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Hello I'm new to this fourm and pretty much new to the Subaru world... I have a question, if I buy a cold air intake do I really need to tune for that? I mean I just purchased a 2017 sti and want to add a cold air intake but don't want to void my warranty. Also what's a good intake brand? Thank you guys in advance.


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The stock intake is a cold air intake. And the best part is that the car is already tuned for it! I know of only one intake that claims it needs no tune -- but conventional wisdom says even that is risky; most here would tell you to get a tune for any aftermarket intake.

Why do you want a different intake?
 

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It requires a tune.

The stock ecu calibration decides fueling from the maf based off the air flow characteristics of the stock housing. When you change that the air is no longer moving through the intake the same and the ecu gets incorrect readings from the maf and causes fueling errors.

Your car will either blow up and you'll pay out of your pocket to replace it because an intake isn't any less likely to get an engine failure warranty claim denied than a tune, or you'll sell it before it blows up and we will have a new member in the future wanting to know why his Subaru has blown ringlands.

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Why do people ask this question so often.. Yes.. you REALLY need a tune... I mean if you read 100 other places that you need to tune for a cold air intake, why will one person saying " I have one and I didn't tune!" change your mind..
 

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Intake=Tune. End of story. Aftermarket intake and an ecu calibration(tune) will jeopardize warranty. Nothing will "void" your warranty. But with an aftermarket modification and a reflash of the ecu, your chances of a warranty claim being granted are much less.
 

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Why do people ask this question so often.. Yes.. you REALLY need a tune... I mean if you read 100 other places that you need to tune for a cold air intake, why will one person saying " I have one and I didn't tune!" change your mind..
A lot of folks come from the n/a world and can't wrap their head around it. It took me forever and a very over my head explanation and simplification by zax for it to make sense in my mind.

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Discussion Starter · #10 ·
Thanks everyone for your feed back and clearing up my question much appreciated!

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A lot of folks come from the n/a world and can't wrap their head around it. It took me forever and a very over my head explanation and simplification by zax for it to make sense in my mind.

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Im one of those, but I accept it. XJ, What was the simple explanation? Is there a link?

An N/A car will benefit slightly or not at all from an intake change and of course benefit more with a tune for it, but this is the first car Ive owned where a Tune was required with any intake change or you risk blowing the motor.
Is the MAF simply not flexible enough or cant react fast enough to the air flow changes, is it a turbo thing or something else? This is my 2nd forced induction car but the first that is so sensitive to airbox changes as to risk damage if you sneeze next to it.
 

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It's only programmed to be accurate in the exact housing the engine was designed with. If you change the housing and location, piping, direction of air flow, speed of the air, then the CPU is using fuel based off of what air it thinks is coming in. In the case of high flow housings many fool the maf and provide more air than it senses causing lean fueling.

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It's only programmed to be accurate in the exact housing the engine was designed with. If you change the housing and location, piping, direction of air flow, speed of the air, then the CPU is using fuel based off of what air it thinks is coming in. In the case of high flow housings many fool the maf and provide more air than it senses causing lean fueling.

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Ok, but why is it like that? It doesnt have to be. MAF and SD sensors are designed (at least in other cars) to measure the amount and velocity of airflow coming in after it passes through the filter, so airbox design and whether there is one or not, should not affect anything as the MAF should see the increased air and then command the ECU to adjust fueling. I guess I still dont understand why Subaru designed a system that isnt flexible.

My HC is supercharged and people change out the intakes all day long on those. Diff size boxes, open air filters, running without a filter at the track etc. No tune and no worries the MAF measures new airflow and ECU sends more fuel, the injectors obey and everyone is happy.

The gains are negligible for the $$$ spent but at least you dont have to worry about it blowing up.
 

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Depending on how the air is moving through the intake it will fall into a different part of the air stream and cool the bulb differently.

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Depending on how the air is moving through the intake it will fall into a different part of the air stream and cool the bulb differently.

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Ah ok. Ive never seen a MAF that wasnt right behind the filter/filter box so that was never a problem. come to think of it now, I never looked to see where the MAF was on this car.
 

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Ah ok. Ive never seen a MAF that wasnt right behind the filter/filter box so that was never a problem. come to think of it now, I never looked to see where the MAF was on this car.
It's all in the fluid Dynamics of the air. You can place the maf in such a way that it's seeing less air. That's how most na intakes spoof the system to get marginal power gains.

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OK, I've typed this up enough so it should be clear at this point.

First you need to understand how the MAF sensor works and how the ECU uses the information to create injection events.

The MAF sensor collects information about the "Mass of air" that "flows" through the engine (MAF). In the case of a WRX or STI, this is a hot-wire sensor. The ECU passes voltage through a wire and the cooling rate of the wire is measured by the resistance in that wire. The higher the resistance, the greater the mass of air that is flowing into the engine. Actually there are a few caveats here.

The MAF sensor doesn't directly measure the total mass of air entering the engine. What it measures is the specific velocity and density of the air flowing immediately around the hot-wire and correlates this value to the total mass airflow into the engine. The ECU correlates the voltage of the MAF sensor with the total mass airflow into the engine by means of MAFv calibration.

But wait, I mentioned resistance earlier not voltage! Well it's just an application of Ohms law -- I=V/R; V=IR. In this case, the current in the circuit is held constant and dV is directly proportional to dR.

So the ECU has a table (the MAFv table) in which the Voltage of the MAF sensor circuit is calibrated against the expected mass airflow. This works great for cars that are not modified. When a car is modified and the hot-wire circuit is placed in a different position in the induction path or the diameter of the induction piping changes, these values become skewed. Turbulence and resonances within the induction piping can cause the MAF sensor to read a relative velocity and density of air HIGHER or LOWER than the surrounding cross section. Even worse, if you change the diameter of the housing, the relative velocity decreases and the reading is significantly skewed. This is the circumstance for any car running an MAF sensor.
 
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Thanks. That's the explanation I was not able to give but understand.

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An example:

If you enlarge the inlet ductwork the air flow will slow down and still flow the same mass. Thus if using the same MAF sensor, it now thinks less air is flowing so the ECU provides less fuel. But in actuality the same mas air flow is happening so now you have a lean condition = bad if too much so for too long.
 
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