performance camshaft. The timing advance gives both higher cranking compression and running compression resulting in more torque and horsepower in the lower rpm range. Furthermore, it prevents lost cam motion or cam wander in the loose roller chains, which results in retarded cam timing on chain-driven cams.
Intake Cam Timing Change
Calculating intake lobe advanced or retarded timing occurs during manufacturing. Let’s assume there ’s an 110 degree LSA and an intake LCA of 112 degrees. The formula is Cam Lobe Separation Angle minus Intake Cam Lobe Centerline equals Advanced/Retarded Cam Timing, or LSA – ILC = Timing Change. When we plug in our numbers we get 110 – 112 = 20 degrees retarded timing. If we assume there’'s an 114 degree LSA and an intake LCA of 110 degrees the formula LSA – ILC = Timing Change becomes 114 – 110 = 4 degrees advanced timing.
Once again, let’s look at the Knight Prowler silent belt system cam specifications, which have an LSA of 101 and an intake LCA of 100 degrees. Using the formula LSA – ILC = Timing Change we get 101 - 100 = 1 degree advanced timing.
Displacement & Compression Ratios
Compression ratios affect cam choice, especially duration specifications, more than the size of the engine although displacement is also a factor. Higher compression and long durations work well together. Longer duration cams do not do well in a low compression engine. This is why many cam manufacturers will specify compression ratio ranges suitable for their long-duration cam profiles. Conversely, long duration cams will allow easy starting of high compression engines because these cams hold the valves open longer, allowing some pressure in the cylinders to escape. Thus, the real life corrected compression ratios are lower than the static compression ratios, which only compare the volumes of the cylinder and combustion chamber with the combustion chamber volume. The formula is: (Cylinder Volume + Combustion Chamber Volume) ÷ Combustion Chamber Volume = Static Compression Ratio
Compression has many names and meanings. There are many ways of interpreting compression measurements, and there is even more terminology describing the principle. The static compression ratio compares the volume of the cylinder and combustion chamber to the volume of the combustion chamber alone. Suppose we have an 840cc cylinder and an 85cc combustion chamber. To calculate the static compression ratio we add 840 and 85 to get a total volume of 925. We then divide 925 by the volume of the combustion chamber alone, which is 85. The result is a static compression ratio of 10.9:1. Static compression ratio is also known as the theoretical or mechanical compression ratio, as well as advertised or nominal compression ratios.
The corrected or effective compression ratio is more accurate for a real-life engine. It recognizes that cylinder volume is lost with open valves. The math to calculate this is complicated, requiring a computer program. JIMS, as well as other motorcycle distributors, has a computer program known as Accelerator for this and many other calculations.
Then we have cranking compression, which measures actual compression in psi with a tool called a compression gauge. The technician screws in or presses a hose connecting to a gauge into the spark plug hole and cranks the engine over to get a dry cranking compression reading in a psi. Wet cranking compression is usually higher than dry testing because the technician squirts oil into the cylinder beforehand so the rings seal better. Running compression is actual, real-time compression under riding conditions where there is heat expansion and rings sealing against cylinder walls with oil lubrication. Higher altitude lowers corrected compression ratios because there is progressively less air with rising elevation. Altitude will not affect static compression ratios, as it is just a comparison of volumes in the cylinder and combustion chamber relative to each other.
To counter the loss of effective compression from longer cam durations and overlap, thinking mechanics will increase static compression ratios.The accompanying charts (III and IV) show generalized effects that cam profiles can have on engine operation. As you can see, certain traits can cause a positive effect in one scenario but a negative one in another. It all depends on the type of engine being built. For example, a tight LCA increases torque, but the idle is rougher and there’s the danger of exhaust reversion coming back into the combustion chamber and diluting the fresh air/fuel charge. A wider LCA causes the intake valve to stay open longer after BDC of the intake stroke. A stock engine receives up to an approximate 10-percent extra fill after intake BDC while performance engines receive more fill, possibly more than 20-percent extra. Greater increases are negated by the now rising piston on its compression stroke. Too wide an LCA produces the possibility of intake reversion where the intake air/fuel charge may be forced back up and out of the intake tract, reducing potential power, specifically torque.
Conclusion
In conclusion, LSA changes are not possible unless it is ground into the intake cam-lobe profile during the manufacturing process. However, you can advance or retard the cam timing in relation to the crank when degreeing in the lobe centerline during cam installation.
As shown in Chart II, advancing the cam to close the intake valve sooner, improves low rpm performance (torque) by reducing cylinder pressure losses. This improves the corrected compression ratio, which suffers by cylinder volume loss through open valves. Retarding cam timing delays intake valve closure and improves high rpm performance. Scavenging, sometimes called ram effect, increases cylinder fill at higher rpms and with the intake valve opening longer. The intake charge air/gas stacks up in the cylinder as fast, exiting exhaust gases draw intake fuel and air into the cylinder.
Next month we’ll continue with cam chrateristics and how they affect engine performance.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Harley Techline July 2008 - Part 3 
Linear Mode
