So the big question in my mind was how the 1982 Kawasaki KZ 750 LTD would perform with the individual pod filters. It was the "hot set-up" "back in the day" to have an individual K&N air filter or velocity stacks fixed directly to the carburetors in order to offer the least restriction and maximize flow and power. Well, maybe that was true back in the 50's and 60's, but that's because the engineering behind those earlier machines was no where near the level that even bikes of the 70's and 80's had. Most people know that an exhaust system can greatly affect how a motorcycle performs. That's why when you open up the catalog of any big parts house (Dennis Kirk for example) you'll see pages and pages of aftermarket/high performance exhaust systems for just about any bike out there. Of course some people want a different exhaust for their bike for aesthetic reasons or because they buy into the old mantra of "loud pipes save lives". (This is a subject best left for another day...) But the industry has traditionally been focused on increasing performance over the stock systems.
Exhaust tuning is a very complex science that uses many different principals to help evacuate the spent gases from the cylinders of the engine and keep fresh unspent air/fuel mixture in. On single cylinder engines (or engines where each cylinder has a completely seperate header and muffler system) header pipe diameter and total length are two of the largest variables. Remember that the air moving through the engine does not do so in a constant pressure, steady state way. As the exhaust valve opens, the gasses are still burning and highly pressurized as they begin to move out of the cylinder and into the exhaust port and system. There are two distinct energy waves or "pulses" moving into the exhaust at this point: The exhaust gases themselves have kinetic energy moving into the pipe, and the sound wave. The oversimplified version is that the length, shape, and diameter of the exhaust can be designed so that these pulses (and their negative "reflections" sent back to towards the engine when the pulse reaches the end of the exhaust system) can be used to draw more exhaust out of the cylinder when the exhaust valve first opens and "stuff" the fresh air/fuel mixture back into the engine when the intake valve begins to open (especially critical as valve "overlap" increases with longer duration cam timing). When you add in the additional variables of multiple pulses from multiple cylinders in 2-1, 4-1, and 4-2-1 exhausts, it becomes an extroadinarily complex task to maximize power (I'll go into more detail at a later date in another post). Also, since the speed of the "pulse" of exhaust gases varies with engine RPM, the exhaust system will be most effective at a certain RPM and less effective at all others. By manipulating length, diameter, etc., a tuner can design a system to work at a desired RPM.
I thought you were talking about Pod air-filters?
Yeah, yeah... I know... Stick with me though. A well designed air intake system works in a similar way as a well tuned exhaust. The suction from the engine created by the intake valve opening is essentially a "negative" version of the pulse seen at the exhaust side. The intake tract length is one variable that can be manipulated in order create a "stuffing" effect (literally creating a positive pressure of air into the cylinder). These energy pulses created by the opening and closing of the intake valves cause a starting and stopping of the flow of air into the engine. This vibration behaves in the same manner as a sound wave which is why the volume of the air intake system and airbox plays an important role in creating the "stuffing" effect. Think of it in terms of a musical instrument... An acoustic guitar has a hollow ressonance chamber to help amplify the vibrations (sound) from the strings. The resulting higher "volume" in the case of a motorcycle engine is how large the "stuffing" effect is at the intake of the motor. As with the exhaust system, the volume of the intake system and the length of the intake tract affect the RPM where the intake system performs optimally.
So that means we can design the exhaust and the intake to work best at peak RPM and we'll have a killer machine right?
Well, with the good comes the bad... These systems will have an RPM range where they work best, but the flip side, is that there is also an RPM range where they will do exactly the opposite of what we want them to do. Remember, we're dealing with waves here... they have peaks and they have troughs. When the peaks line up with eachother, they resonate and amplify their effect, which is what happens at their optimal RPM: In our example, the exhaust is "pulling" it's hardest when the intake valve first opens or the intake system is "pushing" it's hardest when the intake valve first opens. When a peak lines up with a trough, they energy wave will end up cancelling each other out: The exhaust and intake systems are function in essentially atmosphereic pressure. When a trough lines up with a trough, the bad effects are amplified: the exhaust is "pushing" the gases back into the cylinder when the valve first opens and the intake is "pulling" the air out of the cylider when the valve first opens. There will be an engine RPM where the worst case scenario occurs and power output will drop off dramatically. Of course there are countless ways to combat the severity of the problem, including staggering the optimal RPM range of the intake and exhaust systems, but it still will exist in some form(dynamic intake and exhaust tuning is a recent engineering accomplishment).
To be continued...
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