Wow... Absolutely some good readings here!! I tend to agree with synolimit's theory on Helmholtz resonators. I thought they are mainly used to cancel out the drone, but this "restore air pressure" thing totally makes sense to me.
And that's why it makes the HKS Hi Power THE best exhaust out there because it has Helmholtz resonators!
JK JK...
I also agree with Chii on the exhaust piping diameter theory. The bigger piping and less restrictive exhaust systems aren't always good for
NA cars. It's not like the bigger the better. It has a limit. I learned this in a hard way.
2.5 inch piping is close to the optimal for the NA setup on our car.