Dual exhaust
07-12-2011, 01:02 AM
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Dual exhaust
Need to put a new muffler on my 96 xj and figured I might as well put dual exhaust in it as well. Any suggestions for rigging it or parts/systems that work and sound good. Thanks guys
07-12-2011, 07:28 AM
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Year: 1991
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Engine: I-6 4.0 HO
Backpressure is a myth when concerned with MPFI systems. You only need worry about it if you have a carbed system. As for the exhaust, just run a 2.5" all the way back with a high flow cat and your choice of muffler.
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07-12-2011, 10:30 AM
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Explain to me how backpressure allows for any benefit in a MPFI engine? If you had a carb then backpressure was essential because a sudden rush of oxygen into the CC could cause lean burning, which after a period of time burns valves and other wonderful surprises. Introduce electronic controlled fuel injection. With O2 sensors. When the sensor detects a lean burn, the computer calls for more fuel to be injected. Now, Im not saying that you can or should run a FI engine with no header or anything crazy like that. However, backpressure does not relate to torque or performance in the way everyone seems to think.
07-12-2011, 11:53 AM
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Go to the vendor section and find mudfrog's threads. There are 2 of them; one with the exhaust kit (stainless steel 2.5" mandrel bent tail pipe and flowmaster or magnaflow muffler of your choosing [personally, I bought the Flowmaster Super 44, love it]) and the other about the high flow cat. Great prices, fast shipping, and the result is awesome.
07-13-2011, 11:25 AM
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Explain to me how backpressure allows for any benefit in a MPFI engine? If you had a carb then backpressure was essential because a sudden rush of oxygen into the CC could cause lean burning, which after a period of time burns valves and other wonderful surprises. Introduce electronic controlled fuel injection. With O2 sensors. When the sensor detects a lean burn, the computer calls for more fuel to be injected. Now, Im not saying that you can or should run a FI engine with no header or anything crazy like that. However, backpressure does not relate to torque or performance in the way everyone seems to think.
So maybe backpressure isn't exactly the correct term/theory, but for 95% of the people on here who are trying to make a box lifted on mud-terrains go faster, it's close enough.
Besides my brief stint that I had with my cherokee, a lot of my other time was more with performance-oriented platforms... and no, that doesn't mean backyard-engineered camaros/mustangs/etc.
Here's part of an article that I've had saved for a while now. In short, size does matter(oh how I hate when I hear that). A lot of people do have a misconception about backpressure specifically, but the overall idea is "close" in regards to low and high-end power.
III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.
The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of 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 - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.
The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of 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 - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.