A Problem with CAI/SRIs and an Alternative (yes, finally)

[YouRock22]

Originally Posted by williaty

My extremely well educated guess would be that no, it doesn't work on the NA.

Here's why I think that:

Helmholtz resonators are extremely high-Q resonators. What that means is that they suck out vibrations in a tight, narrow frequency notch. This means they have to be tuned very carefully so that their resonance lands right on the mode you need to damp out. Very small changes to their physical design can produce pretty big changes to their resonant frequency. So if you can tell the two resonators apart by eye, they are likely tuned to VERY different frequencies. It's unlikely that the STi one is properly tuned to the mode that needs damped in the NA intake.


Beyond that, why on earth would you bother? The Helmholtz resonator IS NOT a flow limitation. Changing it can't possibly gain you any power. The only thing it's possible to do by going to the STi part is screw **** up with no possible up-side.

Thanks.
The only reason why I put in the sti resonator, is because the hole on the sti duct is larger than on the n/a resonator.
I guess I need to find another intake duct, that would fit the n/a resonator and reinstall again.

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[williaty]

Use the 2.5i resonator and the STi intake trumpet that you have. The fact that the hole is larger on the trumpet doesn't matter. As soon as you start moving forwards, the trumpet is shooting all kinds of air over towards the resonator. Even with the gap between the two pieces, the resonator is going to be sucking in almost entirely air from the trumpet, not from the engine bay. It's a non-issue in the real world.

If it reeeeeaaaaaaly bothers you, use some weather stripping around the inside of the end of the trumpet to make it seal tight against the resonator.

[Demonic13]

thanks for the help. I have a 2007 Subaru Legacy non turbo model same rules apply for a 2005 to 2007 non turbos. I ordered a cold air intake from cosmo's racing. I found out when installing I popped a spark plug which caused my check engine light once the spark plug was put back and check engine light code was cleared the car runs fine fully functional non hybrid cold air intake with no cel. just make sure all the sensors and hoses are connected as on stock and we're done. I'm not sure if it matters mine is a USDM mode

[YouRock22]

Originally Posted by williaty

Use the 2.5i resonator and the STi intake trumpet that you have. The fact that the hole is larger on the trumpet doesn't matter. As soon as you start moving forwards, the trumpet is shooting all kinds of air over towards the resonator. Even with the gap between the two pieces, the resonator is going to be sucking in almost entirely air from the trumpet, not from the engine bay. It's a non-issue in the real world.

If it reeeeeaaaaaaly bothers you, use some weather stripping around the inside of the end of the trumpet to make it seal tight against the resonator.

Ok so I did this set up of sti duct+2.5i resonator, and finally happy.

However, the hole size difference was still an issue and here is why: while the duct is just sort of hanging on the resonator and is not fit tight, it rattles right when the "nasty resonance" occurs between 1200-2000 RPM. You can actually hear the result of this resonance when the trumpet rattles against the resonator. So I did put some weather stripping around it and now it's perfect.

[williaty]

Interesting. Mine doesn't rattle but I can totes see how it could be a problem.

[Donsilvey]

P0420 catalyst deficiency. I have done everything I can think of minus putting my intake back to stock to kill this code I haven't had the car tuned either so this could be the simplest fix but probably the most costly as well. I have changed exhaust set ups intake styles (ram air) and natural. this is a very informative write up do you have any help for me. I have reset the CEL a few times it stays off the longest when it really humid.

[williaty]

Since you don't have a hybrid intake, start a new thread asking about this question and we'll help you.

Your question doesn't belong here.

[gmacmt]

So glad I read through this thread. Well, at least like the first 15 pages anyway. Was about to pull the trigger on a CAI for a 2006 saab 9-2x (impreza 2.5i wagon) to gain ground clearance on the front end.

So this brings me to a dilemma. I already cut up my front fender... Has anyone relocated the resonator? I dont know where you could relocate to...

Still tempted to do a CAI, even though I realize it is not necessarily a good idea. But the damn box is right where I want to put in my tube bumper... Already have the drivers side sorted with the relocated wiper fluid resevoir.

Any advice on the matter? Any way to mitigate potential issues while still running a CAI? I am not interested in getting more performance out of the engine, I just don't to cause any harm with bad fuel/air ratios.

Thanks in advance.

[williaty]

Originally Posted by gmacmt

So this brings me to a dilemma. I already cut up my front fender... Has anyone relocated the resonator? I dont know where you could relocate to...

It's unlikely that it would still be effective if relocated. The port of the resonator has to occur at a specific place along the intake tract to work optimally. If you move it, it won't work as well (and might not work at all).

Any way to mitigate potential issues while still running a CAI? I am not interested in getting more performance out of the engine, I just don't to cause any harm with bad fuel/air ratios.

There is no physical way to mitigate the harm of removing the resonator. I'm not aware of any shops currently offering to put a band-aid on the problem in tuning either that have actually proven they can do so successfully.

[gmacmt]

Thanks for the quick response williaty. Appreciate all your advice in this thread.

I bought the car I am driving for $2800 as a knockaround DD to use for a few years. It has about 60k on the motor. The drivetrain is in good enough shape that I care to keep it that way (bodywork is... otherwise).

I really don't want to keep the resonator where it is, so basically, it is probably going to come off. Here is a photo to illustrate where I am at... (not going to start building tube stuff until I get the lift installed)



Is it really not worth it to run a CAI if I dont mind slightly decreased performance? I mean, I obviously don't want a car that runs poorly, but I am also not going to be logging 1/4 mile times any time soon.

If you were to run a CAI (and I mean theoretically because I know that you wouldn't) what would be the best way to approach this? I imagine that all people running snorkels would come into the same issue as a CAI, although maybe the extended piping would aid in creating more laminar flow past the sensor (which is what is being accomplished by the OEM system, correct?)... Although you would probably need a large diameter to create minimal resonant frequencies... Haven't taken physics in a while so excuse the ignorance.

[williaty]

Originally Posted by gmacmt

Is it really not worth it to run a CAI if I dont mind slightly decreased performance?

The issue is not "slightly decreased performance". The problem is that if you cause the engine to operate in the range where the problem occurs, the engine won't run. In so stalling, it exits the problem area and will re-fire itself. Then it re-enters the problem area and stops running again. Repeat until you take your foot off the gas. The ECU's stock tune will hide some of this from you, but it's still pretty bad.

The thing to remember is that no one should be trying to make the engine operate in this range on purpose. If you get down to significantly open throttle below 1800RPM, it's because you're having an "OH ****!" moment. If already having an OH ****! moment, the last thing you want to do is to have made the car basically want to turn off.

although maybe the extended piping would aid in creating more laminar flow (which is what is being accomplished by the OEM system, correct?)

This isn't a problem with laminar vs turbulent flow.

This is a vibrational mode in the intake system.

[gmacmt]

Gotcha, thanks

[Roke]

I actually do not belong here because I own a Legacy, but this topic made me sign up. I've been using these forums for years, but now it's time to actually communicate. As far as I could find Nasioc doesn't have the 'introduction' rule. Forgive me if I'm mistaken.

Because my 2007 EDM Legacy BP5 2.0R showed some great result on a dyno last weekend with only a modded exhaust (+19 hp / 17 Nm) I decided to take the route of creating my own hybrid intake to maybe gain some more power for a reasonable price. I've already experienced the downsides of running the car without the Helmholtz resonator and a open air filter.



Sounds great, looks great, but the MAF clearly didn't like it. Iddle was ok, but the very low RPM's were bad and I felt less power in the high RPM's. Also no straight pull with WOT. (Is 'dipping' the right word? English is not my native language.) Also, the car was holding back like something was draining power. Everything mounted correcly. Triple checked. Ecu reset, etc. Even removed it for a while to install it a few months later; same bad results. It clearly felt I was not pleasing the great engine I'm taking good care of since 2010.

I think it's important to add that my Legacy has the new 'slot type' Hitachi MAF. Also, the ECU (Hitachi also, if I'm not mistaken) seems to be very unforgiving when it comes to any performance mods, so I have to choose them wisely. (The engine is equipped with AVCS, but I don't think that has anything to do with this topic.) My '07 Legacy is some kind of a scrossover model. Old body, new engine that was used in the facelift model that was introduced in late 2007 here in the Netherlands. Not easy when searching for parts, but that's another story.

I've already bought myself this kit to start off with:



This picture of my engine bay shows the intake parts that are going bye-bye. (Colored red.)



When I receive it I'll get some more materials to make it fit. I'm planning on making it as smooth flowing as possible and I'm also going to try to make it look good.

If you like I'll let you know about the progress. It might be nice to know for a fact if this mod actually works on a mark IV N/A Legacy.

williaty: Many thanks for sharing your knowledge, experiences and maintaining this topic since 2008. For the record: I have read this ENTIRE topic! It was a fun learning experience.

[Roke]

I quess some modding might be required.

I ordered some silicone parts, including a 45 degrees tube. Should work.

[Roke]

This is what I came up with so far:



I just separated the aluminium tube exactly in the middle of the straight part. The silicone bend gives me a lot of room to adjust the intake pipe, so it'll fit without any tension.

Used:
80>76 mm reducer
76 mm 45 degrees bend
76>65 mm reducer

Awaiting:
- 1 meter of 10 mm black silicone hose.
- 3 12 > 10 mm hose connectors.
- Time to install everything (including hose clamps).

I'll post the results when I'm done.

[Soul Shinobi]

It's looking good, thanks for documenting it.

[kraxxus]

I combed through the first few pages till I gained some basic knowledge. CAIs and bolt ons are in my past, but this is my first subaru. I was planning on a CAI/SRI until I came across this doing google/forum research before purchase.
I own an '07 Impreza 2.5i 5 speed sedan. Black.

My only question I couldn't assess a clear answer to, unless I missed it...
Do I need to have the ecu professionally tuned/reset after this hybrid install or when I reconnect the battery for the first time and follow the "hard reset" sequence to clear the ECU and re-calibrate the fuel trims will I be 100% ready to go? Thank you in advance to anyone able to answer. Special thanks to williaty for the original write up.
I literally just asked myself the other day why it looks like I have two air intake areas on the passenger fender under the hood. Now I know what a snorkus is and why it needs to be there

[GrundleJuice]

It's impossible to say for sure if you will need to adjust the MAFv scaling without trying the system and logging to see how the fuel trims change. I say it's impossible because each home made system will be different. With mine I ended up going to back to the stock scaling because they were very close and only had to make some small changes in the D range. If you have the tools to log I would do before and after, analyze and then decide what to do as far as any "tuning".

[kraxxus]

My knowledge on tuning and access to it in general is less than 0. Bolt ons with no calibration are my only experience. No equipment otherwise.
I'll look for a Subaru tuning shop here in Pittsburgh, otherwise I'm SOL. I think I know one, but I'm sure taking a car in for something like that isn't cheap. Thank you, I didn't realize it was a homemade system only, I was wondering why I couldn't find one anywhere

[williaty]

Originally Posted by kraxxus

Do I need to have the ecu professionally tuned/reset after this hybrid install

It's never a bad idea to have a custom tune for an intake. Even the stock intake, since Subaru was damned lazy and didn't even bother to optimize the tune for the hardware they created. However, I have yet to see a hybrid install where the resulting changes to the MAF function were enough to be called dangerous. So, you don't have to, but it's certainly at least a good idea to check up on it and make sure everything's ok.

[Roke]

Has anyone isolated their intake from engine heat?

I chose the course I took because I could fully use the large 45 degrees silicone bend. It's already blocking quite some heat this way. Now I'm thinking of buying al roll of heat reflecting tape to cover the aluminium parts. It's not pretty and it's not cheap, so I'd like to know some thoughts on this please. For example; does the intake get hot?

[kraxxus]

In my experience metal piping dissipates heat more quickly than the thick rubber stock pipes that seem to hold heat. If the metal pipes are holdable in your hand on a decently warm day with the engine at operating operating temperature I wouldn't worry about it. Plus even if the pipes are hot, the air moving from the intake enters the engine too quickly for it absorb any heat from pipes.

[Roke]

Thanks kraxxus!

I'll keep an eye on everything when the intake is installed.

I'm keeping a log of the intake temperature now. I'll do the same with the hybrid intake. Maybe that'll give us some useful data. I'm even thinking of temporarely wrapping the intake with some cloth or something like that to see if that makes any difference.

By the way. If moving air passes the intake too quickly to heat up, doesn't that make intake manifold spacers useless too?

[williaty]

Originally Posted by Roke

By the way. If moving air passes the intake to quickly to heat up, doesn't that make intake manifold spacers useless too?

Manifold spacers are useless for other reasons as well.

[Roke]

Originally Posted by williaty

Manifold spacers are useless for other reasons as well.

List of 'mods in mind' shortened.