I run the same airfilter as rdc767 and it is worth 5-7 HP in top end of HP range. This is dyno check and proved. The bending of the air and smoothing it out helps over not running anything. Same applies for cutting the top choke housing on Holley carbs. For racing and high RPM's this works for low rpms it cost you HP. It's more about bending the air to get to the carb and fuel/air mixture than anything else.
If you like burning up exhaust valves and seats with running straight pipes, it's your motor and money. You can extend the life of the valves by running a motor richer or with a higher octane of fuel to a certain point. I have seen over the years race motor with burnt exhaust valves and seat. Exhaust gas has little to no oxygen in it. If slowed down to get out the the headers you get a little back pressure and this helps. Overlap is also important with how much and again helps cool off exhaust parts. I have seen power improvements with good exhaust and small amount of back pressure in an exhaust system. This is on a motor that runs between 1,000 rpms to 6,800 rpms. This is not a 1/4 mile car. You really need to find a dyno and install a gauge to the exhaust and run power runs and change exhaust until you find max power from your car and see what the numbers are. Again no 2 motors are the same and what works for 1 car will not produce the same power on another car. Rick L. Ps K&N
oil air filter over time will hurt O2 sensors and cause drivablity problem and cause loss of MPG's Use little
oil as possible after cleaning. I have changed my front O2 sensor and picked up 1.2gpm after replacement. I went back to paper with $110.00 O2 sensor per side.