The SST is absolute garbage, you couldn't pay me to run one on any bike of mine.
Originally written by Moto814
The FMF SST pipe is an amazing pipe.
Why is it amazing? Because in just about every case, it will work WORSE than the stock pipe that came on the bike.
The SST uses a restrictive reverse cone and pressure bleed to intensify the return wave and lower the initial flow out the stinger section of the pipe. What that in turn does is increase piston crown temperatures and make jetting difficult at best.
Now increased piston crown temperatures can be compensated for to some extent with richer jetting. But, by doing that you are defeating the purpose of the pipe, and negating any power gain you may have gotten from the pipe in the first place. Why are increased piston crown temperatures bad? Well, in some cases they are not. You see most bikes are jetted horribly rich from the factory, and raising the piston crown temps in this case will "fool" the engine into thinking it is jetted leaner than it is. Which will in some cases improve power. But in most cases, it will just cause slightly better throttle response and an increase in the spooge out the back of the pipe.
After all that you may be thinking: "well this doesn't sound so bad, why are they saying this pipe is no good?" Good question. Here's the answer. Because of the restrictive nature of the pressure bleed from the pipe, it does not evacuate as efficiently as it needs to. This combined with the piston crown temperature increase yeilds jetting that is too rich. So now you have a pipe that "fools" the engine into thinking it is leaner than it really is, but also makes it much richer than it was previously. Here in lies the second problem. It makes jetting a nightmare. If you jet it by feel, you will be jetting richer, and compounding the too rich condition even more, making the bike run like a slug. If you jet it leaner, you will be increasing piston crown temperatures even more resulting in an engine siezure. Catch-22 anyone?
And FYI, pinging is not "firing before TDC". ALL engines fire before TDC, by as much as 40 degrees BTDC at high rpms. "pinging" is the audible symptom of detonation, and is not to be confused with pre-ignition, a very different phenomenon.
Detonation is, in the simplest terms, an explosion, just as it's name implies.
To understand detonation, you first have to understand normal combustion. When the spark plug fires, it creates what is called a "flame kernal" between the electrodes. This tiny flame rapidly spreads in all directions, a smooth wave of burning fuel that continues until it reaches the edges of the combustion chamber, burning all of the fuel/air mix very rapidly, but very smoothly. The burning fuel mixture creates a tremendous pressure rise inside the combustion chamber. It is this pressure rise that propels the piston down the cylinder, turning the crank shaft. The pressures created inside a modern motocross engine can be as high as 30 atmospheres (440 psi).
Detonation, on the other hand, is not normal, smooth combustion. As the fuel/air mixture is being compressed in the engine, the heat and pressure begins to degrade the fuel at the molecular level, creating very unstable molecules called free-radicals. If the concentration of these free radicals becomes great enough, the fuel can spontaneously ignite over a large area all at once, creating a very sudden and violent explosion. This can happen before the plug fires, or it can happen in front of the flame-wave-front, in the remaining unburned fuel/air mix at or near the periphery of the combustion chamber. This explosion creates a much greater pressure rise in the combustion chamber (and much higher temperatures) than normal combustion, and it creates very fast-moving, very powerful shock waves. These shock waves strike the surface of the piston crown and the head with tremendous force, almost like hitting the metal with a hammer. It's this impact of the shock waves that makes the familiar "ping" sound that we associate with detonation.
The shock waves are also what leads to the damaged pistons that we see in a bike with a detonation problem. During normal combustion, the surface of the piston is protected from the raw heat of the burning fuel by a thin layer of unburned fuel, a few molecules thick, that clings to the surface. The shock waves produced by detonation blow this protective layer away, exposing the aluminum piston surface to the heat of combustion. The detonation also causes excessive temps, due to the extreme pressure rise that is well beyond what is created during normal, smooth combustion. The combination of high temps softening the piston, the protective layer being blow away from the piston surface, and the force of the shock waves, literally blows little bits and pieces of aluminum from the piston surface.
Pre-ignition is also exactly what it's name implies. The fuel/air mixture is ignited before the ignition fires, by a seperate heat source, such as a glowing spark plug electrode, hot carbon deposits, red-hot exhaust valves, etc. Pre-ignition does not cause any audible symptoms, but it can (and almost always does) lead to detonation.
Pre-ignition can also lead to the dreaded "runaway engine" syndrome. You've all heard the stories of guys bikes revving uncontrollably on the stand, and the kill switch having no effect. That engine is likely experiencing pre-ignition. Since the fuel is being ignited by a source other than the plug, hitting the kill switch can't stop the engine from running.
So it look as if i'll be putting on the stock kx100 pipe i ordered off of FLEABAY lol. I swear guys why can't a 'rebuild' ever be as simple as it appears. 
