The cam is a very critical part of an engines valvetrain. The cam is what lifts the valves into the combustion chamber allowing air to enter the cylinder. Cam profile selection is very critical on how an engine will run, and how log the cam will live. The cam along with the cylinder heads control how much air can enter the engine. The more air in the engine, the more power that can be made (within reason). This does not mean the most aggressive cam is the best one for you. You can get away with a fairly agressive cam on a carbureted vehicle. Be careful on a fuel injected engine as too much overlap will drop vacuum and will not allow the fuel injection to work correctly.
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A basic car’s engine can be broken down into terms that anyone can understand. The goal of an engine is simple, move the car, which is accomplished by turning the wheels. The wheels are connected through the driveline (transmission, rear end gears), which connects to the engine. The rotation comes from a part in the engine called the crankshaft. The crankshaft is connected to a straight “bar” of metal called the connecting rods. On the top of the rods are pistons, cylindrical aluminum pieces that have rings of metal around them. The rings seal them close to the cylinder bore. The cylinder bore is simply a long hole that the pistons can move up and down in. This up and down motion pushes the rods down and rotates the crankshaft. Make sense so far? Next, the piston gets it’s motion from an “explosion” that happens when it is in the top part of the cylinder bore. This makes the piston move downward very quickly. For an “explosion” to occur the engine needs two things, air and fuel. At the top of the cylinder bore, it is sealed, to force the piston down, with a cylinder head. The cylinder head also allows fresh air to come into the cylinder bore and after the “explosion” allows the used air (exhaust) to leave the engine so more fresh air can come in next time.
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The variable valve timing used the best of two camshaft profiles. Some of the most common abbreviations for this are the Honda vtec and Toyota vvt-i valvetrain. Although I believe the Acura nsx was one of the first to use it. Other systems are now being used by all manufacturers. Some systems actually use two separate camshafts to benefit from low end torque, good emissions, and top end power with the other cam. The vtec uses a second lobe on the same cam with another profile. Others use a wide cam lobe with a different profile on each side of it. They then move the cam side to side. The problem with using one cam is compromise. Most single or dual overhead cam vehicles are tuned for mid range power and low emissions. This means reasonable lift numbers, but not optimal. Lift is simply how high the cam lifts the valve into the head chamber, to let air pass through it. The overlap is the amount of time that the intake and exhaust valves are opened into the chamber at the same time.
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The valvetrain includes the camshaft, lifters, followers, rocker arms, pushrods, retainers, springs and valves. It determines, along with the heads, how much air can pass into an engine. Most valvetrain system are very mild from the factory. This is why alot of horsepower can normally be extracted from a mild factory setup. The cam is usually the best place to extract power and can be purchased for a reasonable amount.
The oxygen sensor or o2 sensor measures the oxygen content in the exhaust. The more fuel placed into the combustion chamber (rich condition) the more oxygen that will be used in the combustion process. When the oxygen content in the exhaust is low, there is a chemical reaction in the sensor. This produces a higher voltage up to 1 volt. Lower voltage is for more oxygen content and can go down to .2 volts. The oxygen sensor will not work when it’s cooler than 600 degrees F. At less than this temp it displays .45 volts same being if there is a problem with the sensor. The computer will show an error (engine light)if at this value longer than what it should take to normally warm up. The o2 sensor continually shifts above and below .45 volts. If you believe the o2 sensor is bad you can test it. First run the car for 5 minutes. Disconnect only the wire running to the oxygen sensor. Then connect a ohm/volt meter with black to ground and red to the o2 wire. Set the volts to MV (millivolt) setting, the o2 should fluctuate between 100 and 1000 mv. If it dosen’t, it is not good.
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The mass airflow sensor’s purpose is to measure the mass amount of air entering the engine. The mass sensor is usually located on the intake inlet running to the intake manifold. Sometimes if there is a problem with it you can tap on it gently in the electronics area, and the engine should surge. There are multiple types of mass airflow sensors. Hot wire airflow meters use a current to keep a wire at a constant temp. The heat is carried away from the wire dependent on the airflow and temp. The ptc resistor varies with temperature change, as the temp drops so does the resistance. As the resistance changes the current has to increase to keep balance. This is sent to the ecu in a form of a voltage change. Another system is the hot film. It is very similar to the hot wire mass airflow meter. It heats a wheatstone bridge board, The air moves across the board and is measured on the back of the board. The thermistor measures the change in temp and thus the airflow change. The intake air temperature sensor also contributes to this equation being that humidity can slightly throw off the density reading. A vane airflow sensor simply rotates a flap as air comes in.
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A knock sensor is generally used on turbocharged or high compression performance engines. The knock sensor is generally screwed into the engine block for all cars. To test the knock sensor, generally you can tap a wrench on it as it runs. The engine should change idle speed as the timing retards. If the idle speed doesn’t change, try again at a slightly higher rpm. If the speed once again dosen’t change the knock sensor probably doesn’t work. High performance engines are alot more prone to detonating when they are under boost. The timing on a turbocharged engine should retard when the boost comes on. If the boost would go too high the timing will be too advanced for the higher boost and will detonate. If the engine detonates it builds very high pressure waves. This can damage the engine’s pistons, bearings and crankshaft. The knock sensor senses the very high frequency pressure waves ocurring inside the cylinder. If the knock sensor is tightened too tight or is too loose it can effect the measurable frequency band. The tightening torque depends on the knock sensor specification for that engine. If it is incorrectly tightened it will sense other vibrations in the engine and incorrectly believe that there is a problem. The knock sensor works by simply retarding the timing until the detonaton quits. The driver should feel the significant decrease in power as it retards. It saves the engine from detonation which will break pistons and burn up head gaskets.
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