Mechanic's Corner - Sept 11
“Get to Know Your Exhaust System”
By Mike Godwin
Before we explore the topic of exhaust systems this month, let me bring you up to date on an important item that Scott Robinson brought to our attention at the July meeting. That is, the use of Ethanol in gasoline year round. I have followed up with the Robinsons, and started researching this problem area. It will make a great article, and be very informative once completed. Truth is, right now I am over a month into the research, and progress is slow. Stay with me though, and we will find the answers and get this important information out to our members in the coming months.
Exhaust systems, for the most part, hang out under the car and are not given much thought. Well, when they fail by becoming a loud exhaust, or cause our Mustang not to run because the system is blocked or obstructed, then we are very aware of where the exhaust system is located.
First, we need to understand that in the Mustang hobby, there are two very different groups of exhaust systems, and know that there are Federal Laws governing what can and cannot be done to these two very different systems. The 64 ½ through 1973 Mustangs have an exhaust system that, for the most part, can be modified any way the owner desires. Starting with the 1974 model year, the use of catalytic converters and the federal mandate for use of unleaded fuel, changes everything, and the owner is limited on what changes can be made.
In that first group of cars that extends through the 1973 model year, an owner can start at the cylinder head and add any type of header that they wish. From the header back, it is also a free-for-all with a long list of options. Local laws on noise abatement might foil some of your efforts, but pretty much the design and installation is not hindered by manmade laws. The laws of physics and metallurgy will cause you some grief, but if you look hard enough, there is plenty of written material on the subject to keep you out of trouble. These cars have been around for years, and are very popular with nearly every form of racing, so many people have gone before us and sorted out the details of what does and does not work.
Here is one tip that I picked up from a book about building a small block Ford for a Street/Drag Strip application. There is NO muffler made that can boost horse power. Now unlike the book, I will not name any manufacturer here, but do concur with the statement. An area of contention to this day is use of the cross-over pipe to connect the two sides of the exhaust system. The theory is that this pipe will provide equal back pressure to both sides of the block. Plenty of studies and many hours on dynamometer test stands, resulting in all sorts of colored graphs, but no conclusive evidence that this layout of pipes is either beneficial or a waste of material.
Starting with the 1974 model year, there are Federal laws that regulate the removal or modification of the catalytic converters. These laws gave birth to a new phrase in exhaust systems called the “Cat Back” system. These units bolt onto the output of the catalytic converter and have been tested in EPA approved labs. They are compliant with all federal regulations for exhaust emissions.
With the popularity of the Fox Body Mustangs, even headers that meet EPA regulations became available as aftermarket pieces. This same 25 year period also saw the return of the cross pipe but this time the entire system forms a large letter “X” under the car. The idea of these systems was to give the car more breathing room and at the same time, provide a distinctive exhaust note that would not violate local noise abatement laws.
With every new generation of Mustang, a new exhaust system is soon to follow. When buying, look for documented proof that the installation gives real results to the rear wheel horsepower numbers. By all means, stay within the law, and if in doubt, contact the local Federal Building and ask for information. The time and effort expended doing this is far less painful than a day in court, or paying off some very stiff fines.
Don’t forget that specialty Mustang builders often offer their redesigned systems to the aftermarket crowd. So look to the Saleen and Roush camps for proven systems that will truly be a built in option.
Watch for another article on this topic in the future, as there is plenty of information on various manufacturers’ products. In fact, there is enough test data on different mufflers alone to fill this column a couple of times.