Jan. - Feb. 2002 Lane Racing And Rodding Article By Jim Kaekel, Jr.
A timing chain set isn't the most glamorous or "trick" item in a racing or street engine, but it is a very necessary piece, and generally very reliable as long as the chain is regularly inspected for wear and replaced when needed. Because of the exorbitant cost of belt drives and the horsepower loss associated with gear drives, many race and performance street cars still rely upon traditional timing chain set-ups in spite of their own inherent shortcomings.
According to Larry Weisenfels of Cloyes, Inc., traditional chain and gear sets have not been completely replaced by gear drives because of the gear drive's tendency to generate large amounts of horsepower robbing friction. Weisenfels also states that gear drives do not absorb engine harmonics because of their lack of flexibility. The gear drive was very popular in the upper echelon of auto racing for several years, but was gradually set aside when belt drives were introduced. Sprint cars and limited budget late models still use gear drives because of their endurance capabilities at high RPM.
High dollar, high revving race engines use belt drives in which cam and crank gears are actually Gilmer type pulleys driven by a Gilmer toothed belt. Efficiency, resistance to wear, low horsepower draw, and ability to withstand high valve spring pressures are all advantages of the belt drive, but $400-$800 purchase prices often place them out of reach of the average racer's budget. Thus, the conventional timing sets - at a quarter of the cost of belt drives – are still in high demand by racers of all persuasions.
Conventional link belt timing sets, for years the standard of the auto industry, employ a multiple link chain secured by a series of small diameter pins. The design creates a significant amount of heat, due to friction, and wear occurs rather quickly in performance applications. Link belt sets earned their bad reputation when major automobile manufacturers began using aluminum camshaft gears with nylon or nylon-coated teeth. Still in use in many of today's popular engines, the nylon gears supposedly provide for quieter engine operation, but after the engine piles up the miles, the nylon teeth begin to crack and fall off, causing a great deal of slack in the chain and eventually the engine will "jump" time. The broken nylon teeth, which usually wind up in the bottom of the oil pan, also cause another problem. Because the broken pieces of nylon teeth are very light, they are easily sucked into the oil pump pick-up screen, restricting oil flow and possibly causing serious engine damage. The shortcomings of the link belt systems inspired manufacturers like Cloyes and Speed-Pro to develop roller timing chain sets, which were ideally suited to performance applications. Roller chain timing sets use a special chain with large diameter, round bearings secured to the chain links with 1/8" diameter pins. The rollers allow the chain to roll across the teeth of the cam and crank gears with little effort, for negligible horsepower loss and improved friction characteristics. Most timing set manufacturers offer sets with roller bearings of different sizes; the larger bearings being associated with a more durable chain. Wear is still a problem as over a period of time the links and pins of the chain will wear, causing it to stretch. The first sign of an excessively worn chain is erratic timing mark movement, easily noted, especially on a race car with locked out ignition advance, when checking the ignition timing. When the chain has stretched to this point, the ignition and cam timing are certain to be unstable and the chain should be replaced immediately.