Jeffy, I was thinking about starting a new post, but since you were talking about it, I will just ask. Does yours easily rev up to 5000 RPM? I was thinking 4300 RPM was about as high as it gets for the 4banger? Mine seems like thats about all she will push without sounding like it is about to blow up. Plus she looses a bit of power after 4000RPM. The powerband drops from there I believe.
Mine will if I remove the venturi and the baffle on the front of the stock box. The nice thing about the box is that it's easier to tune by then to futz around with tube dia. and lengths. The 2.5L seems to struggle at higher RPM's because it isn't able to get enough air. Problem is you need to keep the air speed up as you increase the flow. Basically, it's like the big water pipe vs. a small one. The larger pipe will have more volume but lower pressure. The key is to tune it so you get higher volume but still keep the pressure high as well. This is where tuning comes into play.
At higher RPM's the engine is struggling since most cars are designed for every day driving and most people aren't going to be reving that high. So they tune the engine for lower RPM's. This locks the performance at higher RPM's though. You can really feel it on a stock engine as you try to climb over 4000RPM. You'll hit a wall where, the engine will seem to not make any more power and it's tough to rev higher. That's the engine struggling for more air and tries to compensate by reducing the fuel.
If I remove the baffle and the venturi, I'm able to rev up to 5000RPM's and over. The engine makes more power higher up but this is at the cost of lower end performance. I usually have to rev to 3100RPM's for the power to really start to kick in. This gets old fast when driving around town though as by the time I hit the powerband and shift, I have to start slowing down for traffic again. Currently, I have it where the power kicks in around 2700RPM so I have some power at 2500RPM and I'm not lunging as much.
There are a lot of other factors to consider as well. In short, there is a thing called a pulse-wave which the engine creates as it pulls air into the cylinders. You get a 1,3,2,4 then a pause for 3 strokes then it repeats. The pulse wave is created as it is drawn into the beginning of the intake when it hits the closed valves it then bounced back. What you want to do, on the intake side is to make sure the pulse passes the air filter before the intake stroke starts. When that happens the wave then reenters the intake. You want this to coincide with the intake stroke so it naturally supercharges the intake, at a smaller scale.
This also works in reverse with the exhaust. This is often referred to exhaust scavenging. If you time it right the pulse will hit the exhaust valves right as they are opening. The pulse will then reverse and help suck the exhaust out of the cylinders. This also helps pull the air into the cylinder as the intake valves are open as well.
It takes a lot of tweaking to find what works for you.
