Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian

Stroke

      The piston moves upward and compresses the air/fuel charge. The air/fuel charge is ignited before the piston arrives at TDC on the compression stroke, with peak cylinder pressure taking place just after TDC on the power stroke. Pumping loss is greatest at WOT during the compression stroke, which is increased with a higher compression ratio and with a more-dense air charge.

      More force is required to compact the air charge, so more pumping loss is encountered. However, the pumping loss created during the compression stroke is negated during the power stroke, so the pumping loss generated during the compression stroke really isn’t a problem.

      Power Stroke

      Once ignition has occurred, the piston is pushed downward, applying power to rotate the crankshaft. As the piston moves downward on the power stroke, cylinder pressure drops substantially and progressively as the piston moves farther down in its bore. The power stroke does not create pumping loss.

      Exhaust Stroke

      The piston moves upward as the exhaust valve is open, pushing the exhaust out of the cylinder head. The pumping loss variables that occur during the exhaust stroke are dependent on restriction in the entire exhaust system. If the engine is equipped with nitrous injection, the added oxygen and fuel creates an increase in exhaust pressure. As the piston fights this added pressure during the exhaust stroke, pumping loss increases. To evacuate the exhaust more efficiently, a camshaft with a longer exhaust duration and opening the exhaust valve a bit earlier can help.

      Pumping losses are normal and cannot be eliminated. The best way to reduce the energy wasted by pumping losses is to let the engine breathe, through the use of low-restriction air intakes, relieving crankcase pressure, valve timing that allows the exhaust gas to leave and pull air into the cylinders, and exhaust systems that reduce restriction and scavenge exhaust pulses out of the engine.

Oxygen Sensor Technology

      All production vehicles for the U.S. market made in 1996 and later feature OBD-II diagnostics and require two oxygen sensors: one before the catalytic converter and one after the converter. The ECM uses the oxygen sensor located before the converter to adjust the air/fuel ratio. The oxygen sensor located after the converter is primarily used for catalytic converter efficiency control and monitoring.

      When you see oxygen sensors in a catalog or repair manual, you may notice the terms S1 and S2. The S1 indicates that the oxygen sensor is located before the catalytic converter, and the S2 indicates that the oxygen sensor is located after the converter. On a V-type engine, the bank location of the sensor(s) is indicated by B1 or B2 (bank 1 or bank 2). For example, an oxygen sensor identified as B1 S1 indicates the location as bank 1 and before the converter. Bank 1 usually refers to the left (driver-side) bank and Bank 2 as the right (passenger-side) bank of cylinders.

      The ECM uses the information provided by the oxygen sensor signal to manage the air/fuel ratio. Using this signal, the ECM’s fuel management program adjusts the amount of fuel injected into the cylinder. The two most common types of oxygen sensors are the narrow-range oxygen sensor and the wide-range oxygen sensor, which is also called the air/fuel (or A/F) sensor.

       Although an oxygen sensor functions differently than an air/fuel sensor, the goal is basically the same: to monitor the amount of oxygen in the exhaust stream, so that the electronic control module (ECM) can adjust the engine’s air/fuel ratio by richening or leaning the mixture.

      The A/F sensor appears similar to the oxygen sensor, but features different construction with different operating characteristics. The A/F sensor is referred to as a wide-range sensor due to its ability to detect air/fuel ratios over a wide range, so that the ECM can more accurately meter the fuel in an effort to reduce emissions. While oxygen sensors operate at around 750 degrees F, the A/F sensor operates at about 1,200 degrees F. An A/F sensor also changes current amperage output in relation to the amount of oxygen in the exhaust system, providing the ECM with more accurate air/fuel ratio information.

      The A/F sensor is calibrated for stoichiometry, the theoretically ideal air/fuel ratio of 14.7:1. The ECM uses any deviation from this ideal to adjust the fuel mixture and fuel injection time/duration. When the vehicle features a three-way catalytic converter, the main heated oxygen sensor is temperature-controlled by the ECM. When the air intake volume is low and the temperature of the exhaust gas is low, current flows to the heater to heat the sensor for accurate oxygen content detection.

       A bung fitting can easily be installed on a header collector for oxygen sensor or air/fuel sensor installation either to accommodate an electronic fuel management system or simply to monitor an engine’s air/fuel ratio for fine-tuning purposes.

CHAPTER 2

       EXHAUST SYSTEM COMPONENT DESIGN, FLOW AND FUNCTION

      You need the right plan, exhaust system design, and component selection. You cannot simply select a random collection of exhaust parts and expect to realize top performance. You must take into account your engine displacement, head size, cam timing, intake design, carb size, and other aspects of the engine package. You also need to consider intended operating RPM, application, transmission type, and other factors. This chapter examines specific exhaust system components, including cast exhaust manifolds, tubular exhaust headers, exhaust piping, mufflers, sound-tuning resonators, and catalytic converters, along with the role that each plays.

Exhaust Manifolds