Sorry, gentlemen, but unless an engine builder with a physics background were to walk in here and tell me I am wrong, I am going to have a hard time not standing up on this one.
I am not a ASE mechanic, never pretended to be, but one thing I do know is theoretical and mechanical engineering and looking at the "big picture".
A comment was made that NO exhaust system manufactured or available will/can cause premature wear to the valve seals. I strongly disagree in the truest sense of engineering and physics. Sorry. I respect each member here, but you gentlemen need to fundamentally realize that for every action there is a equal reaction.
We live in a world of natural laws and simply believing something is so, does not make it truth. With this said the core issue is aftermarket exhausts have EVERYTHING to do with physics and engineering. In time, ALL uncompensated modifications WILL impact engine performance and longevity.
When we buy that awesome aftermarket exhaust we are looking to: make appearance and sound changes as well as maximize exhaust flow out of the engine.
The inflow of an air/fuel mixture is a separate issue, but it is directly influenced by exhaust flow, particularly during valve overlap (when both valves are open for "X" degrees of crankshaft rotation). Another topic for another issue.
Perhaps the most important aspect of exhaust flow relative to aftermarket products is the issue of flow volume vs. flow velocity and the savaging that occurs during the exhaust stroke.
Ford spends thousands of hours designing a stock engine to maximize each one of these strokes. Even if their idea of maximization is not OUR idea, they have designed and integrated, fully enclosed and self-actuating environment.
A well designed (or modified) engine needs the highest flow velocity possible for quick throttle response and torque throughout the low-to-mid range portion of the power band. The same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the power band for maximum performance. This is where the issue and fundamental conflict arises.
For "X" amount of exhaust pressure at an exhaust valve, a smaller diameter exhaust component will provide higher flow velocity than a larger diameter tube. Unfortunately, the laws of physics will not allow that same small diameter tube to flow sufficient volume to realize maximum potential power at higher RPM. If we install a larger diameter exhaust component at any portion of the exhaust system, we will have enough flow volume for maximum power at mid-to-high RPM, but the flow velocity will decrease and low-to-mid range throttle response and torque will suffer. Yes, this is where the proverbial back pressure myth usually arises. Stay with me, it gets fun.
This is the primary paradox of exhaust flow dynamics and the solution is usually a design compromise that produces an acceptable amount of throttle response, horsepower and torque across the entire power band.
Now we throw in the process of scavenging in the exhaust system and all hell breaks loose.
In physics, we have inertial scavenging and wave scavenging. Different phenomenons, but both impact exhaust system efficiency and engine performance/longevity and affect one another. Scavenging effects are directly influenced by tube diameter, length, shape and the thermal properties of the tube material. As we modify the exhaust system via headers, mufflers, different style and size tube systems, with or without manifolds or cats, we are adversely affecting the original optimal design produced in a controlled environment by Ford engineers.
We are most certainly impacting reversion and the physical aspects negatively: in that with the loss of flow velocity not all of the exhaust is able to exit during the stroke time. Some of the reverted exhaust moves back toward the exhaust valve on a happy little collision course with the exiting gases where they then pass through one another like a freight train - with a little less energy and turbulence.
What happens when that reversion wave arrives back at the exhaust valve depends on whether the exhaust valve is still open or closed. This is the most pivotal moment in the exhaust cycle because the reversion wave can be beneficial or detrimental to exhaust flow and mechanical aspects of the valves depending upon its arrival time at the exhaust valve.
If the exhaust valve is closed when the reversion wave arrives, the wave is again reflected toward the exhaust outlet and eventually dissipates its energy in this back and forth motion. If the exhaust valve is open when the wave arrives, its effect upon exhaust gas flow depends on which part of the wave is hitting the open exhaust valve and this can cause physical negative affects on valves and their seals a well.
Since these happy little waves are comprised of two alternating and opposing pressures (one compressed and the other rarefied) each wave contains a compression area called a node at higher pressure and the rarefaction area called a anti-node at lower pressure. HERE IS WHERE LOW PRESSURE BECOMES THE ENEMY.... wait for it...wait for it......
………..A well designed exhaust will place the wave’s anti-node at the exhaust valve at the proper time for its lower pressure to help fill the combustion chamber with fresh incoming charge and to further extract gases from the chamber via vacuum. If it is not well designed and not reciprocating into the engine at the proper time you WILL see premature wear on valves and seals. If the stock engine from the factory is already mis-tuned or suffering a weakness in the system, it will happen. 'Tis a matter or physics, Gentlemen. I don’t make it up for fun. It is what it is.
(puts on rain slicker and hard hat for impending flames...
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