Tuning the XK Engine |
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Author: |
Pat Harmon |
Contact E-mail: |
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Disclaimer: Information contained herein is solely the opinion of the author and is by no means to be interpreted as factory authorized procedures. Use of these procedures is at the full discretion of the reader and the author held harmless from any damage resulting from same. Mention of particular products by name or manufacturer is only for reference and not a solicitation of them. |
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Overview The Jaguar XK engine, designed during the WWII air raids over London, was considerably ahead of its time in 1948 and proved to be one of the most durable and enduring designs of the 20th century. As with any machine, it’s performance is dependent upon condition and tuning. This article is a basic primer on tuning this fabulous engine with tips and lessons-learned not included in the factory Workshop Manual. The Basics To properly tune an automobile engine, it’s essential to have a basic understanding how they operate and the various components interrelate. All internal combustion engines operate on the same basic principles: loading a combustion chamber with the correct fuel-air mixture, compressing that mixture and igniting it at the correct time in the cycle so that it develops maximum power for the configuration. The XK engine (as are most automotive engines) is a four-stroke engine meaning that the pistons complete four strokes each cycle, two up and two down. The cycle begins with the piston at the top of its stroke. At this point the intake valve opens and the piston is pulled down (via the connecting rod to the crankshaft) drawing a fuel/air mixture from the carburetor(s) through the intake manifold. At the bottom of the stroke the intake valve closes and the piston travels upwards, compressing the fuel/air mixture. The piston then travels to the top of its stroke (called top dead center) and, the spark plug fires igniting the explosive fuel/air mixture. The exploding gases then force the piston downwards. At the bottom of the stroke the exhaust valve opens and the piston travels upwards expelling the spent mixture through the exhaust valve. At this point, the exhaust valve closes, the intake valve opens and the piston begins the next cycle drawing a new fuel/air mixture into the cylinder. While all this is going on, the distributor (which runs from a geared shaft to the crankshaft) is turning at a rate one-half that of the engine speed acts as a high voltage switch directing current to the spark plugs every other rotation. Factors Affecting Engine Performance With these basics in mind, it should be clear to see that a number of factors determine how well an engine runs. A perfectly tuned engine must have all of them correct. In many cases, if one is off, it will affect the others. It’s really not as complicated as it seems. There are a number of things important to performance that cannot be “tuned.” Typically these are set at manufacture or rebuilding. The engine must be able to adequately compress the fuel/air mixture. If the valves are worn/burned, cylinder/pistons worn then the mixture can leak out of the combustion chamber resulting in less power. The valves are opened and closed in a special sequence by the camshaft which (in the case of the XK engine) is driven by a timing chain from the main crankshaft. The valves opening and closing in relation to the crankshaft and pistons is called “valve timing” and must be correct. We don’t want exhaust valves opening early robbing from the power stroke nor do we want intake valve opening too early (before the piston has reached top dead center (TDC)) or late (after the piston has already started downward creating a vacuum in the cylinder). Before an engine can be tuned to maximum performance, it is important that it be in good condition. A simple compression check (pressure gauge screwed into spark plug hole) is a quick way to check all the internals are in-order. Basic Principles of Tuning There are seven interrelated functions that all must be set correctly for the engine to run it’s best. Spark Plug Gap: A simple, but important parameter. Use the manual specified plug and carefully gap to factory settings. Take care that all plugs are set identical. Sparkplugs must be clean and are so inexpensive they should be replaced at each tuning. TIP: Use anti-seize compound on steel plugs inserted into aluminum heads. Valve Clearance: Obviously, the valves must open and close to allow fuel/air to enter and exhaust to exit. How far the valves open and close is determined by the valve clearance. This also affects the valve timing to a certain extent, as a wider clearance will cause the valves to be opening earlier and closing later. For a smooth running engine, valve clearances should be as identical from cylinder to cylinder as much as possible. Otherwise one cylinder will be operating under different conditions than another. The XK engine is an overhead cam engine. From the front, the left side houses the intake camshaft and valve set. The right side houses the exhaust camshaft and valve set. Camshaft lobes ride directly on top of the valves on followers. Special thickness shims are installed under the followers (on top of the valve stems). The valve clearance is adjusted by installing various thickness shims Point Gap: The distributor is nothing more than a dual function switch. A point contact unit within the distributor rides on a six lobed camshaft, which causes the points to open and close as a follower rides on the camshaft. These points are connected in series with the ignition and the spark coil. When closed the coil is connected directly to the battery. When the points open, the circuit is broken and the magnetic field built-up inside the coil collapses causing a high voltage to be generated in the secondary windings. This high voltage (over 10,000 volts) travels to the center of the distributor cap over a high voltage wire. From there the voltage is directed to one of the sparkplug wires via the rotor. Timing: This determines when the spark
plugs (in
Inside the distributor cap are six contacts – one for each spark plug and a rotor riding on top of the camshaft. This camshaft rotates once per every two engine revolutions and directs the high voltage from the rotor to the appropriate spark plug contact and then onto the sparkplug. By rotating the distributor you can see how the timing of the spark voltage to the spark plug can be changed. Timing can be measured statically (engine not running) by using a light bulb or ohmmeter to measure when the points are just opening. It can also be measured with a timing light, which fires upon high voltage sent to either the #1 (rear of engine) or #6 (front of engine) spark plugs. Idle Speed: To facilitate acceleration/deceleration there are two controls, which vary the timing from its basic setting. One control (vacuum advance) varies the timing depending on intake manifold vacuum. At full acceleration, the carburetors are open, the vacuum is low, and the advance unit does nothing. When the vacuum is high; however, the advance unit advances the timing. These are installed mainly for fuel economy. More important to tuning is the second advance control or centrifugal advance. Inside the distributor there are weights, counteracted by springs, which advance the timing more the faster they spin. On an XK engine, this control begins affecting the timing over 500 rpm. It’s thus important that the engine be at 500 rpm while being tuned at idle. Carburetor Synchronization: Most XK engines have twin SU carburetors with the ‘S’ models having three. For optimal performance, these carburetors must be synchronized so each does an equal part. They are designed so, at idle speed, the throttle plates are fully closed and incoming air is adjusted with idle/air adjusting screws (near the rear of the unit). At idle, the engine speed is controlled with these screws and each is to be adjusted so the carburetors are drawing equal amounts of air. Once the engine is accelerated above idle, the pistons rise within the suction chamber this raising the tapered needle in the jet seat allowing more fuel to flow into the unit. Fuel/Air Mixture: This is set at the carburetor. As air is drawn through the carburetor venturi, a vacuum is created which draws gasoline past an adjustable jet. When the mixture has too much fuel per unit of air it is said to be “rich.” When there is not enough fuel it is said to be “lean.” In XK engines the jet opening is controlled by a tapered needle which fits into the jet opening. Raising or lowering the jet in relation to the needle controls the idle mixture. Lowering will cause a larger gap or opening and thus make the mixture richer. Raising the piston/needle combination a bit thus making the engine run lean checks the mixture. If the engine has been set to rich this will temporarily correct the mixture and the engine will speed up a tad. If the engine was running to lean then this operation will make the situation worse and the engine will slow down. Relationship of Tuning Elements Understanding the above, the setting of one element will affect the other. Any change in one requires the rest to be reset for the engine to be in-tune. For example, an engine that has its fuel/air mixture set too rich can be compensated by changing the timing but at the detriment of overall performance. The best way to approach the tuning is to first adjust elements in the following order: 1. Spark Plugs 2. Valve Clearances 3. Point Gap 4. Timing 5. Idle Speed/Synchronization 6. Fuel/Air Mixture Remember, these elements are interrelated. The first three are mandatory factory settings and need to be set first. Timing should be set to factory specifications first. After initial tuning, you may want to try advancing/retarding in small increments to optimize performance. Bear in mind that, both idle speed and fuel/air mixture will have to be readjusted each time the timing is changed – all three are interrelated. Tuning Procedure Preparation. Clean the engine. It’s much more enjoyable to work on a clean engine than on a dirty one. Additionally parts/surfaces are less apt to become contaminated with dirt. Purchase Materials: Spark plugs, anti-seize compound (Permatex), Cam cover gaskets, Ignition Points and condenser, gasket sealer (I prefer silicone), automatic transmission fluid (you’ll have to read on to find out what this has to do with engine tuning). Tools Required: Timing light or ohmmeter, blade type feeler gauge (for valves), wire type feeler gauge (spark plugs), screwdrivers, socket wrenches, micrometer (conventional), carburetor synchronization device (Unisyn or rubber tubing), carburetor synchronization kit (tubes with indicator wires inserted in the pistons). 1. Gap Sparkplugs. 2. Check Valve Clearances. 3. Point Gap. 4. Ignition Timing. 5. Idle Speed and Carburetor
Synchronization: (adjust when engine is at normal
operating temperature) |
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