Now, before you take Dave's words the wrong way, let me put them into context: As the founder, owner and brains behind Skunk2, Dave has learned a thing or two about what makes a successful business in the SpoCom industry-mainly, that actions speak louder than words. The success of a company's race effort is a strong indicator of their ability to produce effective parts. If they've remained ahead of the pack, along the way something more can be learned; that genuine engineering is at the root of their accomplishment. Any company can put out a product that works, simply by copying the successful ideas of another's, but the ones putting in the R&D and breaking new ground will always set the bar for the others to follow. For them, there can be no greater insult than seeing some fly-by-night "manufacturer" make a quick buck off of their efforts.
Even more insulting is how the knock-offs rely on customers' lack of product knowledge to make sales. "For every company actually designing cams-who genuinely know their product-there are a dozen others who have cams ground to the same published specs and who count on the customer not being educated enough to ask detailed questions about how they're different," explained Dave. "Try getting helpful advice from those guys."
So, aside from the numbers we're used to hearing, what defines a good cam? And more importantly, how can two cams rated the same be different? To understand this, we're going to have to cover the basics-but not into much depth-as Import Tuner readers, we count on you having a slightly more advanced understanding of your cars. If not, you'd be reading one of those other mags' articles cited by Dave in the beginning of this one.
Anyone with basic understanding of reciprocating internal combustion engines knows that they put a vehicle in motion by harnessing the energy released when air and fuel are ignited within their cylinders. In two-stroke engines, air and fuel can only flow into and out of combustion chambers during certain points in the piston's stroke. In four-stroke engines, air and fuel can flow at any time intake or exhaust valves are open-it's the job of the cam to work with cylinder head components to regulate their movement. Most standard four-stroke engines use different variations of the same basic components: Intake and exhaust valves, valve springs, tappets, and camshafts, to transfer energy of the crankshaft in motion to control the timed opening and closing of its valves.
OHC
While cam and valve orientation takes many forms in modern-day engines, we'll be concerning ourselves with overhead cam (OHC) designs, since they are by far the most prevalent in the sport compact world (sorry Hemi and SRT-10 fans-no pushrod coverage here). Inline, OHC engines can be arranged in two designs, DOHC and SOHC. In either case, the cams are held in place above the valves in the cylinder head. As a cam rotates, a tappet rides along each one of its oblong-shaped lobes, moving proportionally to how far "out of round" the cam's lobe is, converting rotational motion to linear and imparting this motion on the valves. At rest, valves are kept closed by the valvesprings when the tappet rolls across the base circle, or the uneventful portion of the cam. In some cases, like the DOHC Ford I4 engine, the tappet is a flat disc, which sits atop the valve, putting it in direct contact with the cam lobe, though more often than not each tappet is a fixed part of a rocker assembly, which connects cam lobe to valve.
(Graphs on next page)
Both cams advertise more lift and duration than the OEM cam but while 'CAM A' brags the most lift and duration, notice how 'CAM B' features more area under the curve.