Not that long ago, you had two choices when it came to cam drives for your race engine – a timing chain or some sort of gear drive. They obviously get the job done, but they’re not 100% infallible. What has replaced them (where legal by the rules) is the timing belt. In fact, you regularly see belt drives in everything from drag race Super Stockers to Pro Stockers (and a lot in between – including endurance applications such as Winston Cup and road race applications). One of the big benefits of a belt drive is the ease of camshaft tuning (phasing), but we’ll look at that in a minute. Why have belts eclipsed timing chains and gear drives in both Sportsman and Pro motorsports? The real key is harmonics.
First, let’s rewind: Decades ago, a few very savvy racers (Dan Jesel was one of them) noticed there was a considerable amount of harmonics, vibration, and resonance transmitted from the crankshaft to the camshaft. It was no big step to discover this harmonics dilemma eventually had a profound effect upon valve and spark timing – to the point where racers had (and in some cases, still have) a difficult time holding the timing stable in their engines. To make matters worse, the only option back then was a gear drive, and it had a tendency to physically tie the crankshaft to the camshaft even more ardently than a timing chain. This meant that every pop, bang and boom seen by the crank went straight to the cam. You can imagine what this does to the dynamics of the valve train. What about the harmonic damper (balancer)? Doesn’t it stop this internal nonsense? Not exactly. Remember the damper is outbard of the timing chain or gear drive on the engine. It actually dampens the harmonics after it passes through the lower cam gear.
Initially, the racing fix was to eliminate gear drives, and believe it or not, use a timing chain set with a nylon upper gear (this was a common Chevy production component at the time). It worked to a certain degree, but as you can well imagine, it wasn’t exactly robust. Both chains and timing gears were replaced on a very regular basis. Due to the nature of the beast, you never really know when the chain or a nylon sprocket would say: “I’m done!” Most often, when a gear or chain disintegrated, the results were catastrophic.
Dan Jesel came up with the ultimate solution. Jesel noted that many foreign cars (and at the time, a very small handful of domestic vehicles) as well as a select few motorcycles incorporated belt drives for their overhead camshaft setups. As the story goes, Jesel was deeply involved in a project that consisted of adapting a pair of overhead cam Cosworth Vega heads to a small block Chevy (eventually destined to reside inside his and brother Wayne’s Camaro gasser). While a break and enter artist cut the project short, the belt drive camshaft concept remained. It was simple: What if a similar arrangement could be engineered for a common pushrod engine such as the small block Chevy? Engineering the system was likely far more difficult than the inspiration, but the process also brought about several innovations that made the belt drives even more appealing to the racer. Aside from the belt absorbing significant amounts of crank-to-cam harmonics, the belt drive also took less power to drive (when compared to a timing chain or gear drive).
Today, Jesel’s belt drive system has evolved considerably. In terms of construction, the belt drive makes use of a cast aluminum plate to seal the front of the cylinder block from the oiling system. This actually allows for improved oil drain back to the pan, and at the same time, eliminates any power loss attributed to driving the camshaft in a pool of lubricant. The belt itself is intended to run dry. Obviously, there is a provision in the cast cover to allow for the cam stub shaft and the crankshaft to protrude. And of course, these openings both incorporate oil seals.
The drive belt is a proprietary 25 mm wide, round tooth or “high torque” job. Jesel selected a round tooth belt for one major reason: The round profile of the “teeth” does a superior job of distributing the load (across the tooth). As mentioned earlier, the belt is designed to run dry, and the overall design is resistant to wear from dirt and grit. While initially configured for the small block Chevy, today, there is a plethora of different configurations available for a wide range of engine combinations (including Chevys and non-Chevy applications). There are even special configurations for engine blocks with raised camshaft locations. A select few of these belt drive kits feature longer belts with tension idlers (there is no need for an idler on combinations such as standard cam location small block Chevys, standard cam LS engines or standard cam location big blocks). For the uninitiated, raised cam engine blocks have been developed to incorporate much longer stroke crankshafts for large displacement engine applications. In these blocks, the cam tunnel is raised so that the block can accept huge strokes (upwards of 5.00-inches), hence the need for special belt drive configurations.
When it comes to idler pulleys and belt drives, they may not really prove to be the best mix. Gates (the maker of many belts used in these drive setups) notes that the use of an idler arrangement leads to heat build up. The reason is an idler pulley can actually spin at speeds in the range of 28,000 RPM. That spells heat, but there’s more, too: Having an idler out in the open just asks for more trouble. Here you have one more component that can fail. But as pointed out above, in some cases with tall camshaft tunnels, the manufacturer has no choice but to include an idler pulley.
More Than A Camshaft Drive…
A big benefit of a belt drive is the fact it allows for extremely easy and rapid cam adjustment. It’s simple too: Four nuts on the cam drive (spider) are loosened (for a closer look check out the accompanying photos). To retard the cam, turn the crank clockwise. To advance the cam, turn the crank counter-clockwise. The timing marks machined into the spider indicate two degrees of crankshaft degrees each. Re-tighten the nuts on the spider and you’re done. The actual range of adjustment is + or – 10 degrees, which, as most of you are well aware, is more than sufficient for any race camshaft tuning operation. Remember too that without a dust cover in place (which is the case for the majority of applications), almost everything is easily accessible. Aside from removing the water pump, the job consumes minutes instead of hours.
Given the above, you can see that changing the cam also proves to be equally simple: Once you yank the water pump and peel out the lifters, there’s nothing left to do except remove the belt and the retaining plate. Then the cam literally falls out. That’s not the end of it, either: Incorporated into the system is a camshaft thrust mechanism. Now, it’s no big secret that roller camshafts tend to move fore and aft in the block. Some means of stopping this movement is a must, and that’s the purpose of this design. The removable thrust plate is also adjustable, allowing easy endplay adjustment as well as cam removal through the cover.
How Good Is The Belt Drive?
Way back (decades ago) when the belt drive system was first developed, plenty of skeptics figured there wouldn’t be much power gain from the system. They figured wrong. Initial testing on a 600 HP, aluminum rod, multiple carburetor engine (an old small block Chevy Modified Eliminator combination) showed gains of 12-14 horsepower. Independent back-to-back tests on a Super Stock style small block saw gains of 17 horsepower when compared to a conventional chain drive. Where did the gains come from? There were three probable areas: Reduced windage, increased timing accuracy (from the reduced harmonics transmitted to the cam) and of course, a lack of wasted motion (timing chains are noted for chordal action which can actually turn the chain into a “S” at high speed).
It’s easy to see that the belt drive system makes more power and the drive system offers a number of advantages. How reliable is it? Very. The belt durability has proven to be greater than both gear drives and chain drives over thousands of race mils of competition in all types of race car applications. As mentioned previously, it sees considerable use in endurance applications. Remember too, there are literally tens of thousands of Harley Davidson and Indian motorcycles out there churning out equally large miles every year. And each and every one of those bikes is equipped with a drive belt rather than a drive chain.
When it comes to maintenance for a belt drive, it’s a simple procedure. Jesel recommends that the drive belt be replaced annually. The belt drive is here to stay, and in the big picture, it’s one of those innovations that have actually made racing easier and actually less costly.
For a closer look at the belt drive system, check out the accompanying photos.