I posted my airbox adjustments in my garage thread, but I want to post it here so we can have a little discussion. Any car that I have, I usually remove the resonator/snorkel and install a high flow panel. However, on this 08 Vibe, I found some super strange things. Forgive me if the 03 is different, maybe someone can chime in.
STEP 1: Crazy screen removal
The first thing I noticed is the "charcoal filter" on the inside of the top of the airbox. My 2006 Lexus had this as well. This is supposedly for emissions. There are 6 plastic rivets that you can use a drill bit to take out. I simply used pliers to brake the frame and yanked it out.
I have looked up the 2003 Matrix Airbox, and it does not have this extra filter. Odd.
STEP 2: New OEM or K&N Panel insert
The OEM filter actually flows quite well. On an MR-S 1zz engine, a fellow dyoned an airbox vs cone filter and found only very small gains near redline. When I say OEM, I mean Toyota. A FRAM, STP, or some other cheap filter may not do as well. The same is true in the lexus community, the OEM filter does flow very well. You can tell the OEM by the soft cotton type appearance vs paper pleats. I opted for a K&N style, so I can wash it when it gets dirty (I drive ALOT). Many people do all of these mods listed and keep their original air filter. So these mods are free.
STEP 3: Snorkel choke pipe removal, air duct
The plastic pipe that connects to the airbox wraps around the battery, then up behind the headlight with a little horn. However, in the middle, it chokes down to what looks like 2". I can't believe that any car could breathe through that hole. Removal is not too bad. You must take off the battery tie down to get some wiggle room. There is a 10mm bolt in the middle holding it to the chassis. Once the battery is moved back and the bolt is removed there is just enough space to snake it out. You will then find a mystery hole next to the battery.
Going a step further, you could run a 2.25" flexible duct from the airbox inlet to this hole and get true cold air feed.
If you do not feel like removing the snorkel, you could simple drill a hole in front of the bottom of the airbox behind the battery, to get some extra flow.
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Just to stir the pot more, TRD offered a snorkel replacement for the MR-S. It was just an elbow that inserted into the airbox to replace the stock one. It was over $100 back in those years, so either there is something to it or TRD was selling snake oil LOL.
STEP 4 (optional) : Intake horn removal. Vane removal.
This is some debate about this. The intake horn in the top of the airbox is removable. It just pops right out. My question is, why is it removable? Some have popped it out, but who knows if this is a benefit or a loss. Obviously a horn shaped duct into the MAF would help with air flow on the low end. There is a screen in the horn though. So in theory removing the horn removes the screen as well. Also, because the horn is so long, you are only breathing from one side of the airbox. The MR-S 1zz does not have a screen, just a shorter horn and a larger airbox. HMMMMM.
By the way, the Matrix internal air horn does not have the slit in the bottom and the screen is right at the front. I *think* the reason why it is removable is because the top of the airbox is shared with the corolla, vibe, and matrix. You can then insert the horn that goes with the application at the assembly plant. Mystery solved?
Some people go as far as to remove the Vanes from the airbox (those plastic ridges). This is a bit extreme, I will not be doing this.
Overall, these modifications may do little to nothing. Maybe 1-2HP is gained at a screaming 6000 RPM which my auto slush box will never see. Still, if we have the same engine as the Celica GT and MR2 Spyder, our airbox should be just as nice as theirs is.
No results on the Vibe, but on a 1ZZ mr2 spyder with the resonator removed and the pre-catalytic converter, the dyno results were as such.
Peak torque at 4300 rpm
Peak power at 6400 rpm
So, you really have to "get on it" to see any gains. This might only be worth it for guys running a manual.