These can be found new or used.
Finally, it’s important to recognize that an exhaust system is indeed a system, and piecing together headers, pipes, and catalytic converters is never as effective as a designed system. You want to keep an appropriate amount of resistance (generally called back pressure) in the system, and make the pipes small enough to keep exhaust gases flowing at the right velocity. The exact optimum setup varies from engine type to engine type, and by the RPM range that makes the horsepower and torque. Unless you buy a complete kit (such as those offered by well-known BMW suppliers) or have a lot of time and resources to dyno-test multiple configurations, you’re generally limited to the commercially available products.
For racing, the only considerations are flow and meeting the noise limits set by the racetrack, and all racetracks have them. A large muffler generally restricts exhaust sound while flowing very well.
INTAKE MODIFICATIONS AND FORCED INDUCTION
At its most basic level, an internal combustion engine is an air pump. Air is sucked into the front and pushed out the back. In this chapter, I examine how air is sucked in through the intake and air filter to the intake manifold and the combustion chambers. In the case of turbo-charged and supercharged engines, I discuss how air is pulled through the turbocharger, then pushed through the intercooler into the engine.
Performance enthusiasts should never lose sight of the fact that all of the air that the engine is going to use to make horsepower has to come in through the air filter, and has to go out through the tailpipe. You need to remove horsepower-robbing restrictions from the entire path. If you were watering your lawn and realized not much water was coming out, and you looked at the length of hose and realized it was kinked in the middle, you’d unkink it and fix the problem, right? That’s basically what you have to do with the engine’s intake and exhaust systems. You must figure out where the kink in the hose is (where the restriction is happening) and correct it. That’s the key to performance.
Here’s a great example of a cold-air box; air is ducted into this box from outside the car. It has no chance to heat up next to the engine before it is used. This means the air is denser and helps make more power. This can be a real improvement.
The cold-air intake is simply better for filtering air and getting it into the engine. The stock system was designed to be inexpensive to produce, and to keep water and particles out of the engine no matter what you do to the car. The CAI is designed to be efficient. The materials don’t matter to the function, but that carbon fiber sure looks great.
A turbocharger offers a big step up in performance, but it costs a lot of money and you’re way out of the safety zone with any kind of forced induction. Plan on spending a lot of money if you go this route.
Within reasonable limits, all products and upgrades discussed in this chapter are aftermarket solutions applicable to naturally aspirated E36 engines. No North American–market E36 models used forced induction from the factory, so any such modifications depend entirely on aftermarket parts. Those projects are generally too involved for the home mechanic to undertake, so although I discuss at them, I didn’t attempt them myself.
An internal combustion engine is a marvelous piece of technology. For the most part, modern engines run longer, smoother, more reliably, and produce more power per cubic centimeter of displacement than any mass-produced engines ever sold to the public before.
A bewildering array of products claim better flow, more pounds per square inch (PSI) of boost, and a cooler intake charge. Generally, more power is what you’re after. But before you dive into your project engine and install a bunch of parts, you need to develop a build-up plan, because each part affects the function of other parts. The engine operates as a system and its parts are interdependent.
And because an engine is a system based on the flow of gases, the tightest point in the system generally governs the total output. The classic demonstration is to attempt to breathe through a drinking straw. It doesn’t matter that there’s a room full of air, or that your mouth could gulp in a lot more air; the total oxygen available in that case is what you can suck through that straw.
What this means in real terms is that you may see an incremental improvement in power by relieving a restrictive component in the system, but real power gains require thoughtful modification to the entire system for the most efficient flow and greater energy output.
The Science of Combustion
A tablespoon of gasoline and a quart of air have a finite amount of energy potential held within them. We can change that potential into different forms of energy such as heat, motion, and light by putting the fuel and air into an internal combustion engine. You can theoretically create perfectly efficient engines and drivetrains, but you can never get more energy out of that spoonful of gas and bottle of air than the native elements hold. So to make more power in your car, you have to put more of those elements through the system, and make the system as efficient as possible.
Inefficiency can include such flaws as not burning all of the fuel you put into the combustion chamber. These are the “unburned hydrocarbons” that are measured in many emissions tests. This happens when the air/fuel mixture is incorrect. The theoretically perfect mixture is called stoichiometric and is about 14.7 parts air to 1 part gasoline. The reason you care is that if you have an imperfect mixture, you’re not getting all the energy you can out of the fuel and air.
To that end, everything you change is about putting more air and fuel through the system. I discuss replacing the stock air intake and filtration with a more direct and less restrictive product, and how to make that air flow faster into the engine with the use of a more advantageous intake manifold. Finally, I look at aftermarket solutions to change the engine to forced induction, with the caveat that such modifications are typically far more expensive than simply installing a better engine or purchasing a later model 3-series that was turbocharged from the factory.
The easiest step in improving engine performance is to look at your E36’s air intake. You want the least restrictive system you can possibly find that provides clean cold air to the engine. You really do need a filter, though, because all kinds of stuff can be sucked into the engine otherwise. The front end of every car is sprayed with road debris all the time. Nothing good has ever been reported about feeding dirty water and rock chips into a BMW engine.
Here’s what most stock air filters look like after a few years. That slab of paper never flowed very well, and now it’s
