Engine and Transmission Tech

I found this discussion on drop base air cleaners. There is a documented case of a 36HP loss from a base with a sharp transition vs a rounded transition. It is on a 455 cubic inch engine but the principle is probably valid for smaller engines as well. Since many British V8 conversions use drop base air cleaners due to limited hood clearance this is something to consider, and it should apply to both carburetors and carburetor-like fuel injection systems. Anyway, a drop base with a rounded profile can be acquired for little extra cost over a sharp-edged base.

[www.v8buick.com]

That is why K&N designed the Stubstack way back when,

[www.knfilters.com]

Most drop bases are on the order of 1-1/8 to 1-1/4" drop from the carb flange - that is what most conversions need in order to clear the hood..
I was fortunate to find an old cleaner base that has a 5/8" drop and a smooth transition profile....both cleaner bases shown below

There's another problem that most people fail to consider. The deeper the drop base, the closer the underside of the air cleaner lid is to the air horn on the carb. If your carb still has a choke, then this really disrupts flow. I have to laugh when people run a tall intake for "performance" then negate that (and more) with a ridiculous drop base air cleaner that totally disrupts flow into the carb. I know one person who paid dearly to get a custom built drop base air cleaner made to allow him to use a tall intake under an Oldsmobile factory O.A.I. hood on a 442. There was hardly any space between the underside of the lid and the top of the carb. That setup probably left 30 HP or more on the table as a result, which is likely all he got from the intake. At least he had the privilege of paying dearly for it.

May not be quite that bad Joe. Not much space on mine, either, nor any others running a drop base with the K&N 2 5/16"x14" air cleaner.

I pulled the lid & air cleaner and left the base in place. Picked up 4 RWHP on back to back runs according to the dyno. Of course, tht was on a puny, little 215. :)

I replaced my 1.35 inch drop base with a Proform 1.75 inch drop base and upgraded from a 2.5 inch tall filter to a 3 inch tall filter. The Proform base was not dropped quite enough so I ground a little bit off the mounting flange with an angle grinder. The top lid is domed so it clears the choke tower; not by much, but maybe slightly more than the original drop base assembly. I have always expected the limited choke tower clearance would cause problems but my last dyno run gave nearly 330hp from my 302 (now a 306) so, as Floyd also found out, it does not seem to be an issue. In the extreme I could remove the choke flap and tower but keep the fast idle cam. I modified a carburetor like that a while back and it worked reasonably well (in that case the air cleaner lid interfered with the movement of the choke flap itself). However, I would only do this if I had dyno data showing a marked improvement with the air cleaner lid removed.

"as Floyd also found out, it does not seem to be an issue."

You increased drop by 0.4" then increased air filter height by 0.5", for a net GAIN in clearance...

Sorry, but this really misses my point. I'm talking about the people who spend hundreds of bucks for a tall intake to make more power, then have to cobble together an air cleaner that puts the lid right down on top of the air horn to get hood clearance.

Joe, I understand that you are referring to a more extreme case, that makes sense.