Here’s the latest reader question, along with my reply!
Stuart asks: Back in the day, I used to flip the air cleaner to get a performance boost. Your thoughts on doing this now?
My reply: I did the same – and it did, if the air cleaner housing was restrictive – which many were. The car companies did the same, for the same reasons, on their performance models, which often came with open element air cleaners or a ram air system of some kind (as my ’76 TA has) that increased airflow to the engine by opening up the air intake system, usually to outside (cooler) air.
To make the most of this, of course, it was a good idea to adjust the air/fuel ratio (jetting, with older carbureted cars) to take account of the increase in airflow. 
The same principles apply to modern cars. Many can be fitted with aftermarket open-element/less restrictive air cleaners and some even have similar-in-concept ram air systems as well.
Tuning is important here, too – more so, arguably, because you might upset the delicate A/F ratio and cause a “code” to be triggered, possibly an issue with the emissions control system. In some cases, you might even have an issue with state emissions inspection, if you have to deal with that. Some states/areas are very strict about “modifications” of any sort, even if the car’s actual emissions aren’t affected. If you decide to buy an aftermarket air-intake system, look for one that is CARB approved as these are usually kopacetic in every state and shouldn’t cause any problems with the car’s ECU/OBD system.
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I did that on every vehicle I bought just as soon as I bought it. In fact, I did it to one after part of a test drive and the difference was day and night.
I had this idiot doing inspections once telling me I was fucking up doing that on a 350 Chevy. It had a tiny horn on it with maybe a bit more than an inch ID. I asked him if he thought an engine that is really nothing more than an air pump, can get it’s maximum amount of air through that tiny hole. He didn’t reply. Cue Forrest’s mother.