Is it possible to use the bottom of a hood as the top of an air-box?

What I mean is, lets say you have an air box that draws filtered air through a cone filter and an air intake tube into an airbox. This airbox is open on top. Can you then place a weather seal on the top of this box to seal to the bottom of the hood and have a reasonable expectation that the seal would seal well enough not to let any stray dust particles in?

Why would I want to do this? Simply because doing it this way would give me more hood clearance for my application than if I made a lid for the airbox in a more conventional fashion. In my case, to make it more complicated, the air box sits on the engine so there is some movement involved(but not very much, the engine mounts don't have a huge amount of give). It seems like this should be possible, the engine vacuum is going to want to pull air from the path of least resistance, through the filter, rather than through a somewhat imperfect weather seal except in extreme circumstances(such as a filter being submerged in water, not that I would place it in such a way that would happen)

Isn't that basically what Shelby did with the big block Cobras and the "turkey pan"?

This how it was done by manufacturers in the '60's.

Any pictures? I don't think I have ever seen a setup like this...mainly because you have to be desperate for hood clearance to consider it.

I would say a lot depends on the quality of the seal and the firmness of the hood mounting. 

It would be a lot of fine work to get the tolerances close enough and you wouldnt want to open the hood in dusty/dirty conditions.  I would avoid that kind of setup if at all possible. 

I think if you were to use a seal with a lot of compression- like 1/2" or 3/4" weatherstripping, it would probably have enough squish to make up for the slop in the tolerance of the hood opening.  As others have pointed out, this was done back in the 60's on some muscle cars. 

It wasn't providing a filter seal though. Which seems to be part of the op's question?

Is this hood interface pre-filter or post filter? Prefilter fine, post, I'd be a bit more worried.

The Neon SRT hood was designed from the factory so that you could remove the 'scoop' insert...

...and cut a hole in the big, flat top of that airbox - and a correspond hole in an oddly-shaped underhood rib which just so happened to pass immediately over it to connect the two dots.  A little weatherstripping and viola.

The K&N units use the hood a lot. 

Duke said:

The Neon SRT hood was designed from the factory so that you could remove the 'scoop' insert...

...and cut a hole in the big, flat top of that airbox - and a correspond hole in an oddly-shaped underhood rib which just so happened to pass immediately over it to connect the two dots.  A little weatherstripping and viola.

 

Read that like Carl Weathers

This is exactly how the 96-02 Camaro SSs work as well. 

The Roush air intake on my Flex works like that, too.

Wicked93gs said:

What I mean is, lets say you have an air box that draws filtered air through a cone filter and an air intake tube into an airbox. This airbox is open on top.

Can you then place a weather seal on the top of this box to seal to the bottom of the hood and have a reasonable expectation that the seal would seal well enough not to let any stray dust particles in?

To directly answer your question:  No.  Especially not without some sort of inside lip to seal to.  Double no if it's on top of the engine.  Remember that engine bay air is slightly pressurized > atmosphere as well, air flows through the front and gets "trapped" in the engine bay, looking for a way to escape (hence hood vents).  So not only is your box under vacuum, it's surrounded by hot (very slightly) pressurized air.  You'd need a compression seal that your hood isn't going to be able provide to a moving object.

The exception where it'd be worth the risk is to answer the question "how long do you honestly want the engine to run?"  You see turbo drag cars with no filters in front of the turbo inlet, because they like to live dangerously :) 

I wouldn't do this for any engine that I cared about, but for something like the challenge it'll probably survive fine.

Make sure that your air flow meter of choice is as far after this box as you can make it, as this'll be a huge vacuum leak which equals large amounts of unmetered air.

In reply to WonkoTheSane :

Its post-filter, which none of the above examples cover. Its an ITB setup, which is where my design challenge comes in....the stacks stick 2.25" above the hood without any filters attached...the edge of the bell-mouth must be .75" away from the lid of the airbox(or the inside edge of a filter), meaning a typical .75" thick filter would add 1.5" of height to that, leaving me a ridiculous sized scoop(4" by the time I factor in clearance) if I wanted to clear it that way. I am fine with a cowl or scoop, but the maximum height I am willing to go is 3"(anything more is an eyesore). So my other option is an airbox...which reduces the total above-hood height to 3.125" if I make a "shaker" type airbox that sticks through the hood...but using the hood itself as the lid would allow a reduction to slightly less than 3", which is the appeal of the whole idea.

What about the weather seals around a door though? They do after all do a great job of keeping air out of the cabin(on a new car at least)...enough so that you can feel the pressure change each time you shut the door. Obviously the door latch itself applies enough pressure to keep the weatherstrip compressed and its not a moving part, but theoretically the same basic principle could apply....especially if you vented the engine bay to some degree(not sure if a cowl hood does anything to vent since its usually used for intake, not exhaust). I do understand there is a certain amount of risk to it...but the long block itself is stock, so its not like I would be risking a built engine.

650 RPM, which is about as much movement as it gets

In reply to Wicked93gs :

Ah, I see now, you are trying to seal post-filter. Yeah, I wouldn't do that. That motor is going to move more than you think at higher RPM's, too. I would do either a fully-enclosed "shaker" style airbox through a hole in the hood, or do a "bathtub" style airbox tray with filter like the Shelby Cobra pictured above. ANother option is simply to put filters on the stacks and cut a hole in the hood for clearance and just let them hang out in the breeze.

In reply to Wicked93gs :

That intake setup gives me some... movement... however

I agree, doing it post-filter is risky. I don't think I would. 

Could you use a wet-molding system (like fiberglass mat and resin) to form a lid for the airbox that is directly molded to the underside of the hood? That way you maximize your space without actually sealing to the hood. 

I believe engine movement nixes the idea, unless you use filters inboard of that seal (as many have said)

Honda does this (sort of) on some of their SUVs.

The air goes up from the grille, to that open area, to the duct down to the bottom of the airbox.  The open area is sealed to the hood with weatherstripping.

I think they did it this way because there wasn't enough room to run a four sided duct, so they just used the hood itself as the top of the duct.  Plus, this way makes it a lot easier to get to the battery.

note that the air filter is downstream of the open box.  I wouldn't trust weatherstripping to be dust-tight.

In reply to Wicked93gs :

How are your motor mounts?

In general, im going to say no, because it seems too likely to me that the hood seal could allow unfiltered dirty air into your engine.

However, depending on how little you like this engine or how short of time you expect it to last, it may be ok. 

Pete Gossett (Forum Supporter) said:

However, depending on how little you like this engine or how short of time you expect it to last, it may be ok. 

Yeah, that's what I was trying to establish above as well.

One other thing to think about is how rigid the hood & its mounts are.  I know on my Mazdas with flimsy (read: light) aluminum hoods, there's a LOT of movement from the hood itself as you change speed and the pressure differential from the engine bay to the top of the hood increases.   Sometimes, in traffic at highway speeds or on track, you can see the hood vibrating some from disturbed air flowing over it if you pay attention.   I wouldn't want that to be what's keeping dust out of my engine.

Thanks guys, I was really trying to get around the fact I am going to have to end up with a shaker type setup...which is a pain because it means I have to also design in some type of drainage system for the rain(not that I intend on driving it in the rain, but its inevitable I will be caught out in it). Nothing on this build can be easy evidently. Working on forming the bottom portion of the airbox now....endless fiberglass work..the stuff really does not like to turn corners.

I used water pipe insulation to seal a heat shield to the hood on my Super 7.  Underhood temps were messing with the IAT in traffic.