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Old 11-12-2003, 11:31 PM   #1
fastenova
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Legacy Building a cold air box...

Hi guys. I am going to build a cold air box for my intake out of sheet metal (I think) and I was curious if anyone had a rough template or plan for the cuts and bends to save me some time...?

So I've looked a little bit, and can't come up with anything definitive re the intake+WOT from start = stall issues... Anyone know more about this? From a stop or slow speed, jam on the gas and the engine drops down to like a few hundred revs, then takes about 3/4 to a full second and picks back up with mad pull... why the stall?

I'm running a stock engine plus Rallitek intake w/ conical filter, grounding mod.

Thanks
-Aaron
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Old 11-12-2003, 11:44 PM   #2
sybir
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Because Subarus have a tubular intake plenum with no real volume, when you run a cone filter on a tube, connected straight to the throttle body, you lose that big black box that holds a reserve of air. Because our motors also tend to pull hard from down low, they pull a lot of air down low.

What happens is you punch it, and it dumps fuel in, but it has to draw all the air from the fender/engine bay through the filter with vacuum, instead of sucking it straight from that large area directly upstream.

I've gone back and forth between a stock airbox and a cone filter mounted on the MAF, both retaining the torque box, and after an initial drop in revs the first couple of times I ever ran the new setups, I've never had hesitation again, and it pulls slightly harder (there's nothing downstream of the airbox, as I gutted the equipment in the fenderwell.)

You can avoid the problem by rolling into the trottle, the problem comes when you crank the butterfly wide open, but there's a vacuum behind the TB instead of atmospheric. That delay/hesistaion you're getting is your motor starving for air and running very very rich for a second, until it builds enough vacuum to pull the air it needs.

I firmly believe that the factory setup on the 2.5's is there for a reason, because the motor pulls too much air to be driveable down low with no air reserves.


YMMV, but that's my 2 cents plus a few bucks more

Last edited by sybir; 11-12-2003 at 11:50 PM.
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Old 11-13-2003, 09:09 AM   #3
KD7000
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Sybir- do you know what it is about Subaru engine design / engine managment that would require this extra volume of air?

Most other manufactures who have 4 cyl. engines in the ~2.4-2.5 litre size do not require a similar thing... and their intake paths aren't that much shorter than the EJ's.

Just curious.

-Brian
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Old 11-13-2003, 10:25 AM   #4
sybir
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It's not that the intake path is too long, it's that it lacks volume. It can flow a lot of air, but it can't hold it anywhere, and a throttle jab when you're idling depletes what's in the runners pretty quickly. If you then have to pull air from outside the filter, that takes time, and is why you get hesitation. The entire intake path may not be much longer than V or inline engine, but the key is the volume between the point of restriction (filter) and the intake runners.

The area in between the TB and the runners can't be more than about 3 cubic inches of air, compared to a lot of more conventional plenums that can hold 3-5 times that amount. Also, on most normal cars, you have a plenum that's maybe a foot wide. From the entry point of the TB, the air has that big swirl area, and the intake valves are maybe 2-3 inches from the plenum, so they start making vacuum and pulling instantly. The Subaru intake runners probably span close to a foot, so there's some lag between when you punch it and use up the very small amount of air in the runners, and when it starts building enough vacuum to pull air. The torque box provides the reserve right there, kind of as a backup plenum, so that when the TB opens, the long runners create instant suction and pull through a hit of air that doesn't empty that torque box, but creates a vacuum there that keeps it full. Most cars have a buffer area in the plenum; because we don't have that volume, the torque box (or an intercooler being crammed full by a snail) is the best engineering hack.

Once you have vacuum, it's like having a normal straw as opposed to a piece of one that's 2 inches long. The normal straw holds enough liquid that it can be full for a meaningful amount of time after the cup is empty. Take off the torque box and you have no volume. You're getting the drink quicker from the cup, but you can't compensate for any interruption in the flow, because you have no reserves. Once you start moving, and especially up high, you can pull air more quickly through an intake, as it's a shorter path with smoother bends, and your motor is gasping for air and building vacuum hard. Down low, nothing can flow enough, at atmospheric pressure, to compensate for the lack of static volume.




Granted, all of this stuff happens quickly, and we're talking fractions of a second difference, but you sure do feel it.....

Last edited by sybir; 11-13-2003 at 10:30 AM.
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Old 11-13-2003, 12:33 PM   #5
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Post wow

Fascinating, That is very interesting and brings up answers to question i didn't even have. Thanks Sybir. Fastenova if you ever get your cold air box built let me know I have thought about building one but I haven't had time to get around to it.

~Mike
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Old 11-13-2003, 12:48 PM   #6
MannySoobia
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Thank you Sybir for Opening my EYE I always wondered why I would get a hesitation when I "Gunned It"

-- Manny S.
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Old 11-13-2003, 01:38 PM   #7
sybir
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I do what I can

FWIW, I don't notice any real difference between a factory airbox and paper filter (with snorkusectomy) and a cone filter mounted in its place. It's a little louder, but that seems to be about it.

If you're having problems with it hesistating for an inordinate amount of time (you were saying a second, mine was only ever rmomentary) try resetting your ECU with the intake on the car. Pull the battery cable, step on the brake pedal a few times to drain the juice from the electrical system, then let it sit for ~30 minutes. Connect your battery, fire it back up, then go drive the car like you hate it. You'll get crappy gas mileage for the first tank or so as the ECU reunes; it starts at a rich baseline and then leans out the car and optimizes curves. Made my hesistation almost nonexistent. It probably won't completely compensate for a tubular intake, but it should make it more livable.


That ECU reset also does the trick if you get a CEL.......
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Old 11-14-2003, 03:03 AM   #8
fastenova
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Sybir, thanks for the wonderful explanations! So for anyone with experience building cold air boxes, what's the best way to seal the box to the fender? And should I build the box so it's big enough to cover both holes on the inside of the fender or should I go the Subaru OEM plug route? TIA!

-Aaron
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Old 11-14-2003, 12:43 PM   #9
sybir
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Sealing to the fender is easy. It's sealing by the headlight area(if you're going to) that's hard. If you're going to have sealed access to the fender, having 2 holes probably isn't that big of a deal. When I have a cone or my stock airbox on, I plug the front hole, because the stock airbox pulls formthe rear fender hole, and my cone slots right into that opening as well.

I have an OB, so I haven't experimented with a cold air box; I just pulled the backing plate from the hood scoop and get cold air getting crammed into the, then it circulates around the bay enough to cool the air a little bit.



For sealing, they make really thick foam weatherstripping, like 1 inch thick. Put that on your sealing edge and you should get a passable seal (it doesn't need to be airtight.....



The thing I haven't experimented with is a cone mounted inside the airbox, of the flow advantages of a cone, but with the smog legality (visually) and cold air draw from the fender.



Hmmm
to the garage!

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Old 11-14-2003, 03:29 PM   #10
sybir
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Scratch that idea, angles are wrong
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