Here's link to the video
Enginemasters Air Filter & Housing tests
It is interesting.
One key factor:
RPM
Of course now I've forgotton the baseline peak rpm in that test, but I think it was somewhere around 6500 - 7000 rpm.
If the engine maxes out at less rpm, and has less displacement, then less surface area and more restrictive filter material/screening can be used with no measurable effect on peak Hp.
That's the basis the guys used at the end to make some rough top of the head estimates of how much Hp each combo they tested could be OK for.*
One effect of the air cleaner shapes that the graphs would show (if we could see them all) is the impact on the bowl vents. Air flow into the venturi is important, but changing the pressure in the bowls can do both good and bad things. With Holleys on a NA engine, its best to have consistant pressure in bowls. I speculate that the downflow only shapes may have upped the bowl pressure; where as the downflow with sideflow created turbulence and inconsistant pressures in the bowl.
*
For example.
15: Dual Snorkle Mopar= -53.1 hp max of 390
means this combo made around 700 Hp,
53.1 Hp less than the baseline of 750.
and it would be fine for any engine up to 390 Hp.
