Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
to help pull in the intake charge. However, with nitrous oxide added, in some cases a longer exhaust duration with a wider LSA may be needed to relieve the higher exhaust heat that’s generated.
Advancing/Retarding Camshaft
Advancing or retarding camshaft timing moves the torque band toward lower or higher engine RPM operation. Advancing the camshaft enhances low-end power while retarding the camshaft enhances top-end power and sacrifices low-end torque. Advancing or retarding the camshaft moves the valve events ahead or behind the piston travel. In a conventional clockwise-rotation engine, moving the camshaft farther clockwise advances the valve events, while moving the camshaft counterclockwise retards the valve events.
Advancing the camshaft timing results in an earlier intake opening, while decreasing intake valve-to-piston clearance and increasing exhaust valve-to-piston clearance.
Retarding the camshaft causes the intake valve to open later, while increasing intake valve-to-piston clearance and decreasing exhaust valve-to-piston clearance.
If you plan to advance or retard the camshaft, you need to check valve-to-piston clearance with the camshaft in the changed position. Therefore, if you advance the camshaft, pay attention to intake valve clearance. If you retard the camshaft, pay attention to exhaust valve clearance.
Special Firing-Order Camshafts
Compared to the factory camshaft firing order, special firing-order (SFO) camshafts are sometimes used in racing applications. This different firing order is also featured on some late-model OEM engines, such as GM’s LS series.
The reason that a different firing order is sometimes used is to obtain a smoother running engine and improved cylinder-to-cylinder fuel distribution. GM adopted a special firing order in its Gen III and Gen IV LS engine series, which feature a 4/7 and 2/3 swap for the same reasons: to smooth out the harmonics in the pursuit of greater engine durability and to potentially generate more power. Changing the firing order in this manner can also offer the benefit of reducing harmonic effects and deflection forces at the crankshaft and the main bearings. Another potential benefit lies in reducing isolated hot spots in adjacent cylinder walls.
Special firing-order (SFO) camshafts alter a traditional firing order in order to smooth out engine pulses and to reduce valvetrain harmonics. Strategically swapping the camshaft firing order helps to achieve a smoother-running engine, which reduces harmonic effects at the crankshaft and the main bearings. It also reduces heat by eliminating two adjacent cylinders firing in succession. This benefits engines that operate at or near peak RPM for extended periods of time. In a V-8 engine a 4/7 swap and possibly a 2/3 swap (depending on the engine) is the most common approach with an SFO design.
As an example, let’s consider the common small-block and big-block Chevy engines, which have a firing order of 1-8-4-3-6-5-7-2. During each engine operation, every cylinder has a “companion” in the firing order. Both companion cylinders reach TDC at the same time, with one cylinder on the power stroke and one cylinder on the exhaust stroke. These cylinders (paired as 1/6, 2/3, 4/7, and 5/8) may be interchanged, or swapped, in the firing order without the need to modify the crankshaft.
A common practice among race engine builders who follow this theory is to swap cylinders 4 and 7, creating a new firing order of 1-8-7-3-6-5-4-2. While some builders who have tried this process report no performance gains, others claim to have picked up as much as 10 hp in doing so.
To maximize the exhaust scavenging effect, the header primary tube length is affected by the engine firing order (based on wave-tuning theory). Experimentation on an engine dyno or chassis dyno is useful when determining header primary tube length when using a special firing-order camshaft. Race-engine builders frequently experiment with different primary tube lengths; this helps to determine the best setup to accommodate an SFO camshaft. Some builders experimenting with SFO cams report substantial gains while others have obtained minimal improvements.
Exhaust manifolds are commonly made of cast iron, although a few exist as short runs of tubular steel that merge into a common exit. Cast-iron exhaust manifolds have been in common use among carmakers for decades for two primary reasons: They’re less expensive to manufacture, consisting of a one-piece casting rather than a tubular header that requires assembly and welding; and because of their compact size, they are easier to install on a production line.
If you want to run cast-iron exhaust manifolds but aren’t thrilled with the design and/or appearance of a stock manifold, custom aftermarket manifolds are available for a limited number of popular applications. Aftermarket manifolds are generally designed to provide superior exhaust flow along with a much-improved appearance.
Tubular exhaust headers provide dedicated primary gas routing for each cylinder, eventually merging into a common collector. Tubular headers, available in both mild steel and stainless steel, also provide a substantial weight savings compared to cast-iron manifolds.
Today, the only reasons that a hobbyist, street rodder, or car collector chooses a cast-iron exhaust manifold rather than a set of tubular exhaust headers are originality, ease of installation, and/or budget. If the vehicle is being restored in terms of historical accuracy, whatever type of exhaust system was originally featured on the vehicle is preferred, and that includes cast-iron or cast stainless steel exhaust manifolds where applicable. The only reason to switch from heavy cast-iron manifolds to tubular headers involves the goal of increasing horsepower.
The various choices of header primary tube diameter and length provide a degree of tuning, in terms of selecting various header designs, whereas a stock cast-iron exhaust manifold offers virtually no engine tuning potential. If your goal is to extract additional horsepower and torque, tubular headers offer choices that allow you to tune the exhaust system to the engine, whereas OEM cast exhaust manifolds provide size and design limitations. The choices available with tubular headers provide tuning capabilities that are simply not possible with cast manifolds.
On the positive side, cast-iron exhaust manifolds are more compact and therefore easier to install and generally require less underhood space. In addition, because of the thicker material, they tend to capture heat a bit better, and tend to lower exhaust noise because of the more insulating properties of the thick and heavy cast-iron construction. On the negative side, they’re heavy, so if weight is a consideration, switching to tubular headers is a plus.
As cast iron ages and is subjected to heat cycles over time, it tends to become more brittle and can be prone to stress cracking. The potential for stress cracking is exaggerated if the exhaust piping and mufflers are not supported properly, as the leverage