Harley Techline July 2008 - Part 2

[Buzz Kanter]

TECHLINE (part 2)

Valve Closing, Overlap, & LCA
As I said earlier, and it bears repeating, the LCA affects valve overlap around TDC. In addition, the LCA establishes the amount of intake or exhaust valve closure delay beyond the theoretical end of the relevant stroke.

As the piston descends to the bottom of the cylinder, but before it reaches the bottom, the exhaust valve begins to open. Most of the fuel and oxygen in the combustion chamber has been burned. Cylinder pressure will begin to push these burnt exhaust gases out into the exhaust port, exhaust headers, and into the mufflers. After the piston passes bottom dead center (BDC), it begins ascending the cylinder on the exhaust stroke. This upward action forces out the remainder of the spent exhaust gases to make room in the cylinder and combustion chamber for the fresh, clean charge of air and gas. As the piston moves toward the top of the cylinder, the already opening exhaust valve goes through maximum lift and begins to close. Just before the piston reaches TDC, the intake valve begins to open but the exhaust valve is not yet fully closed. Theoretically, this should not work unless other forces of physics assist, like the Principle of Momentum. The piston’s exhaust stroke is removing just about all of the spent charge by the time the piston approaches TDC. However, just before this, the intake valve begins to open slowly and scavenging begins.

Scavenging is how the engine flushes out all of the spent air/fuel charge. The remaining exhaust gases rushing out into the exhaust will draw in the start of the fresh intake charge. Conversely, the developing momentum of the intake charge also assists in pushing out the remaining exhaust gases. It becomes a symbiotic affair, first with exhaust pulling and then the intake pushing. Again, some of the new intake charge gases may escape into the exhaust. Thus, the valve opening and closing timing is crucial.

Finding the sweet spot where pollutants fully exit, but the beginning of the fresh intake charge is not lost, is the science, and many times the art, of valve timing. Once the piston passes through the exhaust stroke, ending theoretically at TDC, and starts back down on the intake stroke, the intake charge is pulled in quickly. Therefore, the exhaust valve must close at precisely the right point after the TDC to prevent any burnt gases from reentering. Overlap is this area around TDC where both intake and exhaust valves are open. This is one of the most critical moments in the engine’s 720-degree rotation cycle. Intake and exhaust valve opening and closing must be positioned correctly with the TDC of the piston. To complicate matters, the function requirements of the motorcycle changes this positioning depending on the different types of riding styles, needs, and other factors. As the engine passes through the overlap phase, the intake valve is opening very quickly, while the exhaust valve closes just after the piston starts descending.

As the reader knows, the intake stroke is where the engine breathes in yet another charge of virgin air/gas mixture. The intake valve reaches its maximum lift at some planned point, which is in and around 105 degrees of crankshaft rotation after TDC. This degree measurement is the intake LC or LCA. This refers to the position that the cam was installed at, also known as degreeing, in the engine in relation to crankshaft rotation. All moving engine components time off the position of the crankshaft. LCA determines valve overlap around piston TDC and has much influence on the amount of intake or exhaust valve closure delay beyond the theoretical end of the relevant stroke. Furthermore, tighter or closer LCAs produce earlier intake valve closure after BDC. A high-torque, low rpm cam needs a tighter LCAs. Wider or further apart LCAs extend the intake valve closure beyond BDC. This will move the peak torque output of the powerband through the rpm range.

Cylinder Pressure & Intake Valve Closing
The piston descends all the way to the BDC. As the piston begins its ascension on the compression stroke, the intake valve is fast returning to its valve seat. The closing point of the intake valve will determine where the cylinder actually begins to build pressure. However, it is very important to have the intake valve open long enough to fill the chamber. It is equally important that it close soon enough to yield maximum cylinder pressure. This is a tricky valve-timing point in the camshaft cycle. Compression builds as the ascending piston begins to compress the mixture and when the piston begins to build pressure is crucial to the engine’s power output. Before TDC on the compression stroke, the ignition system fires the spark plug. How far before TDC ignition occurs is the engine’s ignition timing (either advanced or retarded) and ignition timing changes frequently based on the system’s ignition curve.

Advancing and retarding the ignition timing is necessary because of the changing demands of the engine, which means there’s more or less time for the flame front to travel across the combustion chamber. When the engine is accelerating, the piston is moving faster, so the spark plug must fire sooner. For example, a big bore kit -- whether it’s the TC 88 to 95 or TC 96 to 103 conversion -- will increase the diameter of the cylinder bore to create more displacement. The bigger bore of these engines means the flame front needs more time to travel across the extra distance. That means the spark plug must ignite the air/fuel mixture sooner. Conversely, decelerating allows the spark plug to fire later. All the various factors that affect ignition timing will require compromises regarding ignition timing.

I have often discussed with cam manufacturers my dissatisfaction with the grinds they produce, which are just not quite what I am looking for. Powerbands rise away from low-end power and torque. The cams I refer to are wonderful in high-rpm, fast-cruising situations. For me, I would like to lower the powerband a little to retain the higher speed aspects with some off-the-line oomph. Harley-Davidsons are heavy machines and many of the riders are far from lightweights. I want power to get me going in the lower-rpm ranges. Marketing sensitivity on the part of cam manufacturers quite often leads to compromise cam profiles with wide LCAs. This is not necessarily a bad thing, since most riders do not know what they are after in matching a cam to their engines. However, for those in the know, it can be quite annoying.

I feel some companies look to balance horsepower and torque with, of all things, fuel economy and idle quality, while at the same time assuming we all cruise at higher speeds. Very often, the choice of wide LCAs comes with sacrificing some of the potential power increase for a smooth idle quality. Manufacturers will do this by grinding their performance, street profiles on wide LCAs. Me, I like a little rumbling roughness at idle in favor of more power. My Harleys exude toughness as they growl, shuddering with pulsating waves, just waiting to pound the pavement. Bob Wood, with his series of Knight Prowler cams, is an exception to the rule. He concentrates on lowering the powerband with lots of low-end torque. However, the grind profile still allows idling without stalling while only minimally compromising high-end rpm. In short, the powerband is exactly where I want it: in everyday usable territory.

Therefore, wide LCAs are not the only way to go. Not everyone wants the smoothest idle and the highest intake manifold vacuum possible. I personally want tighter LCAs to increase my low-end and maximum torque with more cranking compression for my style of aggressive riding. If you’re a bagger rider and have cammed your bike, you may have noticed that most cam manufacturers offer a mild, high-torque, bolt-in camshaft for heavy bikes. They grind these cam profiles with less duration and with tighter LCAs to produce more low-end torque for this purpose. A tighter LCA is suitable if the LCA overlap improvement and the duration combination is not extreme. The tighter the LCA chosen, the shorter the cam duration must be to preserve idle. This has the effect of developing close to the same overlap given by a wider, longer-duration LCA. Again, the tradeoff (low-end torque gain) will be proportional to the loss of some top-end horsepower. This will be more evident on pre-2006 Dynas and pre-2007 Twin Cams because of their gear ratios. The 2006 Dynas and all 2007 and later Twin Cams have supercharged gear ratios that accommodate these tighter LCA cams wonderfully.

The Knight Prowler BW-408B cams in my current bike have a wild .650” lift, while the stock TC 96 cams have mild .471”/.474” intake/exhaust lifts. The stock cams also have a rather mild 248-degree duration compared with other manufacturers that produce big-lift cams with tight 100/102-degree intake/exhaust LCAs. My Street Glide is fast off the line, and is faster than my previous baggers approaching and going beyond 100 mph. I attribute its speed to Bob Wood’s camshafts, the TC 96 gear ratios, and a few other ditties.

In conclusion, from a broad perspective, LCAs strongly tie in with the flow capabilities of the cylinder heads relative to the displacement needs. Many refer to this as the ratio of cubic feet per minute (cfm) of air per cubic inch of displacement, (cfm per ci).