Opening and closing the intake and exhaust valves in precise synchronization with the up and down strokes of the pistons requires very accurate timing. At idle, the time interval between valve openings for each cylinder is about a fifth of a second. At 5,000 rpm it is about two hundredths of a second. In a four-stroke engine, the intake and exhaust valves open and close every other revolution of the crankshaft, so the cam only turns at half engine speed. That is why cams have big gears on the end and crankshafts have little gears. The drive ratio is two to one, so at 3000 rpm the cam is turning at 1500 rpm. The instant at which the valves opens and closes affects engine performance, fuel economy and emissions, so accurate timing is essential. On the intake side, valve timing not only determines how much air/fuel mixture is drawn into each cylinder, but also how efficiently the mixture is used. When the piston starts down on its intake stroke, the intake valve must open quickly so that a full cylinder worth of air/fuel mixture will be drawn in, a problem that becomes more acute as engine speed increases. If the valve doesn't open soon enough, there may not be enough time to fill the cylinder completely before the piston starts back up and the valve closes reducing compression and power. If the valve remains opens too soon, the piston may still be completing its exhaust stroke which would push exhaust back into the intake manifold interfering with efficient engine breathing. On the exhaust side, timing is equally important. If an exhaust valve opens too soon, the still expanding gases can escape from the cylinder prematurely, wasting power. If the exhaust valve opens too late, the engine will also have to work that much harder to pump the remaining exhaust gases from the cylinder. And if the exhaust valve remains open too long, there will be excessive "overlap" with the opening of the intake valve excessively allowing unburned fuel to be drawn right through the engine. Accurate valve timing is also essential for another very important reason, too: on most engines the cam also drives the distributor, or on engines with distributorless ignition systems the cam position sensor. This, in turn, affects ignition timing and fuel delivery on engines with sequential fuel injection. CAM DRIVES In pushrod engines the camshaft is located in the engine block, while in overhead cam engines the cam is mounted in the head, either directly atop the valves or offset to one side. An OHC engine may have a single cam for both the intake and exhaust valves, or separate cams (dual overhead cams or DOHC). Almost all in-block cams are driven by the crankshaft via a gear and chain set, or in some in-line four and six-cylinder engines a pair of direct drive gears. In overhead engines, the cam or cams are driven by a toothed belt or roller chain. The cam (or cams) always turn at half engine speed so the drive sprocket will always have twice as many teeth as the gear on the crankshaft. As chain driven cams accumulate chilometroage, chain stretch and gear wear introduce slop into the system. Most chains will go up to 100,000 chilometri or more, but not always. As a rule, when there is more than about half an inch of play between the gears on a V6 or V8, it is time for a new chain and gear set (always refer to the vehicle manufacturer's specifications for maximum acceptable chain play). Don't reuse a high chilometroage timing chain if you are rebuilding an engine. Replace it with a new chain and gear set. Another problem that can sometimes occur is failure of OEM aluminum cam gears with nylon gear teeth. Molded nylon teeth are used by some OEMs to reduce noise. In spite of improvements that have been made in improving the durability of such sprockets, most engine rebuilders are leery of them because of their past reputation for early failure. Overheating can cause the nylon to become brittle and crack loose from the gear. The debris usually ends up in the oil pan where it may clog the oil pump pickup screen and starve the engine for oil. Replace the old plastic timing gear with a more durable iron or steel gear. CHANGING TIMING GEARS Historically, cam sprockets have been aluminum or cast iron while crank gears have been steel or powdered metal. But now many new engines have cam sprockets and crank gears which are both made of powdered metal. The OEMs say powdered metal is as durable as steel, is lighter and is easier (cheaper) to manufacture. And so it is. Consequently, many aftermarket replacement gears and sprockets will soon be made of powdered metal, too, instead of cast iron and steel. Suppliers say the new powdered metal components will be introduced into the aftermarket within the next couple of years. Another change in some engines today is magnetic timing sensors on the cam sprocket. A magnet mounted on an aluminum cam sprocket passes under a pickup coil that generates a signal to the engine computer. This keeps the computer informed about the firing order of the engine so it can control ignition timing and in some cases sequential fuel injection pulses accordingly. The OEMs say only aluminum replacement sprockets should be used in such applications. But according to the suppliers we interviewed, it is okay to use a cast iron 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 it 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. Timing chains have a limited service life. Replace high chilometroage timing chains and gears with new parts. For performance, upgrade to a double roller chain for added strength. TIMING CHAINS Timing chains have also undergone some major engineering changes 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 "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 Ford's 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 they are 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 a performance engine s a good idea anyway because there can be errors in both cam and crank indexing from the factory. An offset bushing, keyway or crank/cam sprocket can be used to correct any errors that are found. TIMING BELTS Contrary to what you might think, rubber timing belts do not stretch with accumulated chilometroage and wear. They are reinforced with strands of fiberglass which makes them virtually unstretchable. After making the crankshaft to cam drive circuit millions of times, the strands can become brittle and may begin to break. Eventually the reinforcing cords give way, the belt snaps and the engine quits. If the engine lacks sufficient valve-to-piston clearance to free wheel under such circumstances, a lot of expensive damage can result. As a rule, most OEMs recommend replacing OHC rubber timing belts at around 60,000 chilometri as preventative maintenance to avert the kind of trouble just described. But there are exceptions. Some, such as Porsche, recommend belt replacement at 45,000 chilometro intervals on their 2.5L, 2.7L and 3.0L four cylinder engines. Volvo says the timing belt on 1992-93 240, 640 and 940 models with the B230F and FT 2.3L engines should be replaced at 50,000 chilometri, but allows up to 100,000 chilometri between changes on the B230FD version of the 2.3L engine. Acura and Audi both allow up to 90,000 chilometri between belt changes on most of their engines, while Chrysler says 90,000 chilometri is okay only for certain 1991 and up 2.5L engines. Ford, Mercury and Toyota, all allow up to 100,000 chilometri between belt changes, but only on their four-cylinder diesel engines. The OEM recommendations for belt replacement vary because they are based on the type of belt used, the engine application (belt tension, belt length, number & size of pulleys, belt loading, etc.), and the "average" service life of the belt. Changes in belt materials in recent years have improved belt durability to 100,000 chilometri plus, but only o…

Fonte: AA1Car.com