How a vehicle is used can also affect its octane requirements. If a vehicle is used for towing or some other application where the engine is forced to work hard under load, a higher-octane fuel may be necessary to prevent detonation. If switching to a higher-octane fuel fails to eliminate a persistent detonation problem, it probably means something else is amiss. Anything that increase normal combustion temperatures or pressure, leans out the air/fuel mixture or causes the engine to run hotter than normal can causes detonation. Loss of Exhaust Gas Recirculation The Exhaust Gas Recirculation (EGR) system is one of the engine’s primary emission controls. Its purpose is to reduce oxides of nitrogen (NOX) pollution in the exhaust. It does this by “leaking” (recirculating) small amounts of exhaust into the intake manifold though the EGR valve. Though the gases are hot, they actually have a cooling effect on combustion temperatures by diluting the air/fuel mixture slightly. Lowering the combustion temperature reduces the formation of NOX as well as the octane requirements of the engine.
If the EGR valve is not opening, either because the valve itself is defective or because it s vacuum supply is blocked (loose, plugged or misrouted vacuum hose connections, or a defective vacuum control valve or solenoid), the cooling effect is lost. The result will be higher combustion temperatures under load and an increased chance of detonation. Compression Ratio Keep compression within reasonable limits. A static compression ratio of 9:1 is usually the recommended limit for most naturally aspirated street engines (though some newer engines with knock sensors can handle higher compression ratios).
For supercharged or turbocharged applications, a static compression ratio of 8:1 or less may be required depending on the amount of boost pressure Another point to keep in mind is that boring an engine’s cylinders to accept oversized pistons also increases the static compression ratio. So too does milling the cylinder heads. If such modifications are necessary to compensate for cylinder wear, head war-page or damage, you may have to use a thicker head gasket if one is available for the application or a head gasket shim (a dead soft copper spacer shim) to offset the increase in compression.
• Check for a defective knock sensor many late model engines have a “knock sensor” on the engine that responds to the frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to momentarily retard ignition timing until the detonation stops.
• “Read” you spark plugs. The wrong heat range plug can cause detonation as well as pre- ignition. If the insulators around the electrodes on your plugs appear yellowish or blistered, they may be too hot for the application Try the next heat range colder spark plug. Copper core spark plugs generally have a broader heat range than ordinary plugs, which lessens the danger of detonation.
• Check of engine overheating. A hot engine is more likely to suffer spark knock than one which runs at normal temperature, overheating can be caused by a low coolant level, a slipping fan clutch, too small a fan, too hot a thermostat, a bad water pump, or even a missing fan shroud. Poor heat conduction in the head and water jackets can be caused by a buildup of lime deposits or steam pockets (which can result from trapped air pockets).
• Check the operation of the heated air intake system. The thermostatically controlled air cleaner’s job is to provide a carbureted engine with hot air when the engine is cold started. This aids fuel vaporization during engine warm-up. If the air control door sticks shut or is slow to open so that the carburetor continues to receive heated air after the engine is warm, the added heat may be enough to cause a detonation problem-especially during hot weather.
• Check the operation of the airflow control door in the air cleaner to see that it opens as the engine warms up. No movement may mean the vacuum motor or thermostat is defective. Also, check the heat riser valve to make sure it’s opening properly, as it. too, can affect the air intake system.
• Check for a learn fuel mixture, rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, ‘intake manifold gaskets carburetor gaskets or the induction plumbing downstream of a fuel injection throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt. a restricted fuel filler or a weak fuel pump.
If the fuel mixture becomes too lean, lean misfire may occur as the load on the engine increases. This can cause a hesitation, stumble and/or rough idle problem as well.
The air/fuel ratio can also be affected by changes in altitude. As you go up in elevation, the air becomes less dense. A carburetor that’s calibrated for high altitude driving will run too lean if driven at a lower elevation. Altitude changes are generally not a problem with engines that have electronic feedback carburetors or electronic fuel injection because the oxygen and barometric pressure sensors compensate for changes in air density and fuel ratios.
• Check the boost pressure, controlling the amount of boost in a turbocharged engine is absolutely critical to prevent detonation. The turbo waste gate bleeds off boost pressure in response to rising intake manifold pressure. On mot late-model engines, a computer controlled solenoid helps regulate the operation of the waste-gate. A malfunction with the manifold pressure sensor, the waste-gate control solenoid, the waste-gate itself or a leak in the vacuum connections between these components can allow the turbo to deliver too much boost, which destroys the head gasket as well as the engine if not corrected. Improved inter-cooling can help reduce detonation under boost. The intercooler’s job is to lower the incoming air temperature after it exists the turbo compressor. Adding an intercooler to a turbo motor that isn’t inter- cooled (or installing a larger or more efficient intercooler) can eliminate detonation worries while also allowing the engine to safely handle more boost.
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