Was talking with my techs about exhaust systems in general and bypass valves in particular and that prompted me to develop a better understanding of the topic. I did a bit of research (the Internet is wonderful thing) and found some useful information that I put together and am posting here. Would enjoy getting some comments and feedback. Regards, Steve http://www.forzacomponenti.com/ ====================================== Basic Theory An automotive exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. An 8 cylinder engine motor will have 8 distinct pulses per complete engine cycle and a 12 cylinder engine will have 12 pulses. As more pulses are produced, the exhaust flow becomes more continuous. Back pressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream. With any modern engine, there is inherent back pressure within the exhaust system consisting of the exhaust manifold, the catalytic converter, the system muffler (silencer) and the connecting pipes. While one would think that lower back pressure would be better, that is not always the case when designing an exhaust system for a modern engine. Automotive exhaust designers produce systems that balance exhaust flow capacity with velocity. The objective is to evacuate the exhaust gases as fast as possible. At lower engine RPM and lower exhaust gas volume, the velocity of the exiting gases is lower. If one could artificially impose a restrictor in the exhaust system that would increase the back pressure, the velocity of the exiting gases will increase and this will improve the evacuation of the gases from the engine. A simple comparison is water flowing from a garden hose. If you let water trickle from an unrestricted hose, the water flow rate (velocity) will be low. If you cover the hose opening with your finger, you will increase backpressure and the flow rate will increase. But, at some point, if you cover the opening of the hose too much, you will have caused so much backpressure that the velocity then declines … eventually to zero. Why is Exhaust Velocity Important? The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the gas behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This is why I mentioned exhaust pulses above. It is because of these pulses in the exhaust combined with velocity that tend to produce this scavenging effect. You may have noticed that turbo charged engines tend to have larger exhaust pipes. The reason is simple. Since the turbo is in the exhaust stream, the gas flow spinning the impeller tends to come out of the turbo with pulses greatly diminished, which tend to negate the scavenging effect. So turbo charged cars typically have larger pipes to accommodate gas volume — leaving the turbo to perform the task of scavenging the spent exhaust gases. Dealing with Variable Exhaust Flow Rates in Modern Engines Automotive engineers design exhaust system taking into account many factors, including the number of cylinders (pulses), the power curve they are seeking and environmental considerations. The maximum scavenging effect is achieved when the exhaust velocity is high and time between pulses. So, when engine RPM is low and the total volume of gases is correspondingly low, the designers typically utilize smaller diameter pipes to create a bit of back pressure. But as engine RPM increases, the volume of gas increases and the time between pulses decrease, which in turn reduces the scavenging effect. Now, back pressure becomes a bad thing. In these conditions larger diameter pipes are appropriate to reduce the backpressure and accommodate the increased volume of gases. These variables present a challenge when designing effective engine exhaust systems. At lower engine RPM, the flow rate is low and we want to induce some backpressure to improve engine efficiency and power. While at higher engine RPM, the engine is producing more exhaust gases (higher volume) and we want to reduce backpressure. The trick is to have an exhaust system that will evacuate the gases as quickly as possible (high velocity) at the RPM range that you want your power band located. As discussed above, smaller exhaust pipe diameters are best suited at lower engine RPM when the volume of gases is small. But at higher engine RPM, a smaller exhaust pipe will then create unacceptably high amounts of backpressure and reduce the exhaust velocity when the engine is producing greater volume of gas. This will have a negative effect on power output and engine efficiency. When backpressure is not optimized in the exhaust system, there is a tendency to reduce engine efficiency resulting in decrease in power output and higher fuel consumption. To work around the RPM specific nature of pipe diameters and their effect on backpressure, engineers may use setups that have the effect of changing the pipe diameter on the fly. Advanced exhaust systems (e.g., Ferrari, Maserati) have two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure and improve flow velocity. Since there is greater exhaust volume, there is no loss in flow capacity. Exhaust Bypass Valves Ferraris and Maseratis (and other vehicles as well) utilize exhaust bypass valves to achieve two distinct flows for the exhaust gases. Generally, the exhaust bypass valves are positioned behind the catalytic converters and before the silencer. With the exhaust bypass valve closed, the gases have only one path … through the silencer. This increases backpressure and improves the velocity of the escaping gases through the system resulting in improved power at lower RPMs As RPM increases, the vehicle ECU will open the exhaust bypass valve. This can be a progressive opening that occurs over a range of RPM, but generally the opening of the bypass valves occurs immediately when key parameters are achieved (e.g., engine RPM and throttle position). When the bypass valve opens, it presents a second path for escaping exhaust gases that typically “bypasses” the muffler. This has the net effect of reducing the backpressure in response to the increase in flow rate thereby maintaining (or improving) the velocity. In so doing, the engine is more able to achieve higher efficiencies and power at the elevated RPM. Manually Controlling the Exhaust Bypass Valves If your vehicle comes equipped with exhaust bypass valves and you want to optimize power and engine efficiency for normal driving, you should not employ manual methods for controlling the bypass valves. Let the vehicle’s ECU manage that for you. So, why would you want to manually control the valves? I can only think of two reasons: 1. You are racing or tracking the vehicle. In these situations you are likely going to be keeping the engine RPM in the higher ranges and your throttle will likely be positioned more open than closed. You want the least restrictive exhaust so you can maximize exhaust gas velocity. 2. You want to hear the glorious sound from your engine. The immediate effect of an open exhaust bypass valve is to “bypass” the muffler (at least for a portion of the exhaust gases) with the net result of a louder exhaust note. More than likely, the only reason one wants to manually control the bypass valves is the latter. You want to make some noise. By manually controlling the bypass valves, you can make that noise any time. And, you can return control of the bypass valves to the vehicle’s ECU when you want to quiet things down … e.g., coming into your neighborhood you don’t want to disturb the neighbors … or maybe to deny the local constable from having another excuse to have a conversation with you.