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But restriction is whats also creating scavenging??...
Actually, we have a dual 2.25" exhaust system up to the muffler. ..
2.5" single is good for you tell me
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We don't have a single 2.25" exhaust. We have a dual 2.25" (with no cross over) up to the muffler. What is is in debate is how much of an effect the muffler and tailpipe has on the overall system...
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Why I can't argue with that is a difference in logic, I don't comprehend those illogical thoughts. Scavenging is the relative vacuum created in adjacent branches where two or more air flows come together, pass by each other. Each branch or pulse creates a minor but measurable pull on the other branches. A restriction is a higher pressure at some point, created by something downstream that reduces mass flow. Any tight opening or area creates a restriction.
Every object slows down airflow, the cats, mufflers, resonator, connections(ball sockets), the weld points, and the pipe itself, are all restrictions. Restrictions all slow down the airflow exiting the engine, they are making it harder for the gasses to get out. That's back pressure, and it's all bad, all of it. The velocity of flow at some point along the system is not a measure of power.
Power comes from airflow of/inside/through the engine. The engine converts fuel and air into rotation of the crankshaft. Anything that increases the airflow speed through the engine, increases power. So back pressure in the exhaust system reduces power. You want to know how good your exhaust is, stick a pressure gauge in it somewhere. Check it at the front of the muffler, you'll see more clearly then.
A straight piece of 3" pipe has less restriction than a 2.25" piece of straight pipe. An engine only produces a certain amount of power, it's a closed system, which includes the inlet and exhaust parts(all of them). The front accessories are parasitic losses, as is the transmission, TC, axles etc. That's obvious stuff to car guys, and an engine dyno measures power without those restrictions, and without the exhaust restrictions, but with headers on.
Everything past the headers reduces final output power. If you put a full exhaust system onto the headers of an engine on a dyno, and tested it, removing one component at a time from back to front, the results would change for each item. Why is that, if the tail pipe is irrelevant, or the single muffler makes no difference than twin mufflers etc?
That is my point, every component does matter. The reason each matters is the why of it, each one increases the difficulty of the exhaust getting out. Without any exhaust, maximum power results because the gases are free instantly, nothing else is restricting it, just atmosphere. The cat pipes restrict the gasses, the tiny openings of the collectors(the ball connection(I'll measure that soon from a spare pair I have handy)), the two connections at the back of the cat pipes, the muffler, resonator and the 2.25" pipes.
Why is it hard to grasp what a dual exhaust is? Go back 30 years and everyone understood that it means dual paths for the entire length of the system, end to end. The two banks of an V8 don't constrict down to one pipe at any point, there's two paths throughout the system, which means two flow capacities. With any drop to one path, that cuts the flow capacity in half.
One muffler with a 2.25" outlet(doesn't matter how many input or how big they are), has the total flow potential of one 2.25" pipe. Two mufflers with a 2.25" outlet have the flow potential of two 2.25" pipes, double the flow of one muffler of the same size.
I don't know where you got that single exhaust system chart from, maybe someone made it from a dual chart. But find the common charts for dual exhaust power ratings, and divide those hp numbers in half. So you want to know what a 2.5" single muffler and tail pipe is good for, find the dual 2.5" line listed in those charts, and divide that power by two, that's what a single 2.5" exhaust is good for.
You seem to suggest that the power is determined prior to the muffler, and having two mufflers won't have any effect on power. If that was true, than the one muffler also has no effect on power, and three or four would also do nothing. That should also mean that a single 2" muffler would also not change the power. I don't buy any of that. I bet anything that removing the muffler and tail pipe(which also has a restrictive resonator), will increase power.
Now of course that is after proper tuning of the PCM program, every change alters the A/F ratio. Thus every report of power gains or losses, are inaccurate. The true change isn't known because nobody tunes the PCM(so rarely that it's an accurate generalization). Changes of airflow do not affect the fuel flow in the precise same amount, thus the A/F ratio is too lean or rich depending on what changed. Someone unbolting their exhaust and reporting results of any kind, is meaningless without re-tuning the computer.