With increased power comes need to properly control it
Words and Photos by Sir Brandon Flannery
The difference between making good power and great power most often comes down to proper timing of valvetrain events. With two crankshaft revolutions per one of the camshaft, keeping these two synced, especially at high rpm, is imperative.
Timing sets have evolved through the years, and today’s engine builders have several options to choose from. Obviously, the higher up the horsepower ladder, the more expensive the parts. But, choosing the right one for your application can go a long way in keeping the timing in check. We asked several industry professionals for their input, and the pros and cons of each.
Many early engines used direct-drive timing gears on the cam and crankshaft. As they grew in size, especially in V-8s, timing chains began to appear. While noisy at first, Chevrolet in the late ’60s and early ’70s developed a quieter version with “nylon teeth” that reduced noise. However, they failed to withstand the racing abuse of aggressive cam profiles and related higher spring pressures, or increased rpm. Those teeth were replaced with cast iron and steel sprockets as a result.
The factory link-belt chains were flat stacks of alternating metal links that fit on a sprocket set. While strong enough for racing, the hinged ends of flat metal created a lot of friction. They were replaced by “roller” chains featuring a bushing on an axle for each side of the chain link – very similar to a bicycle chain. Single row chains worked well for factory horsepower, but soon led to a “double row” design aimed at reducing friction and increased durability with higher valve spring and rpm demands of higher performance.
Racing is a constant evolution of finding the next weak spot and soon, the cast iron and steel sprockets were replaced with billet, as CNC equipment gained popularity in manufacturing and competition.
“When they got the heat treatment correct on the sprockets, the billet double roller timing sets, with all of their optional timing features, became very popular,” Crane Cams’ Allan Bechtloff elaborates. “The best ones used a German-made chain with JWIS stamped on each link, and we used them in the 1990s to race 500-mile Winston Cup races. However, there was always a certain amount of chain stretch and deformity when it was dynamically in use.
“Smokey Yunick once told me he filmed timing chains in action with a high-speed camera and found a wave or distortion would form with the variance in tension on the system,” Bechtloff continues. “This ‘slack’ in the chain would then change position and move around as the rpm varied. He theorized that the spark timing event for each cylinder would be slightly altered as this ‘slack”’ in the chain moved its position on the sprocket. That’s why he liked gear drives over timing chains.”
Yunick was splitting hairs and looking for every ounce of power available. For many applications, a double-roller chain is an ideal blend of affordability, reliability, and ease of installation.
“A heavy duty roller chain setup in a gear and chain-type timing set can offer very good durability, as well as being sealed up within the engine for applications such as dirt oval or off-road racing,” Chris Padgitt of COMP Cams adds. “Higher end versions of these offer multiple keyways for making camshaft degreeing easier and more accurate.
“Gear drives can be used in applications where very direct crankshaft to camshaft motion is needed. They have almost no advancing or retarding of the camshaft through the rpm range. However, they can add some harmonic vibrations to the valve train. The gear drives do make a distinct noise that some consumers wish to have their engines make. There is no mistaking the whine of a gear drive.”
Gear drives generally come in two styles: dual-idler or “dog bone” versions, as offered by Pete Jackson, COMP, or Lunati; and single idler drives, such as those from Summer Brothers and Milodon. The latter pair require modification, while the dog bone style usually fits under a stock-style cover.
Another area of concern for gear drives is the noise affecting computer-controlled engines with knock sensors. They can throw off the sensors and cause the computer to retard the timing.
“At the Dairy Queen, a gear-drive can sound really cool,” Bechtloff says. “Though now they make ‘quiet’ versions, gear drives had lots of noise and usually require machining and adaptation to the front of the engine for installation. Fitting one under a stock timing chain cover wasn’t easy.
“Gear drives can transfer vibrations, and in some cases, even create their own vibrational factor to the engine, he adds. “They also have a problem transferring crankshaft and camshaft vibrations, which can be witnessed in the stress on the harmonic damper and throughout the valve train.”
Belt drives were developed in the 1980s for Pro Stock racing. They take less power and generate less harmonics, and some can even absorb some vibration from the crank and cam. Continuing developments in belt technology have solved most of the pre-existing timing problems of noise, accuracy, durability, vibration, and timing stability, but one problem that remains is cost. A belt drive is substantially more expensive that its counterparts.
“A belt drive is best for racing applications,” Padgitt says. “The rubber drive belt helps to dampen some of the harmonics between the crankshaft and the valve train. One of their benefits is the ease of access to make camshaft degree changes, and even change the camshaft in the engine. This is especially helpful for those running superchargers or nitrous who make frequent timing changes.”
Other than high cost, Bechtloff doesn’t think there are any concerns for running a normal tension belt drive on the street. “After all, most all new modern engines have some sort of rubber serpentine timing belt in them from the factory.”
Though some racers are still hesitant about belt stretch, there is a lot of science behind today’s belts, in the materials, belt tension, and the shape (or profile) of the tooth that gets involved in the dynamic functionality of these systems.
“Over the years, many hours of research went into correcting the stretch-like effects seen in testing,” Bechtloff says. “One solution for high-end NASCAR teams was to install the belt with increased tension between the pulleys. It was then measured to see what that tension equated to, almost like a guitar string. The belts were put in with increased tension so at high RPM, there was less distortion or stretch to the belt.”
Several of today’s systems use this “tension fit,” while others offer an adjustable tensioner wheel that acts like an idler. They are particularly popular in engines that have machining (like line boring) that may alter the distance between the cam and the crank.
From mild to wild, depending on how critical your valve timing and rpm range needs are, there are several options from which to choose. Like everything else, the faster you want to go, the more it costs. However, the added investment in the right setup for a specific application can save money between time and damaged parts in the long run. Do it right, do it once. After all, timing is everything.
Sources: COMP Cams, compcams.com; Crane Cams, cranecams.com