Gas mileage and aftermarket exhaust

I will need to replace at least part of the exhaust on my '96 Accord EX 2.2L sedan soon and I would like to improve on my gas mileage as well but I have a few questions. I know that aftermarket exhaust systems can be pricey but how much do they typically improve gas mileage and is it worth the extra money just for mileage gains? Would I need to replace from the header on down to get any real gains? Does the system need to be loud? If so who makes the quietest system or can I use a silencer without robbing too much gain? Sorry about all the questions.

For fuel economy a stock-type system is best...

Opening up the exhaust without intake mods typically gives you a slight DROP in low to mid RPM range performance and WORSE fuel mileage because you'll be stepping into it harder to get the same response from a stop...

The OE system is tuned for economy and sound control. A performance-type muffler with a stock pipe might give you slightly better performance at higher RPM while still preserving some backpressure for lower RPM torque...but I doubt the difference really justifies the cost, unless the OE muffler is really pricey.

It's hard to improve on the stock setup unless you want WOT performance and more noise...which of course isn't going to help your fuel economy...

would modding the intake, like a short RAM, help my mileage or does pretty much any performance upgrade reduce fuel economy?

Cars are engineered for four things;

Performance
Reliability
Driveability+Comfort
Fuel Economy+Emissions Compliance

Every car is a compromise of a mix of these aspects, aimed at what they think the buyer wants. In general when you enhance one aspect, something else has to come up short. So if there way a proven way to enhance performance AND increase fuel economy, they'd already be doing it. With a few slight exceptions.

Any car's engine is basically nothing more than a volumetric pump. The goal of performance is to allow more air (and a correspondingly correct ratio of fuel) to be moved into and out of the engine in the same amount of time, which gives you stronger performance. Any attempt to "open up" a motor and let it breathe requires a similar increase in fuel delivery to have any performance gain.

This is what your engine management system's job is...to sense the amount of air entering the motor, calculate and deliver the correct amount of fuel to match it, and adjust for emissions compliance. Since the goal of compliance is the least amount of combustion by-products possible, a complete burn (and thus the most efficient and powerful combustion possible) satisfies all three requirements, performance, fuel economy, and emissions compliance.

But if you open one end and not another, like intake and not exhaust, or vice-versa, you just create an imbalance in volumetric efficiency that at best does nothing, at worst can hurt performance. But with increased and balanced flow, increased fuel delivery is going to occur as well. If it didn't, you would simply have an overly lean mixture (too much air), which burns too fast, too hot, and can damage the motor. You'd be hard pressed to increase airflow beyond the motor's stock fuel delivery capability with external mods, however, so you don't need to worry about that...

There are a couple of small ways that stock performance AND fuel economy may be improved (slightly), by eliminating the slight bottlenecks at the front and back sides, namely the air filter and the muffler. Both are designed to maintain noise levels with some small amount of performance compromised to decrease cabin noise levels. Most people don't wanna hear the intake "boom" and exhaust rumble when they hit the gas...they just want a quiet, unassuming car.

Without knowing what you actually are willing to tolerate, I'd say your best bang for the buck would simply be a drop-in K+N O.E. style panel filter (about $40) and a less restrictive muffler. This rids you of some of the concessions made for noise levels, but not in a way that will be overly obnoxious. What you will notice is more intake growl on hard throttle applications, and a more noticable exhaust note. Usually the most irritating aspect is the "drone" that 4 cylinders produce at cruising speeds, due to the 180 degree firing intervals. A cheap muffler will make a lot of this, a higher end one "tunes" the exhaust note to a higher frequency, which is much less annoying (inside the car anyway).

Since this combo really makes no apprieciable difference at lower rpm ranges, it should not create a need for increased fuel delivery during normal driving, and may actually allow for lighter throttle application to be needed for moderate acceleration...which can equal a slight mileage increase. At higher RPM ranges it will flow a bit better, which decreases the feeling of the engine's power "falling off" before reaching redline...most engines' power curves turn downward quickly after peaking aound 85% or so of it's accepted safe RPM range...flattening out the curve a bit and raising the RPM where it begins to "hit the wall" will help WOT performance...not a gain you will notice much, unless you are bumping the limiter and barking the tires on every shift :)

I wouldn't go with larger than stock pipe diameter unless you are going to serious beef the motor to need it...doing so will just decrease backpressure and hurt throttle response and torque down in the bottom....overly large exhaust is probably the biggest misguided performance blunder done to most cars today...most don't have the mods to require it. To actually need it, you'd have to have a corresponding increase on the front (CAI, larger MAF and throttle body, ported intake, fuel delivery mods, maybe bumpier cam, etc)...90% of the cars you see driving around with 3" pipe and trashcan mufflers have none of this...