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Hot Air 29/10/2003 (What is the best Silencer Box?) PDF Print E-mail

Silencer boxes make power?

In the last issue I said that an engine cannot produce more power by adding silencer boxes. This is true. However, there have been one or two instances where power-gains were obtained in race prepared cars with the addition of a silencer box.

We must remember that scavenging is not only affected by the branch manifold but the whole exhaust system, which includes the primary header pipes, the secondary header pipes, the exhaust piping and the exhaust muffling silencer boxes.

One function of a tuned exhaust is to scavenge the cylinder completely of exhaust gas. The idea is that the exhaust gas momentum and pressure waves suck the intake charge into the cylinder to “overfill” it to the maximum volumetric efficiency possible.

Another function of a tuned exhaust is to pulse-tune the system so that the intake charge is slowed down in its momentum just enough not to exit the exhaust port as unburned intake charge.

This situation is called over-scavenging.

Keeping in mind that scavenging is also influenced by the silencer system, when over-scavenging occurs, the addition of a silencer box can alter the engine’s flow so less fuel-air mixture is being lost into the exhaust during the cam overlap period, thus increasing power over a particular rev-range.

The point is that this power gain was caused by either incorrect header design or altered cam timing with either longer total overlap period, or longer duration exhaust cam timing (twin cam motors)
Exhaust back-pressure

Before talking about the different types of silencer boxes, we must first look at the subject of exhaust backpressure.

Engines move huge quantities of air in through the inlet tract and out the exhaust valve. The piston forces the air out the open exhaust valve into a pipe already filled with air, causing pressure. This pressure increases with narrower tubing and any restriction introduced into that tubing, such as misaligned welds, kinked bends and any deflection of flow of any kind, including all types of silencer boxes. Silencer boxes are a means to reduce exhaust noise but because silencers increase backpressure this introduction to a system cannot possibly be instrumental in power gaining! As I said before, silencing is a non-performance related requirement: a necessary evil, which can complicate the task of making horsepower.

Yet, almost daily I face the statement by clients with expert friends and advisors, that all engines need varying levels of back-pressure to optimize efficiency and to prevent burning valves and ………

(This is where I switch off and count to ten!)

Installing a performance branch manifold and matching free-flow exhaust system can be seen as a radical change to an engine, and this radical change can sometimes necessitate optimization of ignition timing and fuel-air mixture to obtain maximum benefit.

A misconception exists that a branch manifold relieves backpressure, but a certain amount of backpressure is required for maximum performance. This misconception stems from the fact that people tend to forget that the branch manifold is designed by means of pulse tuning and inertia.

The fact of the matter is that a good branch and system not only relieves backpressure, but creates a vacuum in the system. When the next cylinder’s exhaust valve opens, the vacuum in the branch pulls the exhaust gas out of the cylinder.

Providing, therefore that the air/fuel ratio and ignition timing are optimized, reducing exhaust backpressure always improves power!

It is also important to note that all mods that increase engine power increase exhaust back pressure (assuming the exhaust is unchanged).

A 40% increase in power will double exhaust backpressure! It is advisable therefore to fabricate a branch manifold and matching free-flow performance system according to the engine mods before introducing those mods!

How do silencers work

To a greater or lesser degree silencers attenuate noise by creating backpressure. Some stock exhausts develop pressure sometimes higher that 10 psi. This figure can de reduced to around 1.5 – 2.5 psi with bigger bore exhaust piping and through-flow absorption silencers. There are three categories of silencers: Absorption, Restriction and Reflection. Factory exhausts utilize various combinations of all three to obtain the required noise emission levels.


Absorption method

Noise reduction by means of an absorption silencer is the least effective. These silencers are also known as free-flow or straight through silencers. These boxes are of simple construction consisting of a perforated tube through the centre of a cylindrical or oval casing, which is packed with fiberglass muffler wool, or even steel wool. There are variations to this method involving different casing diameters and lengths, different perforated tube sizes and the introduction of an open chamber along the length of the silencer. This chamber in reality separates the silencer into two separate absorbing units, with a total break within the casing. In theory this design attenuates sound more efficiently than the straight through box because on entering the chamber, the exhaust gases slow down dramatically allowing more attenuation time.

This type of box is also used where the exhaust heat causes excessive expansion, which can crack or break lengthy perforation within the box.

Restriction method

This is the most effective method of noise reduction. As the word “restriction” implies, this silencer attenuates sound by generating restriction by forcing gases through small diameter tubes and passages. Unfortunately, this results in the greatest backpressure and also wastes the most power. This type of silencer is also the least expensive to manufacture, so, even today, vehicle manufacturers use this method wherever possible.

Reflection method

This silencing method is probably the most widely used. It is the most sophisticated type in that it almost always utilizes absorption principles together with reflection methods.

These silencers use internal baffles to reflect sound waves in opposite directions and direct the exhaust gases through perforated pipes surrounded by absorption packing into separate internal chambers. This type of silencer is used widely today in modern cars as rear boxes. In order to maintain the least backpressure possible, the chambers and passages within these boxes are constructed as large in volume as possible. These boxes are even molded to the available space under the car to maximize silencer volume.

Some cars utilize small cylindrical boxes in front or in the middle of the silencer system.

These boxes are called Helmholtz Resonators. They look like absorption type free-flow boxes, except that there is no sound absorbing material utilized. The reflective principle is used here to attenuate specific frequencies.

In the design and development of performance exhaust systems the emphasis naturally is on performance. The obvious silencing method chosen is almost always the absorption method. Whereas nowadays most cars utilize a combination of the absorption, reflection and restriction method, for the true purpose of performance oriented systems the absorption method is utilized throughout for the front box, middle box and the rear box.

Silencer boxes serve other purposes as well: The Catalytic converter besides acting as a catalyst, (more about this later) also attenuates sound quite effectively.

Also, becoming more and more popular is the “JAP-STYLE” box:

This type of box is manufactured mainly in the USA and Japan and is all the rage on weekend only street racers. The cosmetic appeal of these silencer boxes is undeniable; they really look great. But watch out!

Firstly, they are extremely expensive, specially the imported products.

Secondly, they don’t always do your car justice because they are either too short, too long, hang too low or the internal perf is of incorrect diameter for your car.

These Jap-style boxes are available locally at reduced prices. Also, they are available as taylor-made items specific to your car’s requirements. My long-time buddy James Honiball has seen to that. He is able to fabricate any length, diameter and perf combination according to the vehicle’s needs.

Silencer combination

We have established that performance exhaust systems require the use of absorption silence boxes. This can be a huge problem in road cars, because, as mentioned before, these straight-through boxes are not as efficient in noise reduction as restriction or reflection boxes. In order to obtain maximum absorption the following steps can be taken:

Firstly the volumetric size of the box can be increased: i.e. the box must be either longer or wider, or both.

This is usually only possible relative to space availability, which means we are usually limited to doing this in the rear box application.

Secondly, an additional box can be employed: i.e. if the stock system is a two-box system, a three-box system in the free-flow format may be employed.

The problem that we are facing is not so much the exhaust noise output as heard at the exhaust tail pipe. It is relatively uncomplicated to fabricate a free-flow exhaust system with a great sound, which passes the legal decibel limit. This sporty sound is what you hear as the car goes by. However, it is equally easy to create ‘drone’ or ‘boom’ inside the cab that will drive the owner nuts, even though the outside exhaust noise emission is sporty and within legal limits.

A car, which is particularly affected by exhaust droning, is the Golf MK II and MKIII.

In stock form it has a three-box system. The tendency, in the free-flow format is therefore to fabricate a three-box system. I’ve even seen four-box systems on Golf II’s. Removing one of the three boxes would predictably increase noise output and droning, but removing the front box and utilizing one at the axle and another after the axle, although increasing the exhaust noise output, alleviates the head-ache inducing drone to a very acceptable level. In order to reduce the resulting outside noise output, a larger or longer silencer is used in the rear. Problem solved. In this case we end up using a two-box system in the free-flow format. Unfortunately, this set-up does not work on all cars, unless of course there is a turbo up front. (More of this later)

The obvious question creeps up often, “Is it possible to fabricate a performance exhaust system and maintain the stock exhaust sound?”

The answer is, er, well yes, almost.

In the exceptional case of reducing the exhaust diameter to increase power, such as the often-mentioned VW VR6, the answer is an almost YES. Also, in the case of some six cylinder and some eight cylinder cars the answer is an almost yes. In the case of some turbo conversions the answer can also be an almost yes. But is most instances, keeping in mind that silencers introduce backpressure the answer must be no!

However almost every vehicle can have a performance exhaust installed with a sporty, unobtrusive and refined sound, which is pleasing to the ear, including the occupant’s ear!

A lot of exhaust centers who do free-flow systems are simply not aware of a ‘drone’ problem or aware that these problems can be overcome with correct silencer box and tube size application.

So, to conclude, a request to you, the paying customer: Do not be lead to believe that “droning” is an acceptable disadvantage to a free-flow system!

That’s all for now!

Abel dos Santos

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