This is exactly why everyone keeps saying not to buy an open-element ram-air intake for an Impreza. The stock air box is there for a reason. The only way to get rid of the gutlessness at WOT and low RPMs is to put some sort of plenum back in the intake stream. There are no other ways to get around it.

It's not bashing when it's true.

 

Thats the exact problem we kept telling you Weapon-R buyers about... Low-end loss causing bogging. I had a simular intake on my car for awhile and it did the same thing.

 

its not just weapon-r, its all intakes, the stock air box has a reserve that holds air for when you need it at low throttle, any aftermarket tube does not, unless you get one that connects to the stock airbox

 

location is important, has to be right at the throttle body, not sure if you could build one. it probably needs to be designed just like the stock one, seems counter-productive to build one that works just as well as the stock one when you can just use the stock airbox. but i also dont want to use the ugly stock airbox, so i sacrificed the low end torque for launching at a little higher rpm and a little high-end power

 

That is not true in the slightest. Cobb intakes do not have this issue, neither do Ganzflow/PDM's or Ganzflows with the 3" modification, and even the Injen really long Intake seems to have less of an issue than the J-bend "hot air" designed intakes which do have this issue. The Weapon-R's issues with this problem are magnified by the poor quality air filter element.

Ive had personal experience with every intake mentioned exept the Injen and including the weapon-R combined with years of information about all the intakes and modifications for N/A Imprezas. Why do you guys persist on acting like we people who say this stuff dont know anything? Just incredible...

 

ganzflow
pdm
from cobb: Mounted directly behind the throttle body is the mysterious black plastic plenum whose function has been the subject of much debate. This box was completely empty inside, serving only as a chamber, until mid-1999 when Subaru included a secondary air filter inside. While we haven't discussed the details of this design with the engineers at Subaru, we can make some assumptions as to the reasons of this layout based on some theories behind intake systems. One of the primary reasons for this chamber is to act as a supplemental intake plenum to improve throttle response. When you go from low throttle to wide open throttle, the engine's demand for air increases drastically and the stock intake manifold with it's small plenum lacks the necessary volume to satisfy the engine's demands. To remedy this, Subaru uses this secondary air box as a supplemental plenum to supply this sudden demand of air. This gives us the throttle response we want and keeps our engine happy.

you need the stock airbox, ganzflow an pdm keep it. i dont understand how you can call me an idiot when you say that the intakes you listed work fine without it when most keep it

 

There are many reasons why one intake works and another doesn't. The Cobb intake works well because of its filter design- it can supply enough air for good throttle response while delivering more air at high rpm than other j- bend style intakes. The old Rallispec intake was reported to be not too bad either but probably not as good as a Cobb intake as the Cobb intake is a true CAI. You might try just putting a better filter on your intake first and see what happens.

Having a large airbox is good for throttle response but it can be a limiting factor at high rpm because the air wants to stall as it enters the airbox.

Gary at Compass Motorsports made an airbox style intake that had a good plenum size (I'm going to guess around 3 to 4 liters) and it had a straight inlet that the filter mounted on. Keeping the inlet as straight as possible probably helped a little bit at keeping the intake flowing well at high rpm. It looked like a really good design (he went through a few different versions before this one) but it just turned out to be too expensive to produce.

If you want to make your own I'd make it something like the Compass intake but I'd have a full radius inlet for the throttle body. It's also best to keep the inlet for throttle body as far away from the sides of the airbox as possible but given the size limitations it's hard to do. For the filter I'd either use the same filter as the Cobb or the new BMC CAI filter. If that's not possible due to cost, I'd just use a good K&N and make a heat shield and try to get the filter as close to the fender wall as possible.

Guess I'd better go fire up the welder!

 

I also forgot to mention the FLATT racing intake- these guys have been doing NA Imprezas for quite some time. An intake like this wouldn't be that hard to do.
http://www.flatt-racing.com

This is also the same company that modified the intake manfold by creating a bridge across the runners.

 

5Zigen made an intake plenum that looks like what you are talking about (although it was for forced induction.) It was basically just a big round cylinder. The important part is the transition from the air filter tube to your large cylinder plenum- if you can keep the angle of the transition near 15 degrees then there will be much less flow seperation and turbulence and the air will be less likely to stall as it enters the plenum. The problem with this is construction- it gets a bit tricky fitting it in the available space while trying to avois sharp bends in the intake pipe but it can definitely be done.

The other issue is finding large enough diameter tubing for the plenum (assuming you are making it in metal.) Whatever you do make sure the plenum has a full radius inlet for the throttle body- this will make a big difference in how your intake performs.

As far as the filter goes, a filter with proper venturi design will flow more air than no filter at all.