Last summer I had a close to stock 440 mag on the dyno, 8.9-1 compression. I asked the owner to bring some 91 octane and later found out the fuel he brought was 102-103 octane so the numbers were down from what they would have been. With no aircleaner or filter it made 386.3hp, with the factory dual snorkel in place it made 362.9hp. Just flipping the aircleaner lid upside down netted 380.9hp.
On my truck I run a home made ram air system drawing air from the headlite sockets. I have a K&N filter in it ahead of the baseplate. I installed an electric manometer to take pressure readings above and in front of the carbs. Tooling around town at about 25-30mph produced a negative number of -.313psi, at 130mph on my semi-private test facility it generated a number of +.378psi. I'll keep the filter in place on the street and on the track.Dave