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Exhaust System Basics

 

Replacing your vintage vehicle exhaust system

 

When your vintage vehicle needs a new exhaust system or components, you are faced with a wide variety of options and decisions about the parts you will install. Here are some of the critical things you need to know and consider before spending your hard-earned dollars on that next exhaust system.

We are focused here on typical Four-stroke Internal Combustion (IC) engine systems for vehicles built from the earliest days of the horseless carriage up to the introduction of catalyzed systems in the mid-1970s. Newer vehicles utilize the same basic components, but also employ catalytic converters (2 or 3-way), a variety of thermal and oxygen sensing components and active control technologies to meet emissions standards.

Let’s start with the basic elements of exhaust systems. Most single systems consist of an exhaust manifold, exhaust pipe, silencer (muffler and/or resonator) and tailpipe. Dual systems typically use two exhaust pipes or an H pipe, two to six mufflers/resonators and two tailpipes. We’ll start with a brief review of your material choices and then cover each of the major components in turn.

Material Choices

 

When you replace exhaust components, you can choose from a variety of materials. The OEM parts installed on your new vehicle were typically manufactured in low-carbon steel. Beginning in the late 1930’s, mufflers were often made using galvanized steel for improved corrosion resistance. Aluminized steel and stainless steel were introduced into mainstream production vehicle exhaust components in the late 1970’s. Most OEM manufacturers introduced stainless steel components after 1975 with the advent of catalytic converters.

Today, you will find components manufactured from these materials:
- Low-carbon steel (poor corrosion resistance, but “correct” and original for most vehicles). Most “Old Stock” parts are low-carbon, “regular” steel or galvanized parts.
- Aluminized steel (very good corrosion resistance and durability; performs and looks good for many years with typical vintage vehicle usage and care)
- Stainless steel (excellent appearance and durability. “300” Series stainless is more brittle and has poorer thermal shock performance than “409” Series stainless, which is most often used in today’s OEM stainless systems)

Cast iron exhaust manifolds or tubular exhaust headers

 

Typical original equipment (OEM) production manifolds are manufactured from cast iron (inexpensive, durable and tolerant of high-temperature gases). These manifolds are “optimized” to balance production costs, manufacturing simplicity, ease of assembly, durability and performance. They direct the hot exhaust gases away from the cylinder head/exhaust port and into the primary exhaust pipe. They are not generally capable of optimal flow, but they are quiet, durable and strong. In many cases, they incorporate a heat riser valve or pre-heater to aid in cold engine warm-up, emissions control and drivability.

Many high-performance applications in the 1960’s and 1970’s utilized long-tube manifold designs to improve output and engine breathing. The Pontiac HO/Ram Air manifolds, the MOPAR Max Wedge, Hi-Po & Hemi manifolds and the Ford Hi-Po manifolds are all examples of these higher-flow manifolds. These designs improved exhaust gas flow over standard manifolds by utilizing a design that more closely resembles a tuned-length header. Though not as free-flowing or effective at scavenging exhaust gas as tuned headers, these manifolds represented a superior alternative to the basic “log” that most production manifolds resemble.

Performance part suppliers jumped into the performance improvement aftermarket with tubular headers for nearly every engine and vehicle combination. By matching the length of the header tubes, the exhaust pulses from each cylinder can be tuned to reach the exhaust pipes in an orderly fashion. Well-designed manifolds or headers can actually improve the scavenging of spent exhaust gases from the cylinders by using the pulse from previous cylinder output to “pull” the gases from the next cylinder – almost as if the exhaust pulse is being vacuumed out of the cylinder by the previous pulse. A good set of headers can improve torque and horsepower by more than 10% compared to standard factory exhaust manifolds.

If you are using production, cast iron manifolds in your vehicle, follow these steps to help ensure a quiet, trouble-free installation. Make sure that the sealing surfaces are flat and that the heat riser valve (if originally present) is properly installed and operating freely. Treat the manifolds with a high-quality cast iron coating after media blasting them. Use bolt locks where they were originally installed and a high-quality gasket. Follow the bolt installation sequence that is specified in your service manual and tighten the attachments to the specified torque in the proper sequence. (Usually starting at the center and working alternately forward and rearward) This maintains the flat sealing interface between the manifold and the head and minimizes the potential for leaks and cracks.

Exhaust pipes

 

Once the gases exit the manifold/header, they are conveyed rearward under the vehicle by the exhaust pipes. Production pipes were designed to optimize gas flow while maintaining underbody and ground clearances. They were normally press-bent or “push-bent”, with a slight deformation of the circular cross-section of the tube in areas of significant bends. They often include flattened sections for clearance to frame cross members, transmission mounts or other chassis and suspension parts.

Replacing your exhaust pipes with larger diameter pipes can help performance, but it can also cause significant clearance and interference problems. If you are considering a larger diameter system, save yourself a lot of frustration and aggravation by verifying that there is sufficient clearance for the pipes you are considering. Don’t just inspect the system statically; get a helper to move the tailpipe outlets to the limits of the hanger isolator motion and rock the vehicle to full suspension travel if possible. What looks like sufficient clearance can become interference when the engine moves under hard acceleration or when the suspension moves to the limits of its travel.

H pipes, X pipes and competing claims

 

There are many suppliers of performance exhaust systems making claims of huge torque and horsepower gains to be had by utilizing their X or H pipe designs. Do your homework; evaluate the claims in light of your intended use of the vehicle and be certain that the claims are substantiated by hard data, preferably comparisons of vehicle performance with ONLY that specific exhaust system change having been made between comparative measurements. Dynamometer data is useful, but be certain that you understand what was done to the engine/vehicle to achieve the published results. Most of us won’t be operating our vintage vehicle for extended periods under racing conditions, so consider how important the results are for your vehicle driving patterns!

There ARE inherent benefits to improving the breath-ability of your exhaust system. Engines are basically big air pumps. The less restriction there is to exhaust flow, the better the engine can process incoming fuel/air mixtures and the more power the engine will produce. H Pipes provide a balance of the exhaust gas pulses between the two exhaust pipes, equalizing the system pressure in the two “sides” of the exhaust system. This generally provides improved low-end torque and some amount of top-end horsepower improvement. An “X” pipe design may provide a slightly better result than a simple H pipe on identical applications, especially at higher engine speeds. Whichever way you go, be certain to re-tune your engine for the changes you’ve made in the exhaust system. A reduction in exhaust system back pressure can allow improved breathing, which will probably require carburetor re-jetting, idle mixture and idle speed adjustments, ignition timing adjustment and spark plug changes.

Many high-performance vehicles manufactured after 1960 were available with factory-installed duals that incorporated H pipes. In many cases, your car can be equipped with an OEM style H pipe and duals, even if it was originally equipped with a single exhaust system. The factory dual system can be a great performance and sound improvement.

Silencers (Mufflers and Resonators)

 

Exhaust noise silencers come in a variety of shapes, sizes and designs. They are generally grouped into two major types; resonators and mufflers. Let’s look at both of them.

Mufflers

 

Mufflers are designed to reduce the pulsation and volume of the outbound exhaust pulses. This is generally accomplished by a series of perforated tubes installed in a sealed chamber (or “case”) that force the exhaust gases through a series of loops or turns while allowing the gases to cool. The tube diameters, perforations and flow patterns are designed to reduce the sound levels and pulsations while providing the desired tone and sound levels both outside and inside the vehicle. Two-tube, three-tube, chambered and straight-through designs are the most common.

In a two-tube design, the exhaust gases enter the muffler through the inlet tube and are forced to “find their way” to the outlet tube by passing through the perforations or holes in the inlet tube and the sound-absorbing materials or chambers that surround the inlet tube.

In a three-tube muffler, the exhaust gases follow a “double reverse” flow pattern, transferring from the inlet tube into an intermediate tube and then finally into the outlet tube. Three-tube designs are generally quieter than two-tube designs.

In a chambered design, the inlet tube is short and dumps into an expansion chamber. Exhaust gases have less restricted flow paths to enter the outlet tube.

Straight-thru, single tube designs generally are the loudest and lowest restriction mufflers. The inlet and outlet tube are one piece, with a series of holes or perforations that allow the gas pulses to expand into the muffler case area, which is usually filled with a glass fiber or similar material to provide some quieting.

Muffler design is a complex science. Most mufflers are designed to fit a wide range of vehicle/engine applications and you will usually find many options for your specific vehicle. You will probably be happy with any of the products that are offered by the major exhaust component manufacturers to fit your application, but if you have very specific sound and performance requirements, research can save you headaches and unnecessary cost. Read up on any test data that compares the brands and models of mufflers that you are considering. Check websites/forums/tech advice/car club Resources – chances are very good that someone with your same desires has already found a combination that works great for your vehicle! Beware of extremely loud performance mufflers that sound very aggressive; some of these very loud mufflers actually flow no better than quieter, high-flow designs. In addition, many vehicle owners do not like the interior sound levels and the resonance these loud mufflers can create.

Resonators

 

Resonators are utilized on a wide variety of passenger car and truck exhaust systems. They typically utilize a “straight-through” case or tube that attenuates and tunes the sound of the exhaust pulses. Resonators are most often installed just before exit point of the exhaust system, but in some applications the resonator is installed in front of the muffler. Some resonators are designed as simple, hollow expansion or sound chambers with no sound absorbing materials. These are acoustically tuned to deliver the desired exhaust note. They primarily use reflective wave cancellation techniques to eliminate the undesired frequencies while allowing desired frequencies to pass unabated.

Most exhaust system providers will provide replacement exhaust systems with or without the resonator component. A straight pipe replacement can be installed if the resonator is not desired. Without the resonator, your exhaust system will usually sound slightly louder and less mellow, with a sharper and “raspier” exhaust note evident under hard acceleration.

Tailpipes

 

Tailpipes guide the spent exhaust gases to the rear of the vehicle. In the earliest vehicles, they were often no more than a short “turn-down” at the rear of the muffler case, exiting under the vehicle. Over the years, the desire to convey noxious exhaust fumes away from the vehicle occupants drove the exit points to the rear of the vehicles. In many truck and station wagon vehicles, the tailpipe exit point is just rearward of the rear wheel opening. This keeps exhaust gases from being drawn back into the low pressure area at the rear of a station wagon. Interestingly, many high performance vehicles of the 50’s, 60’s and 70’s employed the shorter tailpipe designs of the station wagons with side exits behind the rear wheels. Are you restoring a 396 Chevelle or Impala from 1965-68 or a 1970 Torino GT? Make certain you are getting the correct tailpipe design for your vehicle!

Tuning AFTER you make exhaust system changes

 

We can’t over-emphasize the need to tune for optimal performance after making significant exhaust system changes. If you merely replace an OEM-style muffler with a new direct replacement, you needn’t worry about tuning. But if you have removed a factory single exhaust and replaced it with an OEM style H pipe and dual exhausts with low-restriction mufflers, you owe it to yourself and your engine’s long term health to re-tune your engine to take full advantage of the new exhaust systems’ capabilities!

Summing it up

Your exhaust system plays a critical role in the overall performance, sound and driving comfort of your vehicle. Choosing the correct combination of components and materials will give you the performance and sound that you expect. When it’s time to replace your system, investigate and understand the options available to you; you’ll be glad you did!

 

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