Most of us are aware that to obtain maximum performance from our engines, the exhaust system should be configured to provide minimum resistance to the gas flow. While the exhaust pipe diameter and other exhaust system components should be large enough to reduce backpressure to a minimal level, at the same time they must be small enough to create enough gas velocity to promote best possible exhaust gas scavenging from the cylinders. In a normally aspirated engine, these dual requirements ensure that a minimal amount of backpressure will be present even in the most efficient systems - a necessary evil. In a turbo engine, after the turbo exhaust, the larger the exhaust pipe diameter the better as the need for scavenging is not then a factor.
Wouldn't it be useful if we could actually take backpressure measurements to determine what effect, if any, changing exhaust system components would have on this parameter? Thus, we could compare differences among exhaust pipe diameters, catalytic converters, mufflers and even differences between headers and the stock manifold to find the best setup to meet our needs.
But how can we DIYers accomplish this hot, dirty, task easily, safely and economically? Once again, Julian Edgar of the excellent Australian AutoSpeed online car magazine comes to the rescue. In his article, in Issue 524, March 31st, 2009, "Ultimate DIY Automotive Modification Tool-Kit, Part 1", among several other topics he discusses how one can make exhaust system measurements of the kind dealt with here. For access, go to:
http://autospeed.com:80/cms/A_111109/article.html
To measure total system backpressure, it requires nothing more than temporarily removing the O2 sensor and screwing in a replacement fitting to which a hose is attached and connected to a turbo boost gauge. If extensive testing is planned, first connect a copper or aluminum tube to the fitting so that enough heat can be dissipated not to melt the rubber or plastic hose at the other end. Since the exhaust gas doesn't actually flow through the gauge, it won't be damaged from the heat. Any mechanical boost gauge reading from 15 to 30 PSI (1 to 2 bar) should be suitable.
The Protege O2 sensor uses an M18 x 1.5 thread so it should be easy enough to cobble up a fitting from an equivalent bolt. Incidently, when removing and installing O2 sensors, make sure you use a thread chaser to clean the threads and prevent damage. My thread chaser is made by the Lisle Corporation, part #12230.
It goes without saying that the pickup should be after the turbo when so equipped. Measurements should be taken under full throttle, full load conditions in all cases.
Although Edgar doesn't deal with this aspect of the problem, keep in mind that from thermodynamic principles, exhaust backpressure is dependent on the gas temperature as well as on the mechanical layout of the system. Unless measurements are referenced to exhaust gas temperature, you can't have repeatability of results across testing sessions. The exhaust system must be up to normal operating temperature - but not more. This is one reason there is wide disagreement between exhaust backpressure as measured on an engine or chassis dyno, and backpressure measured on the road where there is significant cooling of the exhaust system.
Consequently, backpressure measured in the way described in this article without taking into account gas temperature, whether stationary or rolling, will only show qualitative differences between components, not quantitative ones. That is, we know that muffler no. 1 is the same, better, or worse than muffler no. 2 according to our measurements, but not how much better in a repeatable, numeric sense. However, this method is more than good enough for our purposes.
Edgar provides some backpressure figures on cars he's measured and you can check them out in his article. He concludes that all the usual approaches to lowering backpressure are valid, and really major reductions in exhaust backpressure of as much as 75% are possible on many cars (however, this figure may be suspect because of the lack of quantifiable data). The catalytic converter is seen as being the most restrictive component, followed by the main muffler. However, unless bends have a very tight angle, for eample 180 degrees, press-bending does not normally cause much measurable restriction. Using the next larger size of pipe and using press-bends is usually more cost-effective than buying mandrel bends.
Backpressure measurements can also be taken of the individual components that make up the exhaust system, but in order to do so, holes would have to be drilled at different points to install pickups, which after the termination of testing would have to be welded closed.
Happy Motoring!
Wouldn't it be useful if we could actually take backpressure measurements to determine what effect, if any, changing exhaust system components would have on this parameter? Thus, we could compare differences among exhaust pipe diameters, catalytic converters, mufflers and even differences between headers and the stock manifold to find the best setup to meet our needs.
But how can we DIYers accomplish this hot, dirty, task easily, safely and economically? Once again, Julian Edgar of the excellent Australian AutoSpeed online car magazine comes to the rescue. In his article, in Issue 524, March 31st, 2009, "Ultimate DIY Automotive Modification Tool-Kit, Part 1", among several other topics he discusses how one can make exhaust system measurements of the kind dealt with here. For access, go to:
http://autospeed.com:80/cms/A_111109/article.html
To measure total system backpressure, it requires nothing more than temporarily removing the O2 sensor and screwing in a replacement fitting to which a hose is attached and connected to a turbo boost gauge. If extensive testing is planned, first connect a copper or aluminum tube to the fitting so that enough heat can be dissipated not to melt the rubber or plastic hose at the other end. Since the exhaust gas doesn't actually flow through the gauge, it won't be damaged from the heat. Any mechanical boost gauge reading from 15 to 30 PSI (1 to 2 bar) should be suitable.
The Protege O2 sensor uses an M18 x 1.5 thread so it should be easy enough to cobble up a fitting from an equivalent bolt. Incidently, when removing and installing O2 sensors, make sure you use a thread chaser to clean the threads and prevent damage. My thread chaser is made by the Lisle Corporation, part #12230.
It goes without saying that the pickup should be after the turbo when so equipped. Measurements should be taken under full throttle, full load conditions in all cases.
Although Edgar doesn't deal with this aspect of the problem, keep in mind that from thermodynamic principles, exhaust backpressure is dependent on the gas temperature as well as on the mechanical layout of the system. Unless measurements are referenced to exhaust gas temperature, you can't have repeatability of results across testing sessions. The exhaust system must be up to normal operating temperature - but not more. This is one reason there is wide disagreement between exhaust backpressure as measured on an engine or chassis dyno, and backpressure measured on the road where there is significant cooling of the exhaust system.
Consequently, backpressure measured in the way described in this article without taking into account gas temperature, whether stationary or rolling, will only show qualitative differences between components, not quantitative ones. That is, we know that muffler no. 1 is the same, better, or worse than muffler no. 2 according to our measurements, but not how much better in a repeatable, numeric sense. However, this method is more than good enough for our purposes.
Edgar provides some backpressure figures on cars he's measured and you can check them out in his article. He concludes that all the usual approaches to lowering backpressure are valid, and really major reductions in exhaust backpressure of as much as 75% are possible on many cars (however, this figure may be suspect because of the lack of quantifiable data). The catalytic converter is seen as being the most restrictive component, followed by the main muffler. However, unless bends have a very tight angle, for eample 180 degrees, press-bending does not normally cause much measurable restriction. Using the next larger size of pipe and using press-bends is usually more cost-effective than buying mandrel bends.
Backpressure measurements can also be taken of the individual components that make up the exhaust system, but in order to do so, holes would have to be drilled at different points to install pickups, which after the termination of testing would have to be welded closed.
Happy Motoring!
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