Timing Chains, Gear Sets and Belt Drives

Timing Chains, Gear Sets and Belt Drives

The short answer to which type of cam drive is best depends on the application, what your customer wants and how much they can afford to spend. A stock link style timing chain is fine for everyday driving and normal use. “Silent tooth” link chains are quiet, long-lived and adequate for stock cams and valve springs. But if you’re building an engine with a hotter cam, stiffer valve springs and a higher red line, upgrading to some type of aftermarket timing chain set, gear drive or even a belt drive may be in order.

Obviously, there are a lot of ways to spin a camshaft. For most V6 and V8 engines, the auto makers have used a simple chain drive setup because its cheap, quite and relatively durable (although chains do stretch as the miles add up). For inline fours and sixes, the stock setup may be a chain or a helical gear set.  With overhead cams engines, the long distance between the crankshaft and cam(s) on top of the cylinder head(s) dictate a chain or belt drive. Some OHC race engines have used gear drives but it takes a lot of gears to transfer drive torque all the way up to the cams.

The least expensive and simplest upgrade for a stock link style timing chain is to replace it with an aftermarket performance timing set: either a beefed up link style chain or the ever popular single or double roller timing chain. Link chains with larger diameter pins and stronger heat treating can add durability and strength to the cam drive.

Roller Chains

A roller chain is even better because the rollers reduce friction, and the larger pins and stronger links improve durability and longevity. A double roller chain is the ultimate chain setup because the two rows of rollers effectively doubles the strength of the chain — which is essential if the engine has significantly stiffer valve springs compared to stock. Many roller chain kits come with billet steel cam and crank sprockets for added strength and wear resistance.

Double roller chains are used successfully in all types of performance applications, including street, drag, circle track, off-road and marine. Roller chains are available for most engine applications, including new double-roller chain kits for Chevy LS engines (previously, only single roller chains were available for the LS).

Installation is as simple as replacing the stock chain and sprockets with the roller chain and roller sprockets. No other modifications are needed except to advance or retard cam timing as desired to dial in cam timing for the application. Most roller chain timing sets have crank sprockets with multiple offset keyways and/or cam sprockets with multiple mounting holes that can be repositioned for up to 4 degrees (or as much as 8 degrees with some) of cam timing advance or retard.

Installing a timing chain (roller or link) is relatively simple, but there are a few things you have to keep in mind. Line boring a block can change the distance between the crank and cam. This distance is critical for fitting a chain because if the distance has changed, the chain may be too long or too short. Some chain suppliers “select fit” their timing sets to assure the chain fits with minimal play (no more than about 1/8th inch deflection between the sprockets). Special chain lengths may be provided for blocks that have been line bored, or for aftermarket blocks that have a raised cam location to provide extra clearance for a stroker crank.

You want the chain to be tight with minimal slop, but not as tight as a banjo string because too tight can overload the chain and cam bearings.  A certain amount of play is needed to compensate for thermal expansion as the engine heats up. But too much play can have an adverse effect on performance.  A quarter inch of chain play can retard valve and ignition timing as much as three degrees.

A degree wheel should always be used to check cam timing as well as chain play. Rotating the crank back and forth to see how much movement it takes before the cam starts to move will tell you how much play is in the timing chain. If it is more than 3 degrees, the chain is too loose. Ideally, it should be half that or less.

Chains will loosen up with use, and some chains may show noticeable stretch after only a few hundred miles or a few dozen runs down the strip or laps around the track. The better the quality and heat treatment of the chain, the less it should stretch. If a chain has stretched, it may be necessary to readjust cam timing to compensate. Adding a few degrees of advance can offset the stretch in the chain. The other alternative is to replace the chain with another better quality chain that hopefully won’t stretch as much — or to dump the chain drive entirely and switch to a gear drive or belt drive.

Gear Drives

For those who want to go a different route, another upgrade option is to install some type of gear drive. There are gear drives with one or two idler gears between the crank and cam gears, as well as “noisy” and “quiet” versions to choose from.  The three gear drives mount the idler gear on the block. The four gear versions use brackets to mount the idler gears between the crank and cam gears.

Gear drives have some advantages and disadvantages. Most gear drives are a simple bolt-on installation that require few if any modifications. Most fit under the stock timing cover while others may require a special cover. Multi-position crank and cam sprockets with some sets allow as much as 12 degrees of cam timing adjustment.

The main advantage with a gear drive is that it provides a rigid link between the crank and cam (depending on the amount of play and backlash in the system). Valve and ignition timing won’t change once the system is installed because there is no chain stretch. This eliminates variations in cam and ignition timing as well as any worries about timing changes over time. Many heavy-duty diesels have gear drives for this reason. They are rugged and long lived, and they can handle lots of valve spring pressure. However, gear drives create more friction than a chain or belt drive, which may take a bit more power to drive the cam.

Some automotive performance magazines have run dyno tests comparing various types of cam drives back-to-back on the same engine. Most of these tests have found very little difference from one cam drive setup to another, usually no more than a couple of horsepower difference. At really high RPM (over 7,000 RPM), belt drives do show an advantage with a power gain that may be as much as 10 to 14 hp over a chain or gear drive. The difference is attributed to reduced friction with the belt drive, and and better dampening of crankshaft harmonics that can disrupt valve and ignition timing at higher engine speeds.

Another characteristic of gear drives is noise — lots of noise, even with some so-called “quiet” gear sets. Straight-cut gears produce a lot of clash when they mesh together, unlike helical cut gears which are much quieter and smoother (and more expensive to manufacture). However, noise can be minimized by carefully machining the teeth during the manufacturing process to assure the best possible fit.

Some people want a gear drive just  because of the “whine” it produces. It sounds like a old school blower under the hood. Others who have tried a “noisy” gear drive (one with dual idler gears) have found the noise is too loud for everyday street driving and have gone back to a double roller chain. Gear drives are fine for the drag strip, but a noisy version might not be the best choice if a customer prefers a quieter cam drive.

Another drawback of the noise produced by a noisy gear drive is that it can play havoc with the knock sensor on an engine with electronic fuel injection. The sound frequency generated by the gears knocking against each other may trigger the knock sensor, causing the engine computer to retard spark timing. That hurts performance or fuel economy, so some makers of gear drives do not recommend using their systems on late model engines with EFI. Even so, some people say they have successfully run gear drives on late model EFI engines with no issues, but it depends on the location and sensitivity of the knock sensor as well as the design of the gear drive itself. If there’s no knock sensor, or if the sensor has been disconnected or disabled, even a noisy gear drive should be no problem.

Some makers of noisy gear drives do not recommend their drives for high revving engines (over 7,000 RPMs) because of the concern over crank harmonics being transmitted to the cam and valvetrain. Other gear drive makers say they’ve successfully run their products on all kinds of racing applications and do not have any limit on engine speed.

If you do choose a gear drive, follow the installation instructions provided by the manufacturer as far as gear lash and clearances are concerned.

Belt Drives

For really high output, high revving engines with lots of valve lift and spring pressure, many racers prefer a belt drive — especially drag racers. It’s hard to find a Pro Stock drag car that isn’t running a belt drive. Same with NASCAR.

A machined cover plate seals the front of the block because the belt drive runs dry. A cam adapter and shims are installed behind the cam pulley to achieve proper alignment and camshaft end play (.008 to .012˝ of end play is recommended for flat tappet cams, but as little as .001˝ of end play can be used with roller cams). Reducing camshaft end play reduces side loading on roller lifters for longer life.

Belt drives are expensive, but they are also super easy to adjust because the belt drive is exposed on the front of the engine. All you have to do is loosen a few nuts on the cam pulley, rotate the crank to advance or retard cam timing (8 to 10 degrees of adjustment is available depending on the pulley), then retighten the nuts. There are no covers to remove — unless the belt drive has been fitted with a dust cover to keep dirt and sand out (which is recommended for dirt tracks, off-roading or mudding).

Belt drives are quiet and efficient, with minimal friction compared to chain or gear drives. Also, belts do not stretch — and they seldom break as long as they are inspected regularly, kept clean and replaced according to the supplier’s recommendations.

Belt drives are also offered in custom lengths to fit aftermarket blocks with raised cam bores.

One leading supplier of performance belt drive systems recommends replacing the drive belt every other year for street performance applications, annually for circle track applications, or after 250 runs for drag racing. A new belt is also recommended if the engine has failed for any reason. If a belt has been contaminated with grease or oil, it should be replaced immediately. Cleaning a belt with engine degreaser or other harsh chemicals is not recommended.

Other recommended maintenance for a belt drive includes replacing both seals and thrust washers annually.

Front-mount distributors with a crank trigger ignition system are available for belt drives. Repositioning the distributor to the front of the engine allows more freedom with intake manifold runner design and makes it easier to remove the manifold because the distributor is out of the way. A front distributor with a larger diameter cap can also be used to reduce the risk of crossfire with high output ignition systems. You also get more accurate ignition timing because the trigger signal is being read directly off the crank.

Pitfalls to Avoid

The key to choosing the “best” cam drive is to make sure it meets your customer’s expectations and needs. A hotter cam and stiffer valve springs in a performance engine will require some kind of cam drive upgrade, either a stronger chain or a gear or belt drive.

Is easy cam timing adjustment a requirement? Then a belt drive should be considered in spite of its higher cost.

Is noise a factor? If a customer wants that cool old school “whine” from the cam drive, then a gear drive should make him happy. On the other hand, if you’re building a motor for a street machine and the customer likes listening to his sound system without a lot of background buzz, then a quieter roller chain timing set or belt drive would be the best choice.

Another pitfall to avoid when buying a cam drive system is no-name cheap quality knock-offs. It’s a highly competitive market and everybody is looking for the lowest price on parts. The internet offers a lot of deals, but many of these so-called bargains are too good to be true. Some of these low cost offshore timing sets look the same as a name brand timing set. In fact, they might even copy the look of the brand name packaging (outright counterfeits!).  But the offshore knock-offs typically use inferior steels, inadequate heat treatments and pay minimal attention to quality control.

Cheap offshore manufacturers may be  experts at copying all kinds of things, but they have yet to master the art of copying quality (or metallurgy or material science for that matter). For them, it’s all about making a quick buck. It’s not about brand recognition or reputation or delivering a quality product to the end user.

Cheap timing sets may be tempting for a stock low-dollar rebuild but these kind of products have no business going on a performance engine. A cheap chain will stretch more quickly than a quality chain, and it may not even fit correctly when it is installed (too much slop in the chain is a common complaint). Is a cheap timing set really worth the risk of a comeback or engine failure? The answer should be a resounding no!  ν

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