Lew is correct. Dual fours will raise manifold pressure (provided the total venturi area is more than whatever single four setup you would compare it to) and raise HP (slightly). This will be accompanied by an increase (slight) in total airflow to the engine, but that increase will be small in comparison to the doubling of filter inlet area. So dual filters with whatever carb setup is better than a single filter of the same type.
However... If you are using the stellings (are those the little bitty ones I see so often?) air filter with a 1 inch element or something that small, even doubling it by going to two filters (with dual carbs) will likely not produce optimum horsepower.
Now... I am talking optimum HP here. A post above indicates little change was seen with hood open or closed. I believe that. Removing the turkey pan provided increase, but the scientific method was disrupted because a gasket was changed at the same time and may have affected the results... My point here is this: The increase in HP is likely to be small unless the inlet path is very restrictive. However, racers live and die by slight advantages over the competition. As pointed out earlier NASCARs use huge filters. How much HP does this add? Well if it adds 2 or 3 it was well worth the money to use it in a restrictor plate class... Many people spend major bucks on headwork that gains 7-10%. If a $50 filter can gain 1%, don't the economics work?
The flip side: These cars are space limited and we may have to live with less than optimum intake systems including filters. Heck, some folks even run webers - look great, but too small in series with each cylinder for best HP. Even a 750 on a 428 is way too small for optimum HP at 6000 RPM. Yes, I know about the VE calcs blah, blah, blah, but those are not for optimum HP, they are for best VE or good driveability, which are not the same as best HP. Each of us has to decide where to draw the line and make the compromises we choose. My discussion is only intended to provide an understanding of what the factors in the compromise are and what each otion may or may not add.
I do agree with an above comment (and I see this all the time) where folks spend what I know has to be $15,000 - $20,000 for an engine setup with all the trick bells and whistles and fuel pumps and on and on and then puts a little air filter on it and takes out about $2000 out of the performance. Why put it in there in the first place. I think its mostly for braggin rights and just because the owner likes it. WHich is why we build these things in the first place.
