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Old 02-27-2010, 12:27 AM   #1 (permalink)
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Default Intake silencer fell off - hurts fuel economy ?

Hey all,
my main reason for posting this was to say - if you havent looked at your 2.5L H4 recently, pop that hood and make sure that all the intake silencers are still fully attached. I was hunting down a P0303 and found that the big one was resting on the fender...with a few mm gap between it and the flange where it is supposed to connect to the intake pipe.


Still has the P0303, I'm gonna replace the coilpack, plugs and wires becuase its time to do it anyway- hopefully that'll get rid of that.

None of the threads about intake silencers talk about the 'extractor' effect and the resonant volume helping the engine breathe better (If we think that suby intended such - has anyone dyno'd a 2.5l with the silencers on and off ?

so I'm thinking - if (one of) my intake silencers was disconnected and open to the engine bay - shouldnt that hurt power more than just from the hot air ?
2001 OBW, 2.5l 4EAT, 100k miles - Normally hauls a wife and 2 kids with 200lbs of toys/food/strollers in the back - She gets 19-20mpg in mixed city/highway driving and a light foot (got to be careful with the kids y'know =) I was thinking that this mileage was slightly worse than average but maybe I just found the culprit =)


-John
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Old 02-27-2010, 03:08 AM   #2 (permalink)
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In general, other than sucking hot air ... I'd go with "not really". The resonators don't supply "extra air" as the volume stays relatively the same and there is not enough vacuum in the intake tract to "extract" that air on demand.

There may be something to turbulence the intake tract but, with all that junk in line and the curvy path, it's not very "smooth to start with. The designers, I'm sure, allow for a "maximum demand" and probably increase that by a percentage. Look at the throttle plate under your average pedal input, it's barely cracked open.

If your car had a MAF, yeah, intake tract leaks are huge but, unless they changed something ... yours doesn't have one.

As far as the misfire, I'd check or change the plugs and wires and see if that solves the issue. There is not a "change' schedule I'm aware of on the coil pack, might just be throwing money away.

BTW, I pulled the huge plastic resonator box off mine a long time ago and put a plastic cap over the opening [ a black spray paint can top snapped right on ] and haven't noticed any decrease in mileage and honestly can't tell the difference as far as noise is concerned.
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Old 02-27-2010, 10:30 PM   #3 (permalink)
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I hate to prolong this discussion (I'm sure its been rehashed on this site many times and I just cant find it...

But it seems to me that Subaru put a lot of effort into intake silencers if they dont do anything. I can't find anyone who dyno'd their car before and after. I can't find any quantitative sound level tests. Its been my experience that since 1950 or so, anything a manufacturer could do to make a car cheaper - they will.
Those intake silencers are not expensive to produce ($30k injection molding die, $.01 of plastic per piece) but when you add up the man-hours it takes to add these things (oh, and you need a new intake hose with a big T-connection, and you need a rubber grommet to attach it to the fender, and a bolt to bolt it down to the frame...) I must assume that Subaru put it there for a reason. I think that we (all the threads I've found on this) are missing something.

maybe I'm mis-reading what people here say (I found more than a few threads about these things) but when I say 'extractor' effect it has nothing to do with supplying extra air because the intake silencer holds air. What those (intake silencers) do is make the intake volume larger, which resonates at a lower frequency and allows the pulses of air (air is flowing into the tube at all times, but the engine has 4 or 8 intake valves that open and shut, so even if you had an infinitely long intake tract, you'd never have fully laminar flow).
If you plug this stuff into a computer, you find that the intake tract resonantes at a certain frequency, and that vibration helps (or hurts) air to move into the engine. Same as a guitar - the soundbox is sized to amplify frequencies of a certain range, so if you play really high notes, they arent as loud (for many reasons, but some due to the fact that the guitars soundbox is too large to resonate well with higher frequencies. Intakes can be designed like camshafts to give more power at a specific rpm range(usually taking some power from another rpm range)

Generally, manufs. want a larger volume so they can resonate more air in at lower rpms (lower frequencies). Some cars (BMW?) have variable-length intake runners - lets the ECU mess with intake volume depending on rpm. It wouldnt surprise me one bit if Subaru did this (tuned those intake silencers) and it hurts performance (maybe it only affects the flatness of the torque curve) when you delete the silencer.

or maybe I should just give up, and import a diesel for better fuel economy =)

-John
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Old 02-27-2010, 10:50 PM   #4 (permalink)
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The silencers are actually functional to a point, they do cancel out a lot of intake noise from about 3,000-4,000 RPM. I removed all the silencers from the intake snorkel and sealed the holes, and acceleration is noticeably louder, especially when under load.

Despite having a lot of extra plastic in the engine bay and the extra costs associated with producing the extra plastic, it is what the customers want. The vast majority of people view their car as an appliance, and want it to be as quiet, comfortable and "numb" of an experience as possible. A car that is too loud is probably a deal breaker for many shoppers.
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Old 02-28-2010, 07:05 AM   #5 (permalink)
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Hey, It's your thread feel free to drag it out as long as you like ;]

In my mind, the boxes are "intake mufflers". Most new car buyers want their cars as quiet as possible. These boxes tend to muffle the sucking noise the intake makes under heavy acceleration as well as fill space in the engine bay which has an added deadening effect.

As far as intake air volume and resonance effect on actual flow to the intake runners I can't comment as I don't a degree in fluid dynamics ... anything I might babble out would be a random conglomeration of theories and speculation from years of spending way too much time on car, motorcycle, sandrail and atv sites ;]

I could speculate that the boxes have an added job of dispersing condensed moisture [notice it has a little hole in the bottom] or that the box itself adds to heat sink issues with the IAT's which would only serve to lower performance.

But, ultimately I yanked it off because it was a PITA when I check my air filter, pull the air box to check the diff fluid level and, because after I took it off I didn't see any performance losses [with my butt dyno] or consistent mileage losses [with actual calculations].

Here's a great thread about intakes [Ducati motorcycles] plenums, volumes, velocity etc. More than most people will need or want to know about intakes and their effect on engine power and an powerbands.

http://www.ducati.ms/forums/showthread.php?t=18854
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Old 02-28-2010, 08:00 AM   #6 (permalink)
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I think they do two things - they quieten down the induction "roar" and they smooth out the air flow and optimise it for certain rpm/throttle openings..

Some want their car to sound louder, that's their call.

Induction tuning is used extensively on non-turbo race engines - but these have very narrow power bands - the resonator chambers do their best to optimise the power/economy delivery of a wideband road engine which is why manufacturers spent so much time/money developing and producing them.

Unless you're building a race engine, or trying for max dyno power at one specific rpm to impress your friends - then they should be fitted the way the manufacturer intended.
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Old 02-28-2010, 08:29 AM   #7 (permalink)
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This isn't a WRX or a ZO6 vette, I doubt they put too much in to the development of the intake as far as performance is concerned. If the intake was routed in something like a direct path [straight over to the wheel well might be a good idea] I might be convinced other wise ... the engine/chassis rubber accordion isolator doesn't seem like the most advanced piece of engineering either.

If volume related to specific resonance at specific rpm/powerband was the deciding factor in the design ... couldn't they have just used larger diameter tubing for the whole thing ? isn't volume ... volume ?

I'll look around and see if I can find some real world [not parts manufacturer] dynos of CAI's to see what if any variations there are between the two.
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Old 02-28-2010, 08:55 AM   #8 (permalink)
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Quote:
Originally posted by ETC
This isn't a WRX or a ZO6 vette, I doubt they put too much in to the development of the intake as far as performance is concerned. If the intake was routed in something like a direct path [straight over to the wheel well might be a good idea] I might be convinced other wise ... the engine/chassis rubber accordion isolator doesn't seem like the most advanced piece of engineering either.

If volume related to specific resonance at specific rpm/powerband was the deciding factor in the design ... couldn't they have just used larger diameter tubing for the whole thing ? isn't volume ... volume ?

I'll look around and see if I can find some real world [not parts manufacturer] dynos of CAI's to see what if any variations there are between the two.
It's just not that simple - power at high rpm comes from short intakes with a high volume plenum - torque and economy at low/medium rpm comes from long intakes and pulse-assisted induction - but nothing is single state on a road car.
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Old 03-01-2010, 03:52 AM   #9 (permalink)
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I understand that but, he is talking about the ducting to the throttle body ... not the intake manifold runner length/diameter/design. While this ducting may have a slight effect on the air delivery to the throttle plate, the effect you describe are primarily effected after the air metering area which is the throttle plate to the cylinder itself.

If the ducting played this role in intake runner design, you would not see or need different length/diameter intake manifolds as you would be able to modulate this effect solely by lengthening the intake duct tubing. In a MAF car this tubing becomes much more important as the fuel mixture is in part effected by the perceived volume of air entering the motor and that perceived volume is effected by the diameter of the tubing and the velocity of the air.

While I would agree that air delivery has an effect, the fact that the tubing [in this particular case] delivers air to a box which then must pass through a filter which completely changes the flow path/velocity and in essence renders the delivery system [within reason] mostly irrelevant [other than the IAT and as long as the volume is sufficient].

In this case I don't believe it would matter much if the intake tubing was 50 ft long with 50 turns in it as long as the required volume to the box was met and the intake air temperature was not raised appreciably.

But. like I said ... I'm not an automobile engineer
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Old 03-01-2010, 11:17 PM   #10 (permalink)
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Yep... Long Thread...

There is much discussion on this on some of the Impreza boards out there.

The design of the Subaru Intake track is very deliberate and designed to do a few things:

-Reduce noise:
This seems like a normal thing for car makers to do. Keep it quiet, and make the customers happy. My NA Impreza and NA Outbacks aren't designed as aggressive performance machines, just great utility vehicles that need to sound a bit subdued.

-Tune the intake track for specific intake performance/economy:
As others have noted this isn't a deal on our cars, but Subaru does tune these to deliver a baseline for the MAF sensor and give a certain volume of air to the engine.

-Avoid resonance interference with the Mass Air Flow (MAF) sensor:
This is the biggy for our H4 NA engines with a MAF. The EJ25 engine will begin to cause resonance waves in intake track between 1200 and 2000 RPM. This is caused by the valves opening and closing, and the pressure differential they cause. At about 1800 RPM, the resonance gets very bad.

The Mass Air Flow sensor (MAF) senses air flow into the engine and helps the ECU come up with how much fuel to add to the air entering each cylinder. When the air in the intake resonates, the MAF can't correctly determine how much air is entering the engine. This causes the ECU to overestimate the air entering the engine and dump too much gas into the cylinder. This can cause a bit of a flat spot in performance.

The resonator that sits in the fender helps cancel out the waves in this RPM range. Without it, the engine can get a 'weak' spot around 1800 RPM, and over the long term the engine will figure out it should add less fuel. (long term fuel trim takes a big negative value). If the ECU didn't double check the MAF data with the upstream O2 sensor, there could be issues (stalling).

The resonance issue and it's relation to aftermarket intakes is best described here:

http://forums.nasioc.com/forums/show....php?t=1471819


With the aftermarket Intake I have on my Impreza, I get to experience all the fun things mentioned above. Above 4000 RPM the intake gives my engine a bit more power, and makes a noise that sounds like my wagon is trying to kill something. I do see a flat spot in performance from about 1500 to 2000 RPM when under 75% to 100% throttle. What is very interesting about the aftermarket intake is what it does for throttle response. I used to think the Throttle by Wire on my '05 was causing the throttle response to be slow. It turned out the intake was a bit of the problem. With the new intake the throttle is more responsive.

IMHO... the Subaru intake is simply designed to let the EJ25 make a certain amount of power/torque, emit a lower level of sound, and do it without issue from idle to redline. Aftermarket intakes can help with a bit more power and louder acceleration, but they do so with a few drawbacks (low RPM flat spot, ECU having to correct)
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