After the discussion in another post (Flowmaster post) I decided to do some research. Tim was correct is saying backpressure is bad (for all RPM's). No backpressure is ever good (for HP or Torque). I may have mislead some by saying a little backpressure at lower RPM's is good....it is NOT and I was wrong.
That being said, running open headers or "open exhaust" on the street WILL hurt low end performance but it has nothing to do with backpressure, and won't necessarily cause burnt valves (although it can). Again as Tim stated richening your mixture when running open exhaust is needed to keep valves from burning as running open exhaust causes the mixture in the combustion chamber to be lean.
It is based on pressure differentials, volumetric efficiency, and scavenging (pressure waves). Searching the web will provide hundreds of articles discussing the issue (some factual, some not), but after reading and looking at formulas it does make sense. At high rpms the gasses exiting the combustion chamber are moving at a much higher velocity than at lower rpm's, valve timing is reduced (open for the same amount of crankshaft rotation, but since the rotation is faster the actual time in Milliseconds is reduced), and the physics of fluid (gas is a "fluid") combine to make exhaust size/shape irrelavant as long as it is not restrictive. At lower rpm's it becomes important to maintain volumetric efficiency (VE). Lower VE for a given rpm = less power and higher VE for a given rpm = more power. Running an open exhaust or very big chambered exhaust (size if most important at and near the head) lowers VE at low rpms and will cause a reduction in power (torque/hp). Sizing the pipes for low rpm so that they "scavenge" and create a high VE without restricting the flow at high rpms is a delicate balance. Ideally a variable exhaust along with variable valving would net the best power over a large rpm range, but until that becomes a mass market reality, we have to live with a comprimise. For any street driven vehicle, running an open exhaust is not an optimum setup. Even those running exhaust "cutouts" for occasional drag racing are not an optimum setup as they are either running lean on drags, rich on the street, or a little of both (unless they know how to tune for each occasion AND do so).
Stock motors (especially older carbureted ones) were built to work in a variety of settings (both ideal and unideal). Many performance enthusiests refer to this around here as a sloppy motor. They are great for the street as they will work well in varying enviroments and don't need as much tweeking to keep running right, but not great in a specific enviroment (i.e. racing or all out performance). A "tight" motor will work well in a specific environment but is easily thrown off by changes in the environment and is very sensitive to even minor tuning changes. Because of this, changing a component on an engine (such as manifolds to headers) will not net a large amount of power as the rest of the motor is not "setup" to notice the gains.
In looking at some articles of manifolds vs headers, it looks like headers win out in the performance range from anything at or just above idle rpms (assuming stock street motor). The reason many manufactures used (and continue to use) manifolds is that they provide a common chamber close to the head (great for very low idle efficiency) is that they performed well for idle smog checks, sealed better than headers, reduced flex, and lasted longer (at least longer than the warranty period). For a rig built for longevity, ease of maintenance, and not built to put out every ounce of power it can, manifolds are an excellent choice. Headers in and of themselves don't provide a huge power gain unless complimented with other items (mainly the rest of the exhaust). For example, put headers on a BBC that has a Y pipe and single 2.5 inch exhaust most likely will not see much if any performance gains over the stock manifolds. Many people around here still use stock manifolds (ram horns if they can find them) with custom duals. I went with ceramic headers wanting the most power I could get for pulling, but even they have suffered heavily from the salt/sand of New England. The 78 K10 already had headers installed, but when they rust out, I am planning on putting manifolds back on for ease of maintenance as it is a D/D plow truck that rarely sees high RPM use.
To each his own (as that's what makes the world go round), just wanted to let everyone know I was incorrect with what I said before (not a first and won't be the last /forums/images/graemlins/blush.gif)
That being said, running open headers or "open exhaust" on the street WILL hurt low end performance but it has nothing to do with backpressure, and won't necessarily cause burnt valves (although it can). Again as Tim stated richening your mixture when running open exhaust is needed to keep valves from burning as running open exhaust causes the mixture in the combustion chamber to be lean.
It is based on pressure differentials, volumetric efficiency, and scavenging (pressure waves). Searching the web will provide hundreds of articles discussing the issue (some factual, some not), but after reading and looking at formulas it does make sense. At high rpms the gasses exiting the combustion chamber are moving at a much higher velocity than at lower rpm's, valve timing is reduced (open for the same amount of crankshaft rotation, but since the rotation is faster the actual time in Milliseconds is reduced), and the physics of fluid (gas is a "fluid") combine to make exhaust size/shape irrelavant as long as it is not restrictive. At lower rpm's it becomes important to maintain volumetric efficiency (VE). Lower VE for a given rpm = less power and higher VE for a given rpm = more power. Running an open exhaust or very big chambered exhaust (size if most important at and near the head) lowers VE at low rpms and will cause a reduction in power (torque/hp). Sizing the pipes for low rpm so that they "scavenge" and create a high VE without restricting the flow at high rpms is a delicate balance. Ideally a variable exhaust along with variable valving would net the best power over a large rpm range, but until that becomes a mass market reality, we have to live with a comprimise. For any street driven vehicle, running an open exhaust is not an optimum setup. Even those running exhaust "cutouts" for occasional drag racing are not an optimum setup as they are either running lean on drags, rich on the street, or a little of both (unless they know how to tune for each occasion AND do so).
Stock motors (especially older carbureted ones) were built to work in a variety of settings (both ideal and unideal). Many performance enthusiests refer to this around here as a sloppy motor. They are great for the street as they will work well in varying enviroments and don't need as much tweeking to keep running right, but not great in a specific enviroment (i.e. racing or all out performance). A "tight" motor will work well in a specific environment but is easily thrown off by changes in the environment and is very sensitive to even minor tuning changes. Because of this, changing a component on an engine (such as manifolds to headers) will not net a large amount of power as the rest of the motor is not "setup" to notice the gains.
In looking at some articles of manifolds vs headers, it looks like headers win out in the performance range from anything at or just above idle rpms (assuming stock street motor). The reason many manufactures used (and continue to use) manifolds is that they provide a common chamber close to the head (great for very low idle efficiency) is that they performed well for idle smog checks, sealed better than headers, reduced flex, and lasted longer (at least longer than the warranty period). For a rig built for longevity, ease of maintenance, and not built to put out every ounce of power it can, manifolds are an excellent choice. Headers in and of themselves don't provide a huge power gain unless complimented with other items (mainly the rest of the exhaust). For example, put headers on a BBC that has a Y pipe and single 2.5 inch exhaust most likely will not see much if any performance gains over the stock manifolds. Many people around here still use stock manifolds (ram horns if they can find them) with custom duals. I went with ceramic headers wanting the most power I could get for pulling, but even they have suffered heavily from the salt/sand of New England. The 78 K10 already had headers installed, but when they rust out, I am planning on putting manifolds back on for ease of maintenance as it is a D/D plow truck that rarely sees high RPM use.
To each his own (as that's what makes the world go round), just wanted to let everyone know I was incorrect with what I said before (not a first and won't be the last /forums/images/graemlins/blush.gif)
