Gas Mileage

[vertigospiral88]

Backpressure creating torque is a complete myth, and has been proven so several times. Most of the time low-end torque is reduced by changing the frequency of the exhaust pulses from that which is usually designed into the exhaust manifolds and in some cases the downpipe depending on factory R&D (on N/A cars anyway, on turbo cars the exhaust pulses are effectively terminated by the turbo itself, so the only scavenging effect that you can really generate has to be done with just the exhaust manifold) and terminated by the cat (if you still have one).

Low end efficiency is helped by specially designed exhaust that allows the frequency of the "pulses" of exhaust to help to draw out exhaust in between compression strokes of the crankshaft (by creating distinct pulses, the areas of low pressure in between help to move the exhaust out of combustion chamber because gasses follow the path of least resistance). This effect is known as "scavenging." At higher RPM, the volume of exhaust gas out flows the area allowed with factory designs, so in order to create more horsepower you have to open it up. Because you (probably) don't have the time, money, tools, or inclination to R&D a larger exhaust that can maintain the proper frequencies for optimum scavenging effect, you loose some low end efficiency to gain top end horsepower.

"Backpressure" is a completely different physical phenomena, and is bad for exhaust flow and horsepower (top or bottom end), period. Otherwise you would feel a "low end power drop" when you install a flowmaster muffler on a stock exhaust, or cut the cat out of stock exhaust. If you think you do, then you're wrong, because dyno's have proven time and again that you gain horsepower AND torque by reducing backpressure.

By the way, you're correct in that oxygen sensors have an average service life of 80k miles, but if you run too rich (fouled plugs, injector's failing open, etc.) you can easily ruin one much faster than that.

I don't have time/focus to read through this whole thing, but you have a similar setup to what I have on my '01, so a couple of quick observations.Opening up the exhaust reduces low-end torque, which also reduces efficiency at lower engine speeds due to too little backpressure. I think this is where my efficiency went shot, and probably where yours went also. (I get similar mileage)I have not found a solution to my problems yet, nor do I anticipate finding one any time soon. I have no money to sink into R&D on the subject, preferring to send it all to Iran and the Middle East via my gasoline purchases.Replacing an O2 sensor will do nothing unless the sensor is high mileage (they usually go at 75k-100k).

 

[krakhedd]

I know decreased backpressure helps at the top end, but I've heard for my entire life, and even more recently from some of the more "intellectual" individuals on here, that opening up the exhaust too much causes problems, I believe for the same reason you claim that properly tuning an exhaust helps to move the exhaust gases out. If your exhaust tubing has too large a diameter, the exhaust gases 1) don't stay as compressed, and so lose their heat more quickly, and 2) have more surface area of the exhaust pipe to lose that heat to. Thus, there is an optimal diameter for exhaust to run. Plus, I have a Borla cat-back, so I don't think I need to do any of my own R&D. I certainly have the inclination - if I were so rich I didn't have to work, or if I even made more money than I need to get by on, I would build vehicles all day long - but you're right, I am certainly lacking in time and money. For the record, engines are a compromise. You compromise between low- and high-end torque. Opening up the exhaust (btw, I drilled out my CC, leading to a further reduction in low-end torque, but at the gain of plenty of mid- and high-end) will certainly help the motor to create more power at higher revs, but sacrifices low-end, and therefore, low-end efficiency. (NOTE: Can somebody please tell me why I'm having so many script errors preventing me from separating my words by creating paragraphs? I also cannot use italics/bold/change font size/etc., thanks)

 

[vertigospiral88]

Hmmm, not sure on your script errors, are you placing tags inside the "[" "]" characters? As for seperating for paragraphs, I just hit the enter button like you would in word and it seems to work ok for me....

At any rate, thermal expansion is definitely an issue, but that has nothing to do with backpressure. I'm not going to argue with you about whether or not you lost low-end power when you eliminated you cat, but I will say that most people can be proven wrong about that on the dyno, and usually it's because they only gained 3 lb/ft at 1,500 RPM at 18 lb/ft at 4,000 RPM. Since you gained MORE power up top, it feels like you lost power down low when in reality you only gained LESS power.

When I was talking about designing exhaust, I wasn't talking at all about cat backs, mostly because as I pointed out the scavenging effect is pretty much lost by the time you get to that point in the exhaust (the pulses or the exhaust "wave frequency" breaks down due to turns and bends in the system... If you had a long-ass streight pipe, on the other hand....). For the most part, the best scavenging effect has to be designed into the exhaust manifold, since that's where the frequencies are doing the most good.

I wasn't trying to be desparinging towards how much money/time you have, I was speaking in generalities since 99% of the population couldn't afford the time/money/tools/engineering degree required to optimize theoretical frequencies in exhaust.

I am well aware that "engines are a compromise," however modifications that change the fluid dynamics within a motor can be made to increase power across the powerband, the question is where you want to make the MOST power. Internal engine components that change the timing of the motor (most notably camshafts) are a different story, because they can change the powerband itself and therefore remove power from one place and add it somewhere else (with proper supporting modifications).

I don't disagree with the end result of using too large an exhaust that you posted originally, you just happened to put in there that reducing backpressure causes you to loose low end torque, which is an automotive myth (or misunderstanding, if you prefer), that is mostly propragated by car/truck forums because people think it's easier to type that in rather than explain what's really happening to the exhaust, and that causes people that only learn from these forums and have little/no real-world experience to think that's the way that it works, who then tell their friends etc. etc. etc. I just wanted to set the record streight, in that it's not backpressure (or lack thereof) that causes people to loose low end power, but other factors that can do so by overestimating the diameter of the exhaust that is needed.

At any rate, yes, thermal dynamics and the volume of exhaust can effect when/how much power is there, but neither of those things are backpressure. Backpressure is a direct obstruction of exhaust flow (like a cat or a muffler), that causes the exhaust to "push backwards" towards the engine again, which must be overcome by the flow of exhaust that is coming from the motor. This, obvisouly, can hurt power/performance and is under no circumstances a good thing.

 

[krakhedd]

No script errors, typing on my parents' computer, so I can actually make paragraphs - yay.....(pressing enter doesn't work on the work computer that has the script errors)

Anyway, my father explained it to me once, about backpressure/whatever, when I was arguing YOUR side against the one I'm arguing now. He doesn't recall exactly what he read, but he is a very skeptical man, so doesn't read "sketchy" publications. Being that the guy is incredibly intelligent, despite being almost 72, I tend to believe him when he starts off a sentence with "I read ______".

I will say one thing on this issue, and I don't know if it's my IAT relocation or what (my Jeep's in pretty bad mechanical condition right now, almost everything except the motor and the tranny, but especially the rest of my drivetrain, needs fixing/replacing, so I try not to floor it), but my Jeep in the past has felt like it has less off-the-line than my brother's bone-stock 99. Now, my IAT relocate robs me of a little low-end torque, and a hell of a lot of mileage, so that could begin to explain it. However, once I hit about 2k revs, my brother's stock motor has nothing on mine.

Ok, that's a thousand yards off the port bow of the original topic. I'm done.