Flipping the upper lid will reduce the desired ram-effect of the OE designed air cleaner lid, as well as reduce the amount of air-flow.
The newer design aftermarket FLAT upper lids actually reduce air-flow by taking away the ram-effect.... but some say they look nicer... and you get to spend/waste money!
In other words, a contoured upper lid (matching a same contoured lower base) will always perform better than any air cleaner without the desired ram effect (up, over, down into the carb).
Yea, that's all nice and stuff, but back when I was playing on the streets, nearly 40 years ago, we didn't do timed automotive events in our town. The nearest drag way was 45 minutes away and they only ran on Sunday. We did our racing by who got from point A to point B first, and we judged the HP by the seat of our pants. I'm pretty sure the difference between the flat lid and the contoured lid wasn't detectable by either of our testing methods. I suspect loosing the base of the air filter housing, or pulling the air in from more area then through the single snorkel on the filter housing probably off set the flat lid, but there were no time slips to prove either direction. There were things that could be done, some were pretty cheap if you knew about them, and those things actually did improve things with both testing methods available to us.
Modern times have taken all the fun out of playing with cars. Now everyone has to do the same thing and gets the same results by spending the same large amount of money. Back in even as late as the 70s and 80s, ingenuity and thinking outside of the box was still the best way to go fast. Some things you could read in a book that would give you an advantage over learning things by yourself and was definitely better then having no clue, but many things were still learned by trial and error. You got out and broke stuff to figure out what worked and what didn't. If you didn't have the guts to break stuff, you probably was not going as fast as you could have been. One of the fastest cars I ever had would have never seen the light of day because the books (even back then) would all say it wouldn't work and would be a dog. It probably would have been if I would have driven it like "the book" said. The way I drove it with the combination of parts it had worked pretty well in the surroundings in which it was expected to perform.
I no longer feel the need to prove myself, but if I did, I'd just go to the dealership of my choice, and plop down more money then any house I ever bought, and order a new 800+ HP go fast new car, like everyone else is doing, then send it to the local tuner guy and pay him to do what he does. Then I would probably be concerned about how many 100ths of a second that flat lid on the the air filer might be costing me.
