Cam timing plays a huge role in an engine’s power curve. Advancing cam timing shifts the power curve more toward the lower rpm range, while retarding it moves the power curve higher up the rpm scale. If an engine is being built for a particular application (stock, street/strip, drag race, circle track, you-name-it), how the cam is timed relative to the crankshaft will depend on the cam grind selected, the valvetrain, the gearing and final drive ratio, and what kind of power curve the customer wants.
With stock engines, it’s a no brainer. You install the cam in with the timing marks aligned in the stock position. But this doesn’t mean the cam has zero degrees advance or retard because most aftermarket cams are ground with about four degrees of advance built in with the stock timing marks aligned. Because of this, it’s important to use a degree wheel to check actual cam timing if you are doing a performance build and want to know the exact position of the cam.
With performance engine builds, cam timing adjustments are usually required. You may want to tweak the timing a few degrees one way or the other depending on the cam grind, drivetrain gearing and application. This requires a cam drive setup that has some type of adjustable crankshaft and/or camshaft sprockets with offset keyways that allow you to choose various degrees of advance or retard.
Most aftermarket timing chain set suppliers include offset crank sprockets that have multiple keyways (usually 3 but up to 9 on some sprockets) machined into the ID to allow this type of adjustability. The maximum amount of advance or retard is usually around 6 to 8 degrees in 2 or 4 degree increments, but some allow as much as 12 degrees of advance or retard. The only limitation with this setup is that once you’ve picked a particular timing setting for the cam and buttoned up the engine, the only way to change the setting is to pull the front cover, remove the timing chain and reposition the crank sprocket. If the timing chain set has an adjustable cam gear rather than a multi-keyed crank sprocket, adjustments are somewhat easier. You still have to pull the front cover, or install a two-piece cover that allows easy access to the cam gear for adjustments.
For many years, most pushrod engines have used some type of “silent” timing chain to drive the cam, with cast iron, powder metal, aluminum or nylon coated sprockets. Though adequate for stock applications, most performance engine builders replace the stock timing components with either a single or double roller chain and heat treated steel cam and crank sprockets. Roller chains are a better choice for a performance application than a silent chain because they are lighter, stronger and can handle higher rpms. Upgrading to a double roller chain is usually recommended if an engine will be running at speeds above 6,000 rpm. Billet steel gears are recommended for engines that are upwards of 600 or 700 horsepower or use heavy (over 200 pounds of closed seat pressure) valve springs.
The difference in friction between a single and double roller chain is negligible. On some applications, like a Chevy LS engine or many late model overhead cam engines, internal clearances do not allow enough room for a double roller chain. But on applications that do have the space, a double roller chain can improve reliability. If a break occurs in one row of rollers, the other row will hold the chain together preventing what could be a catastrophic chain failure. A broken timing chain is not something you want to happen in an “interference” engine that has close clearances between the pistons and valves.
On Ford’s modular 4.6L V8, Ford uses an American Standard (ANSI) roller chain. Unlike BSI (British Standard) roller chains that have been used on many import applications over the years, ANSI chain does not have a freely rotating roller, only the fixed bush. The bush is larger and stronger than that used in BSI chain, however, making it more suitable for heavy-duty applications. Aftermarket upgrades for this type of application include full roller chains made with stronger materials.
The Chevrolet family of LS engines is one of the hottest applications for all kinds of performance parts these days, and cam drives are no exception. One timing component supplier we interviewed for this article said they now offer over 40 different variations of timing chain sets for the LS applications, including stock blocks as well as various aftermarket blocks with raised cam locations. Their chain sets are up to 33 percent stronger than the stock chain, and are run by 24-hour endurance racers as well as everyday drivers who want the ultimate in durability.
One rule of thumb with respect to timing chains is that the chain, crank and cam sprocket are a matched system and should always be replaced as a set. Installing a new chain on worn sprockets is not going to provide accurate cam timing or long term reliability. The same goes for overhead cam engine applications that also include chain guides, tensioners and other components in the timing set. Replacing all of these components is essential to assure reliability and correct timing adjustments.
Something else to keep in mind is that all timing chains are not the same. Differences in price usually reflect differences in quality. A cheap chain intended for a stock application won’t have the same heat treatment, metallurgy or strength of a quality chain that’s been engineered for high performance use.
Gear Drive Systems
Gear drives have been around for many years. The first performance gear drives appeared back in the 1960s and 1970s as an alternative to timing chains. One of these was the Erson Cam Drive, which was produced until about 10 years ago.
The main advantage of a gear drive is that it eliminates the timing chain and all the worries that go with it. Timing chains can stretch with use, which retards valve timing and hurts engine performance. Chains can also break with disastrous results. Slack in the chain can also cause variations in cam timing at high rpm.
Gear drives use one of two basic designs: a three gear setup (one idler gear between the cam and crank gears) or a four gear setup (two idler gears between the cam and crank gears).
The makers of gear drives say their products provide precise cam timing that won’t change once the cam has been dialed in. Beyond that, they disagree about the relative advantages of three gear versus four gear drives, and gear drives versus belt drives.
One maker of gear drives says the do not recommend using their two idler gear system for racing applications that will rev beyond 5,000 rpm. Another supplier said gear drives are good for street or strip applications up to 7,000 rpm. Their explanation? “At higher rpms, the gear drive creates too much friction, and there is a risk of transmitting crankshaft harmonics to the valvetrain.”
In spite of these recommendations, gear drives from some suppliers are run successfully on many Top Fuel dragsters with no reliability or performance issues whatsoever. One supplier who makes a three gear setup says harmonics is not a problem with their design. They say their “precision gear tooth profile” eliminates backlash and chatter issues that can be a problem with other types of gear drives.
Gear drives are available that allow a distributor or magneto to be mounted on the front of the engine.
Belt Drive Systems
The hot cam drive setup with many drag racers is a belt drive system. Instead of using gear sprockets and a chain to turn the cam, a belt drive uses ribbed pulleys and a ribbed synthetic rubber belt reinforced with carbon fiber. Belt drives are available for most common pushrod engines (including Chevy LS), as well as most common aftermarket performance blocks. Various belt lengths and pulley diameters are available to accommodate different applications and variations in belt tension.
Belt drives are expensive, costing up to six to 10 times as much as a chain and sprocket cam drive, and three to eight times as much as a gear drive because of the machining required for the front engine cover. However, belt drives do offer a number of important advantages.
Easy adjustability is one. A belt drive runs dry and requires no lubrication as chain drives or gear drives do, so it can be left open and exposed. For drag racing, this is no problem and allows quick cam drive adjustments as well as belt inspection between runs. Adjustment bolts on the cam pulley allow the cam to be advanced or retarded as needed to fine tune the engine’s power curve for prevailing track and traction conditions. For other types of racing that allow belt drives, a dust cover is usually necessary to protect the belt from dirt or debris that may be kicked up into the engine compartment.
A belt drive does require the installation of a front cover seal plate and oil seals behind the crank and cam sprockets to keep engine oil where it belongs. The front plate is machined to fit specific block applications, including standard blocks and aftermarket blocks with a raised cam bore. The more complex the cover, the higher the cost of the kit. Some belt kits are available with torrington bearings or even ball bearing thrust bearings for the cam to precisely control cam end-play.
Belt drives are also quiet (much quieter than gear drives) because of the rubber belt. Another advantage is that the rubber belt minimizes harmonic vibrations that can be transmitted from the crankshaft to the camshaft (a problem that twin idler gear drive systems can experience at high rpm). The reduction of harmonics means more stable valve timing and no loss of power.
Belt drives also create less internal friction than chain drives or gear drives, which reduces parasitic horsepower losses inside the engine. Another advantage with belt drives is that they make it easy to use a front-mounted distributor. This gives you more flexibility in planning the overall configuration of the engine, and comes in handy if an engine will be equipped with a long supercharger that would otherwise interfere with the location of a stock distributor. The belt drive can also be used to drive a front-mounted fuel pump.
The trade-off with belts is that they are not as long lived as chain drives or gear drives. The belt has a limited life, and should be replaced after a certain period of time depending on use. For drag racing, one recommendation is to replace the belt every 200 runs. For circle track racing, replacing the belt at the end of the season is recommended. For endurance racing, most racers will change the belt every race.
Is belt breakage a risk? Breakage of any cam drive or timing component in racing is always a risk. But there’s no greater risk of breaking a belt than a timing chain. The high tech belt materials (including carbon fiber reinforcing fibers in some belts) can safely handle loads created by even the stiffest valve springs.
One belt drive supplier said their 25 and 27 mm width belts can handle up to 500 pounds of closed seat pressure, and 1,200 pounds of open pressure. For more demanding applications, wider belts are available (up to 36 mm).
The crank and cam pulleys for a belt drive system are typically made of billet heat treated steel. So in spite of their relatively high cost, belt drives have proved to be both popular and reliable with professional drag racers.
OHC Timing Belts
On overhead cam engines with timing belts, the main concern when rebuilding the engine is to install a high quality belt that won’t fail prematurely. Back in the mid-1990s, most belt suppliers made an upgrade in the type of materials they were using for timing belts. This allowed the belt replacement intervals to be safely extended from 60,000 miles up to 100,000 miles or more.
Unfortunately, some offshore belt suppliers didn’t make the shift so their belts are still 60,000 mile (or less) belts. One of the worst examples of this has been the belt used in the Chevy Aveo. The factory timing belt on these cars has had a terrible reputation for premature failures, often snapping in as little as 30,000 to 40,000 miles!
Accurate belt tension is extremely important in OHC engines. Too much tension may cause idler pulley bearing wear, premature water pump failure (if the water pump is driven by the timing belt), or belt breakage. Insufficient tension may allow the belt to jump time, or may allow slippage at the water pump causing the engine to overheat. For these reasons, engine builders should always use a belt gauge to set the tension of the timing belt to factory specifications when the engine is assembled.
For more information about timing components as well as supplier information, visit our online Buyers Guide.
Latest posts by Larry Carley (see all)
- Picking a Racing Oil - Jan 16, 2015
- Consider a Dyno/Flow Bench for Your Shop - Dec 23, 2014
- Selecting Pushrods, Rockers and Springs for Your Build - Dec 23, 2014