Firstly, you should know why we want to change the exhaust to a high performance setup.. Every vehicle of any make/model that rolls off of the production line has an ill equipped air intake and exhaust system. Restrictions are put in place on both of those systems to limit the amount of noise a consumer will hear. I suppose auto manufacturers have to appeal to a very broad audience as far as buyers, so they make the vehicle as quiet as can be.. The problems with that is: IT REDUCES YOUR VEHICLES POWER AND GAS MILEAGE!
A simple understanding of VOLUMETRIC EFFICIENCY (VE) can help you understand why you can get better power and/or fuel economy by helping your engine breathe better. Volumetric efficiency is basically a ratio (or percentage) of what amount of fuel/air is being drawn into your engine as compared to the actual capacity that is possible. Think about breathing through a straw. Sure you can do it just fine while your sitting down on the couch vegging out, but if you decide to go jogging then its just not going to work out so great is it?
When we eliminate the restrictions on the air intake system, we allow the engine to BREATHE much easier. We do this by putting a high flow performance air intake system on the vehicle. The amount of air that it will draw in naturally will be increased because there are no more intake resonators (restrictions used to quiet suction noise) or restriction stock paper air filters for the air to try to get through. By having more air entering easier than it used to you will not have to apply as much throttle to get up to speed. This is where your mileage can increase. Now when you want to have some fun and mash that pedal to the floor - you will have much more air available to create more power which is just plain fun. Gas mileage does not really increase when you drive like this though :(
Alright. Now, the engine can breathe in easier. Next order of business: HELP THE ENGINE EXHALE. The design of factory exhaust systems is horrible for the most part. All sorts of restrictions built in to keep that SUCK-PUSH-BANG-BLOW quiet. Mufflers are usually the most restrictive part of a stock exhaust system. The exhaust gasses basically have to maneuver its way through a series of walls, chambers, louvers, baffles, and other maze-like flippety-doo's.. This needs to be fixed!
**A note on catalytic converters: Many people are under the mistaken impression that the converter is the 'bad-guy' that is restricting your power and want to remove it. DON'T! Converters on almost all late-model vehicles are designed with HIGH-FLOW catalyst biscuits. They offer just the right amount of back-pressure needed to maintain some bottom range torque, yet flow free enough to not hinder performance at the higher RPM range. Do your research and if you find that your converter is of a bad design then do the smart thing and upgrade - Not remove! I won't even get into EPA mandated laws and fines and violations etc...
There are lots of other factors that help with the power/mileage aspect of VE such as exhaust scavenging and valve overlap, but if you want to learn more you should do some surfing.
Step 1: Select the right performance muffler. Do some research!
What we need now is a great muffler for our project. For the best performance I always choose Magnaflow mufflers because they are just plain awesome! They are stainless steel constructed inside and out and carry a lifetime warranty. They offer the same performance as straight-piping does, yet they have superior sound control and they are not going to get you pulled over..NO BACK-PRESSURE WHATSOEVER. Another fantastic thing about them is that they do not sound like a "rice-burner" or a "bumble-bee in a coffee can". You know what I'm talking about.
When selecting a muffler for your project you need to probably take some measurements to find out what size to buy. You will also need to determine what "offset" you will need - meaning: Where does the muffler inlet and outlet sit. Magnaflow mufflers are reversible so they are pretty easy, but note: NOT ALL MUFFLERS ARE REVERSIBLE. About 80% of cars/truck have a side/center offset. The inlet is either in the center or the side and the same goes for the outlet... The mufflers I am using on this Challenger are "side-side" offsets meaning that the inlets/outlets are staggered opposites from each other.
Mufflers come in standard pipe sizes and are measure I.D. (inside diameter) while pipe sizes are measured O.D. (outside). A 2" muffler inlet is made to slip over a 2" pipe. The most common pipe/muffler sizes are: 1-1/2", 1-3/4", 2.00", 2-1/4", 2-1/2" and 3.00". Diesel trucks will usually have 3-1/2" to 4.00" stock sized pipe.
MUFFLER BODY SIZE: When it comes to Magnaflow mufflers and (most others too) the larger the body of the muffler the better sound control will be. There is usually a body length and body thickness measurement. Pick one that fits and also decide how loud (or not) you want your hotrod to be.