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Discussion Starter · #1 ·
Wasn't sure if this was the right place to put this, but I figured it would be better than in the engine section, and it's not really anything for the off topic, since it deals with my Versa. Anyway...

I stopped by Car X today to get an estimate to fix my exhaust. While I respect everyone that does automotive work, the owner said something that kinda surprised me. He told me that aftermarket exhaust systems actually decrease fuel mileage, because the oxygen sensors work off of the back pressure in the exhaust system, and when the back pressure is reduced, the oxygen sensors tell the computer to add more fuel to increase it.

I'm sorry, but, from my education from WyoTech, that is completely wrong, or did one of the top ranked automotive schools in the nation give me the wrong explanation of what an oxygen sensor does? I highly doubt it.
 

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If you have been educated at WyoTech you know way more about this kind of stuff than I do. My days of working on cars goes back to the days when cars were not computer controlled (1960's to early 1970's vintage). Back then adding a freer flowing exhaust could pay huge dividends in performance and mileage, especially if you tweaked the intake side (carburator).

I am not taking the side of the guy from Car X (your post), but what I believe I know about today's computer controlled cars, is that there are many inputs coming from sensors (like the O2 sensor) to the computer that controls many aspects of how the car runs. The computer is programmed to operate within specific parameters. If something is changed that would greatly affect what a sensor should be reading, the input going from the sensor is telling the computer something isn't quite right, causing the computer to adjust something (like more fuel). To gain the most advantage from an upgrade in today's cars, the programming within the computer may need to be adjusted so it knows how to properly deal with the inputs. I am sure you understand this at a much deeper level than I do.

I am not exactly sure what the O2 sensor does, I suspect it is monitoring some aspect of the exhaust gasses to provide feedback to the engine computer. Whether backpressure is a property being monitored, I wouldn't know but it could be possible. Perhaps with less backpressure there is less contact time of the exhaust gas with the O2 sensor because it is flowing by the sensor quicker. Less contact time would possibly cause the sensor to detect less of whatever gas it is monitoring and that would affect the input going to the computer.
 

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On small motors, if you have a large diameter exhaust you can actually lose performance. This is true for say an exhaust system with a header, test pipe, and 3 inch exhaust. But, this is for NA motors that are small because they DO need backpressure to run correctly. However, any sort of reputable brand of catback will have already tested such things.

On my Honda the biggest you want to go on a NA setup is 2.5". I went with 2.25 and now have a 2.5 and notice zero difference between the two...but a 2.5" exhaust is huge for a little 2.2.
 

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The old e36 318ti, and the new touring are 2 different cars. I would have to say the old 3dr ti is faster than the new 5dr, but I have not had the chance to drive the 5dr.
 

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Don't forget how much more the Beamer costs over the Versa. Granted, the interior is probably a bit nicer and it probably has more standard features, but still.
 

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Discussion Starter · #12 ·
If you have been educated at WyoTech you know way more about this kind of stuff than I do. My days of working on cars goes back to the days when cars were not computer controlled (1960's to early 1970's vintage). Back then adding a freer flowing exhaust could pay huge dividends in performance and mileage, especially if you tweaked the intake side (carburator).

I am not taking the side of the guy from Car X (your post), but what I believe I know about today's computer controlled cars, is that there are many inputs coming from sensors (like the O2 sensor) to the computer that controls many aspects of how the car runs. The computer is programmed to operate within specific parameters. If something is changed that would greatly affect what a sensor should be reading, the input going from the sensor is telling the computer something isn't quite right, causing the computer to adjust something (like more fuel). To gain the most advantage from an upgrade in today's cars, the programming within the computer may need to be adjusted so it knows how to properly deal with the inputs. I am sure you understand this at a much deeper level than I do.

I am not exactly sure what the O2 sensor does, I suspect it is monitoring some aspect of the exhaust gasses to provide feedback to the engine computer. Whether backpressure is a property being monitored, I wouldn't know but it could be possible. Perhaps with less backpressure there is less contact time of the exhaust gas with the O2 sensor because it is flowing by the sensor quicker. Less contact time would possibly cause the sensor to detect less of whatever gas it is monitoring and that would affect the input going to the computer.
From what I remember from WyoTech (It was back in 08, and there's been a lot of alcohol between then and now), the O2 sensors work by measuring the amount of oxygen (Hence 'O2 sensor'), and some of the other gases, depending on how complex the sensor is. I am almost positive that it does not measure backpressure though. Therefore, just cutting off the muffler shouldn't affect how the O2 sensors operate, unless you cut the catalytic convertor out.

Anyway, the catalytic convertor provides backpressure too, since the honeycomb inside it provides a restriction, so, even without my muffler, it's not like it's truely straight piped.

Before the exhaust pipe broke off of the flange, I was getting about 24 mpg combined. Now that it's not restricted by the muffler, I'm up to around 26 mpg, so I don't think that it adversly affect the fuel mileage any.

On small motors, if you have a large diameter exhaust you can actually lose performance. This is true for say an exhaust system with a header, test pipe, and 3 inch exhaust. But, this is for NA motors that are small because they DO need backpressure to run correctly. However, any sort of reputable brand of catback will have already tested such things.

On my Honda the biggest you want to go on a NA setup is 2.5". I went with 2.25 and now have a 2.5 and notice zero difference between the two...but a 2.5" exhaust is huge for a little 2.2.
Yea, I know that too big of an exhaust system can adversly affect fuel mileage. The biggest that I would even want to do on my Versa is a 2", if I even go that big.
 

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On my Honda the biggest you want to go on a NA setup is 2.5". I went with 2.25 and now have a 2.5 and notice zero difference between the two...but a 2.5" exhaust is huge for a little 2.2.
Exhaust size depends on the motor and supporting modifications... 2.5 is not huge for a 2.2, its ideal.
 
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