F-Body Modification Guide

This is a starting page for everything related to modifying your F-body. It is divided up into sections for each generation, and from there into type of modificaiton (performance, safety, appearance, etc). Note that Engine Modification is covered in a different section.

To get started, choose the generation you are interested in modifying:

• First Generation Modification Guide
• Second Generation Modification Guide
• Third Generation Modification Guide
• Fourth Generation Modification Guide

Or, read on for non-specific information.

General Modification

Engine

When modifying an engine, it's best to think of the entire engine as an air pump. Air is sucked in, and exhaust is pumped out. The more air your engine can suck in and pump out, the better. Practically every engine modification you make will fall under one or more of the following:

• increasing the amount of air being pumped through
• preventing the engine from damaging itself while pumping so much air
• improving efficiency

Intake

Intake modifications generally fall under increasing airflow and/or improving efficiency. During the intake stroke, the downward motion of the piston creates a vacuum. Air rushes through the filter, some plumbing, the throttle body, the intake manifold, and the intake ports of the heads in order to fill that vacuum. Each of these items provides some resistance to airflow. Reducing this resistance increases the amount of air that can rush into the piston before the intake valve closes.

Another common intake modification is the cold air intake. Colder air is denser, which means it contains more oxygen. More oxygen means more power from combustion. Most cars simply draw air from the engine bay, which is usually well over 100 degrees Fahrenheit. By relocating the air intake to draw air from outside the car, the engine gets access to the coldest air available.

The final intake modification is the ram air intake. This is a variety of cold air intake that takes advantage of the fact that the car is moving forward through the air. By locating the air intake on the front of the car (usually on the front of a hood scoop), the engine can take advantage of aerodynamic pressure. At speed, air is being "rammed" into the intake.

Exhaust

Like intake modifications, exhaust modifications generally fall under increasing airflow and/or improving efficiency. During the exhaust stroke, the upward motion of the piston builds pressure in the cylinder. Air rushes past the exhaust valves, the exhaust ports of the heads, the exhaust manifolds, and through a series of pipes, catalytic converters, and mufflers in order to relieve that pressure. Each of these items provides some resistance to airflow. Reducing this resistance increases the amount of air that can be removed from the piston in a single exhaust stroke.

One of the first and most common performance modifications on any car is the installation of a high-performance muffler, or, occasionally, a "cat-back" exhaust. "Cat" is a reference to the catalytic converter, and a cat-back exhaust is a complete kit that replaces the factory exhaust piping, starting after the catalytic converter and going all the way back to the muffler(s) and tip(s).

Another popular modification is the replacement of factory exhaust manifold(s) with aftermarket ones, usually called headers. Headers improve exhaust flow in two ways. The first is obvious -- the pipes are bigger, and have more room for the exhaust gases to flow. The second, called scavenging, is less obvious, but more important. When an individual piston performs the exhaust stroke, it generates a "pulse" of exhaust gases. By tuning the length of the primary tubes (the smaller ones, before the pipes come together) so that the pulses reach the collector in an orderly fashion, without colliding with one another. As each pulse travels down the tubes, it creates a low-pressure region behind it. This low-pressure region "pulls" the next pulse down the tube.

The last common exhaust modification is the replacement of the catalytic converter with a high-flow aftermarket model, or with an "off-road" or "test" pipe (a plain pipe that replaces the converter). Catalytic converters in general are restrictive, so removing or replacing them can improve performance. Note: it is illegal in the United States to drive on public roads without a catalytic converter. In states with emissions inspections, you may not be able to register a vehicle without the factory catalytic converters in place. Specific laws about catalytic converters are documented [here].

Exhaust modifications often have a significant effect on the volume and character of the exhaust sound, especially while accelerating.

Power Adders

Forced Induction

Forced induction systems add power by adding an air compressor to the air intake system. As soon as the valve opens, the pressurized air rushes in to the cylinder -- no need to wait for the piston to generate a vacuum, and no need for the engine to do the work of generating that vacuum. In fact, the air pressure can actually help push the piston down. The air pressure ("boost") is generally measured in PSIG, which is gauge pressure relative to open atmospheric pressure. If you have an engine running on 10psi of boost, that means that the pressure at the air intake is 10psi higher than the local atmospheric pressure.

Adding forced induction to an engine that was not designed for it from the factory (or significantly increasing the maximum boost on one that was) is one of the surest ways to destroy the engine. Increased temperatures and pressures throughout the entire system will stress every component. When turbocharging or supercharging, be sure to make sure that your engine can handle the added stress.

In general, high performance naturally aspirated engines use a high compression ratio to maximize horsepower. Compression ratios in the 10.0:1-11.0:1 range are common. Modified engines get even higher, especially when using high-octane race gas. With a forced induction engine, however, it's common to reduce the compression ratio (generally around 8.5:1). This allows for more boost pressure without predetonation (a.k.a. knock), which yields more total horsepower.

Superchargers

A supercharger is an air compressor that is driven off of the crankshaft. Because of this, maximum boost is always achieved at maximum RPM, and boost builds smoothly and steadily as RPM risesm, and there is no lag between when the driver applies the throttle and when the boost takes effect. There are two major types: centrifugal and screw-type (a.k.a. Roots-type). A screw-type supercharger typically mounts like an intake manifold, on top of the engine, whereas a centrifugal usually mounts at the front of the engine, alongside other belt-driven accessories like the alternator or power-steering pump.

Both types of superchargers introduce a nontrivial amount of parasitic loss at the crankshaft.

Turbochargers

A turbocharger is a centrifugal air compressor that is driven by a turbine. The turbine is in turn driven by exhaust gas pressure. Because exhaust gas pressure is related to engine load rather than RPM, a turbo generally takes a moment to start developing boost pressure once the driver applies the throttle. This is known as turbo lag. Turbo lag can be mitigated by selecting an appropriately-sized turbo (smaller turbos spool faster), reducing friction (for instance using ball bearings) on the shaft connecting the turbine to the compressor, and by using advanced technologies such as variable vane geometry.

Aftermarket turbo systems are generally more complex and cause more fitment issues than aftermarket supercharger systems, because the turbo needs to be plumbed into both the intake and the exhaust.

The additional power from a turbocharger is generally considered "free power." This is because the additional exhaust backpressure requires minimal additional work on the part of the engine, especially when compared to the work required to turn a supercharger. It is not truly free, but the parasitic losses are trivial.