Cams, Chains And Gears
Since the earliest days of the internal combustion engine, a crankshaft-driven camshaft has been used to operate the engineís valves. The camshaft turns at half the speed of the crank (a two-to-one drive ratio) so the drive mechanism has traditionally been fairly simple: a set of intermeshing gears, a pair of sprockets connected by a chain, or in the case of an overhead cam (OHC) engine, a pair of sprockets connected by a long chain or rubber belt.
In the 1970s, some thought camshafts, cam drives and valves would soon be obsolete when the revolutionary new Wankel rotary engine went into production. But rotary engine technology never caught on and only saw limited production.
Today, valvetrains have become more complex than ever. Weíve gone from simple pushrod engines with a block mounted cam to overhead cam engines with one to four cams. Some of these OHC engines have one, two or even three timing chains, while others employ a combination of timing chains and belts. Some OHC engines also have "variable valve timing" (VVT) to vary valve overlap, duration and/or lift. These systems use hydraulic and electronic controls to advance or retard cam timing, or to change the position of the cam.
The latest technology that is just now going into production will take cam drive technology even further. It controls valve lift to vary engine speed, thus eliminating the need for a conventional throttle!
BMWís next generation "Valvetronic" technology adds a computer-controlled stepper motor to the valvetrain. The stepper motor moves a pivot that changes the relative height of the cam to alter the amount of intake valve lift. When the driver steps on the electronic gas pedal, the engine computer sends a signal to the stepper motor to increase valve lift.
BMW says its new valve control technology eliminates the restriction and pumping losses created by the throttle and improves fuel economy 15 to 20 percent. The system also employs variable valve timing, too, to further enhance engine performance.
The Valvetronic system is currently being manufactured at BMWís new factory near Birmingham, England for four cylinder engines in BMW Series 2 cars in Europe. BMW says its Valvetronic system will eventually be adopted to its other engines and will come to the United States.
Other manufacturers have been developing new technologies that would eliminate traditional camshafts and cam drives altogether by using solenoids to actuate the valves. The main advantage of a camless engine is that valve timing and lift is controlled 100 percent by a computer, which means lift and duration can be changed almost infinitely to suit changing loads and driving conditions. The promise is less pollution, better fuel economy and performance.
But building a camless engine has proven to be a challenge because of the limitations imposed by a 12-volt electrical system. Twelve volts canít generate enough magnetic force to open and close the valves quickly enough to keep pace at higher engine speeds. This obstacle is about to disappear, though, because the auto makers are preparing to change to 42 volt electrical systems within the next three to five years. The first European 42 volt electrical systems will be in production in 2002, and it wonít be long until the new high voltage electrical systems spread here.
Siemens Automotive in Europe has developed what it calls a second-generation "Electromechanical Valve Train" (EVT) system that should be production ready within three years. The EVT system takes full advantage of the new 42-volt electrical systems and has been proven to work at maximum rpm on a 16-valve four cylinder engine.
A key component in the Siemens computer-controlled EVT system is an armature-position sensor on each valve actuator. This provides feedback to the computer so it can not only keep track of the position of each valve on the engine, but also the rate at which it is opening and closing.
The individual valve actuators use what Siemens calls the "free spring mass oscillator principle." The computer varies the current to each actuator coil so that the valve decelerates to near zero as it seats. This provides a soft landing and prevents the valve from pounding the seat as it closes. This approach not only reduces valve noise and wear, but also improves the durability of the valve actuators.
Siemens says its next step is to eliminate the armature-position sensor and use feedback from the actuator coil itself to keep the computer informed about the valveís position and speed. Power for the current EVT system is provided by a starter/generator integrated into the flywheel.
Another new technology thatís been under development for quite a number of years is Orbitalís direct injection system that mixes air with fuel and injects it directly into the engineís cylinders. The system requires less pressure and improves fuel atomization compared to other direct injection technologies. Originally conceived as a way to bring back lower cost, two-stroke engines (which have no cams, cam drives or reciprocating valves), the technology has since been adapted for use on conventional four-stroke engines with valves.
Cam Gear And Sprocket Metallurgy
Over the years, vehicle manufacturers have used a variety of materials for cam gears and sprockets: fiber, cast iron, steel, aluminum, aluminum/nylon and powder metal.
Softer materials are quieter but not as durable. Aluminum cam sprockets, especially those with nylon coated teeth, have been documented with more than a few premature failures. Molded nylon teeth reduce noise and friction. But, engine overheating can cause the nylon to become brittle and crack loose from the sprocket.
The same can happen to molded fiber timing gears. The debris usually ends up in the oil pan where it may clog the oil pump pickup screen and starve the engine for oil. Because of this, many rebuilders prefer to replace original equipment aluminum sprockets and molded fiber gear sets with ones made of cast iron or steel for added durability.
Almost all late model engines now have cam sprockets and crank gears made of powdered metal. Such parts are made by molding powdered metal in a press at extremely high pressure and temperature to melt (sinter) the particles of metal together into a solid mass. The result is a material that is more durable than a machined steel part, and is lighter and easier to manufacture.
Paul Cunningham, vice president of sales for Cloyes Gears, Paris, AR, says 100 percent of the timing gears and sprockets his company is supplying the vehicle manufacturers are powder metal. He says Cloyes also uses mostly powder metal parts in its premium timing components line. "If the OEM uses powder metal, we do too," he said.
In some cases, aftermarket suppliers may use different materials than what the OEMs used for other reasons. Some engines like GM 3800 V6s have a magnetic timing sensor on the cam sprocket to keep the computer informed about the firing order so ignition and sequential fuel injection can be controlled accordingly. GM says only aluminum replacement sprockets should be used in this application. But several aftermarket suppliers say itís OK to use a cast iron or powder metal replacement sprocket as long as the sensor magnet is correctly mounted on the sprocket when it is installed.
The cast iron sprocket will not affect the magnetic properties of the sensor. If the engine is equipped with a camshaft thrust button assembly, and the button is not reinstalled (or is weak), however, the sensor magnet may make contact with the sensor on the front timing cover causing a no-start condition.
A camshaft is a hefty piece of iron or steel that represents a fair amount of weight in an engine. In recent years, Ford, Chrysler and some import manufacturers have gone to "assembled" or "composite" cams to reduce weight and manufacturing cost.
On Fordís 4.6L V8 and 2.5L V6, for example, the cam is made by mounting molded powder metal lobes on a hollow steel shaft. A rod is then forced through the shaft to expand it and lock the lobes in place.
The composite cam design is about 30 to 40 percent lighter than a conventional one-piece cast iron or steel camshaft, and is also about 1.7 times as rigid as a cast iron camshaft, which reduces flexing that can lead to cam damage or breakage. The hardness of the powder metal lobes is also equal to or greater than that of a hardened steel cam for improved durability. The use of powder metal lobes which can be precisely molded to near perfect shape also eliminates or reduces the amount of grinding needed to finish the cam.
Unfortunately, nothing is perfect and composite cams are no exception. On some of the early (1992) Ford 4.6L engines, the pressed-on cam lobes can loosen up and slip on the shaft causing a loss of compression in the affected cylinders. Since thereís no way to reposition or reattach the lobes, the camshaft must be replaced. A similar problem also has been seen on 1983 to 1986 Toyota 2.0L 2SFE Camry engines with composite cams.
Regardless of what type of camshaft an OHC engine has (cast iron, steel or composite) as original equipment, replacement cams should be the same material as the original. Steel and powder metal followers should only be used with steel or composite cams and vice versa.
Composite cams often show little wear and may be reused. But care must be taken to clean out the hollow camshaft as this can become a hiding place for sludge and debris.
Variable Valve Timing
Variable valve timing (VVT) is a trick thatís used on some engines to squeeze more performance out of a given cam profile. By advancing or retarding cam timing, valve duration and overlap can be altered to improve engine performance over a broader range of speeds and operating conditions. At lower rpm, VVT is typically used to advance cam timing for more low end torque. At higher rpm, VVT may be used to increase valve overlap and duration for added power.
Most of the VVT applications to date have been on high end import luxury engines (Honda, Lexus, Nissan and Mercedes), but Ford is also using it on its Zetec 2.0L engine in the Contour.
Most of these VVT systems use some type of hydraulic mechanism on the end of the cam to alter cam timing. A computer controlled solenoid routes oil pressure into the VVT cam mechanism at a predetermined rpm to change cam timing. Mercedes, though, uses a magnetic clutch rather than a solenoid for this purpose.
Hondaís "Variable Valve Timing and Electronic Lift Control System" (VTEC) on the Acura NSX 3.0L DOHC V6 is unique in that it has extra lobes on the cam and extra rocker arms that come into play at higher rpm to increase valve lift and duration. Oil pressure routed through a solenoid pumps up the extra rocker arm for each pair of valves to change valve lift.
Problems with a VVT system typically involve the control solenoid and electronics that route oil pressure to the VVT mechanism. Oil leaks in the unit itself, as well as mechanical wear or cracks, can also prevent the VVT system from working properly. The key here for engine rebuilders is to thoroughly clean and inspect the VVT hydraulic mechanism and to test the control solenoid to make sure they function properly.
Belts Vs. Chains
For many years, most OHC engines were built with rubber timing belts. Though quiet, belts are not as durable as timing chains and eventually need to be replaced ó a job which involves quite a bit of labor and expense on many engines. The recommended replacement interval of 60,000 miles for belts on older engines is not adequate for todayís long-lived engines.
Since about 1995, improvements in belt technology have stretched the replacement interval for OHC belts to 100,000 miles. The longer mileage belts use a special high temperature grade of neoprene called "highly-saturated nitrile" (HSN). But the newer materials are not always available for older OHC belt applications. So depending on what type of replacement belt you install when rebuilding an engine, you should let your customer know what the recommended replacement interval is for the belt ó especially if the engine is an interference design that lacks sufficient clearance between the valves and pistons to survive a belt failure.
The current trend in new engine designs is back to chains. Virtually all new OHC engines that have entered production within the past few years, as well as those that are planned for future production, have timing chains rather than belts. The Quad Four was one of the first to reverse the trend, followed by Fordís 4.6L V8 and the Cadillac Northstar V8. GMís new 3.5L DOHC V6, which will eventually replace the 3800 V6, also uses chain-driven cams.
The OEMs have learned how to make chains quieter by using revised tooth profiles, and prefer chains because they can go 150,000 miles or more before they need to be replaced.
Most of these OHC engines with timing chain drives also have hydraulic tensioners to maintain proper chain tension. As a rule, the tensioner assembly should also be replaced along with the timing chain and gear set when the engine is rebuilt. If a tensioner is reused, it must be carefully cleaned because it can become a trap for debris.
Timing chains have also changed in recent years. Historically, the domestic OEMs have used an inverted tooth or "silent" type of timing chain. Most European and Japanese OEMs, on the other hand, have used a British Standard (BSI) roller chain (similar to a bicycle chain).
The domestic OEMs have preferred the silent chain design for V6 and V8 applications because it provides a very smooth, quiet drive. Tooth links engage the cam and crank sprockets almost like a flexible gear. Most older silent chains were the "rigid back" or "stiff back" design which allowed the chain to flex only one way. Most silent chains in newer engines are now a "fully flexible" design that allows the chain to bend both ways. The fully flexible design is easier to install and is somewhat more durable than the stiff back design.
Some of the newer domestic engines with overhead cams, such as Fordís modular 4.6L V8, are using American Standard (ANSI) roller chain. Unlike BSI roller chain which has been used on most import applications, 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.
Double roller chains have traditionally been considered an "upgrade" for replacing silent chains in performance applications. Roller chains perform better in such applications because theyíre lighter, more durable and can handle higher rpms than a silent chain.
Some aftermarket double roller chains also come with an offset cam sprocket so the cam can be advanced or retarded as needed to dial in the engine. Advancing cam timing up to several degrees is a common trick that improves the low end torque and throttle response characteristics of performance cams in street-driven engines.
To dial in the cam, a degree wheel is needed to index the cam to the top dead center position of the number one piston. Most aftermarket street performance cams come with 4 degrees of initial advance already built-in (a fact which must be taken into account when degreeing in the cam). Using a degree wheel to verify correct cam timing in an engine takes time, but is a good way to verify accurate cam indexing and valve timing. An offset bushing, keyway or crank/cam sprocket can be used to correct any errors that are found.
Cam Drive Problems
The classic symptom of a broken timing belt or chain, or stripped timing gear is an engine that cranks but wonít start due to no compression. But, cam drive failures can also cause an engine to lock up, especially if the engine lacks proper valve-to-piston clearance to free wheel if the cam stops turning.
A broken cam drive can also be a symptom of additional problems, like a seized cam. Severe overheating, for example, can cause an aluminum cylinder head to warp and bind the overhead cam. This, in turn, may cause the timing belt to jump time or break. So just because the cam drive has failed, donít assume thatís the only problem youíre dealing with.
Harder to diagnose are cam drive problems that donít involve a complete failure. A stretched timing chain (or worn gears) will affect both valve and ignition timing, which in turn affect intake vacuum, compression, and idle speed and quality. If the wear is severe and the chain (or a belt) jumps timing, the engine may run rough or possibly backfire ó or it may not run at all.
By the same token, if the timing belt, chain or gears are not aligned correctly the engine may run but run poorly because of advanced or retarded cam timing. If timing is off by more than two teeth (the equivalent of about eight to 10 degrees depending on the application), the engine probably wonít run at all.
On GMís 3.4L DOHC V6, for example, the procedure to reset cam timing in the early service manuals (1991-í95) omitted a critical step (oops!). This engine does not use a key way or dowel pin to locate the engineís four camshaft sprockets. The only reference marks on the camshaft are two flats located near the front or rear of the end journal. When the camshafts are correctly positioned in the cylinder head, those flats will be facing up on one bank and down on the opposite bank. This requires rotating the crankshaft 180 degrees after one set of cams has been aligned. Special tool #J38613 is also required to set the cams.
The information published in the original service literature indicated that all four camshaft flats should be in the up position when installed ó which is incorrect. The engine will start and run, but will lack power and performance because valve timing is incorrect.
If you have to replace a cam on this engine, mark each cam prior to disassembly because all four are different. Finding a correct replacement cam can also be tricky because GM has used 14 different cams in this engine between 1991 and 1997. Intermixing of different year cams is not recommended because the lobe configurations vary. To properly identify the cam, you may need the casting number as well as the year, make and model of the vehicle.
Changing Chains, Gears
Steve Tucker, vice president of sales & marketing for Dynagear, Downers Grove, IL, says timing sets are more complex today, and so is their installation. "A typical timing set today may have two chains, four to six sprockets and a couple of guides and tensioners. Itís not the same as installing a typical small block Chevy three-piece timing set. Thereís more chance for making an error during the installation."
Timing gears and chains should always be replaced as a complete set. It makes no sense to use worn sprockets with a new chain or vice versa. Timing gears always come as a matched set and must likewise be replaced as a set.
If a V6 or V8 pushrod block has been line honed, it can decrease the distance between the centerline of the crank and cam. This can create unwanted play in a new timing chain set ó unless you install a set with oversized gears that compensate for the change in dimensions. The same applies to OHC engines that have had the head milled. Unless head height is maintained by using a thicker head gasket or head gasket shim, it may be necessary to use an oversized cam sprocket.
When installing a timing chain and gear set on a pushrod engine, the new crank gear goes on first, followed by the cam gear with the chain on it. On OHC applications, there is no set procedure since chain tension is controlled by a tensioner or guide.
On many OHC engines, there are additional components that should also be replaced. These include chain tensioners, guides and/or rails. These components all play a vital role in supporting the chain as well as keeping it taught, so donít overlook these items.
One common installation error is misalignment. This can be caused by something as simple as installing the cam sprocket backwards, using the wrong thickness of washer under the sprocket, failing to press a cam gear all the way on, etc. If the cam and crank sprockets are not perfectly aligned, the result will be rapid chain and sprocket wear, or interference problems.
When installing sprockets on the cam and crank, do not hammer directly on the sprockets or chain. Press both sprockets on evenly, keeping them parallel. This prevents stretching or damaging the chain or sprockets.
On some applications, a separate spacer that goes behind the cam sprocket is used to control end thrust. Replacement sprockets for some of these applications have the spacer built onto the backside of the sprocket ó which means you should not reuse the old spacer.
Press fit gears (except fiber gears) should be preheated to about 250į F to make installation easier. Attempting to press on a cold gear may ruin the interference between the gear and shaft, causing the gear to work loose later.
Itís a good idea to double check the alignment of all timing marks on every engine application to make sure theyíre right ó especially on engines with balance shafts. This can be tricky on some engines (especially certain imports) where multiple sprockets and/or multiple timing marks are involved.