Exhaust System

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Exhaust system - FADM Stern GNSF

The exhaust system on these bikes is an integral part of the bike, and required for the proper optimum operation of the engine. The components of the exhaust system are designed to act as a single "tuned" unit to optimize engine efficiency. It does this by not only providing "back pressure" (a term very grossly misunderstood) but also "tuning".

    • Back pressure

People often think "back pressure" is the only reason for an exhaust system (aside from noise), but while it is important it is not all the exhaust system does. For the engine to operate properly, a specific amount of back pressure (resistance to exhaust air flow) is required to "dampen" the piston on the exhaust stroke. The reason for this is due to stresses on the connecting rod when the piston has to "switch" directions. While the stresses on the rod are basically the same during reversal at TDC as is it when it hits bottom of travel, the stresses are "different". At the bottom of the stroke, the rod undergoes "compression" (rod being squished) along its length, while at the top it experiences "expansion" (rod being pulled). While the magnitude may be equal, object fail easier when being pulled than when they are squished. The small amount of resistance (back pressure) "cushions" the piston as it moves up, reducing the stress on the connecting rod. Now, what about the upward motion in the other part of the cycle (compression stroke)? Well that is cushioned even more since both valves are closed. With no back pressure, the rod stresses are a lot higher and can lead to rod failure.

    • Tuned wave

This is an aspect of the exhaust system that most people just don’t a clue about, but is VERY important for optimum engine performance. This type of exhaust system is designed to have the exhaust gases create a "tuned" wave within the exhaust system. This "wave front" does something very important; it creates a momentary "low pressure" area behind it, which aids in pulling in fuel during the intake down stroke. If you are having trouble understanding this phenomenon, go to you-tube and look for a video of "nuclear testing" and watch it. You will see the effect very dramatically while watching the blast wave move over objects (like trees). As the wave approaches, it will shove everything in the direction of its travel ... but then.... everything gets "sucked back". This is a negative pressure induced by the wave front and is present in ALL explosions (remember, internal combustion engines work by creating a timed "explosion" within the cylinder. This is also how the principle of "venturi action" works. The benefit of this “vacuum” is it helps do two things. As the wave moves, the low pressure "sucks" more products of combustion out of the cylinder, while aiding in drawing in fuel/air during the intake stroke. This makes the engine "breath" more efficiently and helps prevent pre ignition (fuel/air being ignited by left over hot gases) which shows as "popping" or backfiring, especially during deceleration of the engine. Without this function the engine can sometimes miss (spark doesn’t ignite the fuel/air mixture due to too much exhaust gases still present which drops the air concentration) and backfire during the subsequent ignition cycle (hot gases run into unburnt fuel already in the exhaust system and ignite.

The exhaust system consists of three sections;

  • Header pipes
  • H-Box
  • Mufflers
    • Headers

The headers connect to the exhaust ports on the cylinder and feed the exhaust gases to the H-box. During this process, the headers also help "cool" the gases before they reach the H-Box, helping prevent the H-box from seeing the full temperature of the ignited gases which helps to slow corrosion and burn through. While the headers perform this function, there lengths are also part of the "tuning" the system as a whole provides.

    • H-Box



The H-Box is a very important part of the exhaust system, as this is where the "tuned" wave is created. Also called a "crossover box", it is engineered to "balance" the pipe "length" of each section, and to create "back-pressure" to the other cylinder to aid in wave creation. As one Wave front is generated during the exhaust stroke, it sends this pulse back up the other header and creates a slight "pressurized area" in that header, which helps dampen that cylinder (back pressure) when it fires and creates its wave front. This is extremely effective especially at high RPM, as the faster the piston moves the more stress on the rods (momentum). at lower RMP's this "pressurized area" outside the exhaust valve will start to dissipate before the cylinder valve opens, while at high RPM's, it has no time to dissipate when the cylinder opens. This adds "progressive resistance" (or call it "progressive back pressure") as the RPM's (and piston rod stress) increase. This is one of the reasons these bike can operate at such a high RPM value.

    • Below are pictures of a Motad 2 into 1 exhaust that has been cut in half for viewing. Thanks to Jimuk for the pictures.

It came with a Motad 2-1 Exhaust which was rusted and full of holes. So now that I've bought a replacement I thought I'd cut up open the old one so you guys have have a look.

My new exhaust is actually second hand off ebay, but is the exact same thing. The only difference is the previous owner has drilled out the rear baffle. So now that I've seen how the internals are organised I can see the new one is almost an open pipe. Sounds good though and MPG is still ok.

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1981 CX500B

    • Mufflers

The mufflers are used to cut out a lot of the noise generated by the "wave" generated in the system. Mufflers, like their name, are designed to dissipate or "muffle" the sound waves generated from the high and low pressure wave fronts. Some are tuned to allow more of the low frequency band to pass (deep rumbling sound) while other block all of the spectrum (making the bike very quiet).

Below is a drawing of the H box and mufflers and how the exhaust flows:



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