Engine Building Fundamentals: Valve Timing
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Valve Timing
04.53
| 00:00 | - The position in the engine cycle when the intake and exhaust valves open and close is critical to the engine operating efficiently and reliably as well as making the intended power and torque. |
| 00:12 | The timing of these events is controlled directly by the camshaft or camshafts. |
| 00:18 | However, these camshafts are driven by the crankshaft and there are a couple of ways that this can be achieved. |
| 00:25 | The most popular techniques include chain drive and belt-driven camshafts where either a timing chain or a timing belt is used to transfer crankshaft rotation to the cams. |
| 00:37 | Although relatively uncommon it's also possible to use a gear drive system to operate the camshafts. |
| 00:44 | With a cam chain system we will have the actual chain itself. |
| 00:47 | And this will be driven by a sprocket that locates positively on the nose of the crankshaft. |
| 00:53 | The chain will then pass over a cam pulley that drives the camshaft. |
| 00:58 | Typically a chain guide or pair of chain guides, will be attached to the front of the engine block to help guide the chain between the adjacent pulleys. |
| 01:08 | These guides use an engineering plastic material that the cam chain runs against which offers minimal friction and wear characteristics. |
| 01:18 | The cam chain is housed within the front cover of the engine and is lubricated by the engine oil supply. |
| 01:26 | The chain requires the correct tension to ensure there's no slack in the chain during operation. |
| 01:32 | This is achieved with a cam chain tensioner that may be either a mechanical or a hydraulic design. |
| 01:39 | A mechanical tensioner uses spring pressure and a ratchet system that allows the tensioner to extend and take up the slack in the chain. |
| 01:48 | However, the ratchet system prevents the tensioner from retracting again. |
| 01:53 | A hydraulic tensioner, on the other hand, uses hydraulic pressure that may come from the engine oil to extend the tensioner and provide the correct tension against the chain. |
| 02:04 | Particularly in V-8 performance engines it's often common to fit a double row timing chain which as its name implies, consists of a cam chain that is double the width. |
| 02:15 | Essentially two chains joined together. |
| 02:19 | This provides a significant increase in strength which can be important when running large cams with aggressive profiles. |
| 02:27 | It does however require suitable sprockets on both the crankshaft and camshaft to suit. |
| 02:33 | And may also require modifications to the oil pump and the front cover assembly to provide sufficient clearance. |
| 02:42 | With a cam belt the system is very similar to the cam chain however the belt will be external to the front cover of the engine and doesn't require lubrication. |
| 02:52 | In fact, oil can break down and damage the cam belt material. |
| 02:55 | And this must be avoided. |
| 02:58 | The cam belt will again be driven by a cam belt pulley attached to the nose of the crankshaft. |
| 03:03 | And then in turn drive the pulleys on the cam or cams. |
| 03:07 | It's typical with cam belts to include a number of idler pulleys that bolt to the front of the engine block. |
| 03:13 | These pulleys guide the cam belt and also ensure that there's sufficient contact with the crank pulley and the cam pulleys. |
| 03:21 | These idler pulleys consist of a sealed bearing that the cam belt runs against and depending on the orientation they may be either smooth when they run against the back of the cam belt or toothed when they run on the toothed side of the belt. |
| 03:36 | The idler bearings do wear and it's common to replace these components as part of a cam belt replacement or any time you're building a fresh engine. |
| 03:46 | You can tell if these idlers need to be replaced as they'll be noisy if you spin them by hand. |
| 03:52 | As with a cam chain a cam belt also requires a tensioner system in order to achieve and maintain the correct cam belt tension. |
| 04:01 | Again, both mechanical and hydraulic tensioners are used. |
| 04:06 | Mechanical tensioner system will consist of an eccentric idler pulley that can be moved by hand in order to achieve the correct amount of tension prior to tightening the idler and fixing it. |
| 04:19 | Hydraulic tensioners which are also known as auto tensioners are installed in the retracted position and normally held there with a locking pin. |
| 04:28 | After the cam belt is installed and correctly timed, the locking pin is removed and the tensioner will extend and provide tension for the cam belt. |
| 04:38 | While the hydraulic tensioner is maintenance-free in performance applications it's common to replace them with mechanical after-market tensioners to allow more control over the cam belt tension. |
