03-17-2010, 06:48 PM | #16 | |
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Location: SW Ontario
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It's useful in covering up for what is an otherwise incorrectly designed exhaust for the application. |
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03-18-2010, 12:04 AM | #17 | ||
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03-18-2010, 09:26 AM | #18 |
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Beg all you want.
You're still wrong. |
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03-18-2010, 10:18 AM | #19 | |
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Location: Volcano, Ca
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03-18-2010, 11:24 AM | #20 | ||
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I can appreciate where it seems like a certain amount of back pressure is beneficial, because it seems like that if the rest of the system is not optimized for a free-flowing exhaust. I gotta agree with suprf1y on this; an optimized induction system does not benefit from exhaust restriction. As an extreme example, Top Fuelers and Funny Cars don't use anything other than straight pipe that is tuned to the application: wide open throttle for 1/4 mile. A regular carburated passenger car or motorcycle that has had the muffler removed (with no other changes) will not run as well as it did with the muffler, because the system is not optimized for the additional airflow. Modern fuel injected vehicles are another matter. Due to advances like mass airflow sensors, computer controlled vehicles can adjust on the fly and increase pulse width (injector "on" time) to add more fuel to suit the application.
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Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer |
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03-18-2010, 11:40 AM | #21 | |||
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03-18-2010, 11:45 AM | #22 | |
Join Date: Mar 2010
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In a ‘perfect world’ or the new, modern world of computerized engine controls and sequential port fuel injection adjusting the entire vehicles manners ‘on the fly’, yes, you are correct that zero back pressure is best. With the ability to constantly adjust injector pulse width and a continuously changing timing curve, all babysat by sensors, actuators and ECM’s, regardless of altitude and temperature…..zero backpressure will optimize your overall performance across the power curve. Thanks to modern science, the new vehicle is capable of maintaining a 14.7/1 ratio, richening and leaning it as you accelerate and decelerate and maximizing burn and power by that and timing adjustments due to demand, MAF sensors, MAP and BAR sensors, knock sensors, etc……..all while also keeping a zero back pressure, ‘tuned’ exhaust system silent for the modern, quiet running vehicle. Now….let’s get to the ‘real world’ of what we are dealing with here, fixed timing curves and antiquated carburetion with preset jetting. Of course, opening up the stock exhaust system to be totally free flowing will create more top end power. It will also lean the heck out of it, especially if you open up your breather. Hence….the need to richen the jetting or you eventually cook exhaust valves, piston domes and your bank account. Unfortunately, this freeing up of the intake and exit breathing also eliminates an inherent, planned ‘design flaw’ of the older engines and that’s that a little back pressure was actually designed into these old systems to in a sense, double charge the air/fuel mixture by reversing the flow a fraction of a second in the carburetor bore when demand is increased….creating the side effect of midrange torque and also, cooler running exhaust valves, seats and piston domes. But yes, you are correct…..in today’s world of modern vehicles….or specifically designed racing vehicles, zero back pressure is, by far, best. |
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03-18-2010, 11:45 AM | #23 |
Join Date: May 2007
Location: Altamont, Kansas
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Modern systems can adjust, but we need to focus on a vintage dirt bike with old technology.
How do we get the results of peak power and torque where it is most useful?
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You meet the nicest people on a Honda Clone. |
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03-18-2010, 12:44 PM | #24 |
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There's no such thing as zero back pressure. Aint gonna happen! Even gas has mass and it takes energy to set in motion. Add to that the drag as the gas moves along past the exhaust valve(s), throught the head and down the pipe... All that will add up to backpressure.
Think of the gas column as a big spring that stretches and compresses with each exhaust pulse (which is how it works). According to the timing that spring will either work for or against you. A "tuned system" is made so that the ex. valves open when the "spring" is stretched and can pull the waste gasses straight out of the cylinder. Back pressure is important in that it can be used to tune the timing of the pulses where they will be effective in the lower RPMs where most street engines are used. A race engine usually needs it's power on the top end, the bottom end dosen't matter so we go with a WFO exhaust with little backpressure. |
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03-18-2010, 12:55 PM | #25 | |
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Other than that I have nothing to contribute, I've never engineered an exhaust system.
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Happy to serve. |
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03-18-2010, 01:16 PM | #26 | |
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I've considered welding in an O2 sensor bung in the exhaust pipe and experimenting with a wide-band O2 sensor and a voltmeter. Of course, being a cheap China Rider, I'd need to get an O2 sensor for free.
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03-18-2010, 01:24 PM | #27 | ||
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This thread has been very informative for me, although I still don't know how to properly optimize the exhaust system in the bike we're building. Trial and error, I suppose.
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Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer |
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03-18-2010, 01:34 PM | #28 | |
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03-18-2010, 04:14 PM | #29 | ||
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Happy to serve. |
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03-18-2010, 09:24 PM | #30 | |
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Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer |
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