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Old 03-12-2013, 04:06 PM   #1
steve2788
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Default Bad idle boost 2013 wrx

Hi, i my 2013 wrx which is currently stage 1+ cobb sf intake
my idle boost is sitting around -11 to -10psi boost and i thought that was perfectly fine but i was told today that i might have a minor leak causing this.
i just wanted to get a little feedback on if those numbers are abnormal and what/where the problem might be.
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Old 03-12-2013, 05:07 PM   #2
2011boostdwrx
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What is idle boost? Check your AP, what does it say or is this off the AP?
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Old 03-12-2013, 06:59 PM   #3
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Originally Posted by 2011boostdwrx View Post
What is idle boost? Check your AP, what does it say or is this off the AP?
Vacuum.

Vacuum leak somewhere
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Old 03-12-2013, 09:30 PM   #4
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cobb intake issues i bet. do a search. there is a reason soo many are for sale here.
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Old 03-12-2013, 09:41 PM   #5
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My AP says idle is at -10 psi, ill do my research on the Cobb intake, I'm taking it to a car shop and if they can't find the leak I might try a new intake
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Old 03-12-2013, 09:50 PM   #6
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Originally Posted by 424wrx View Post
cobb intake issues i bet. do a search. there is a reason soo many are for sale here.
Are you serious? Is this like the guy that was saying Rotella T6 was causing engine failures?
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Old 03-12-2013, 09:57 PM   #7
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I'm still confused. You're complaining that you have negative boost sitting at idle? So you're car is fine because you don't boost at idle?
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Old 03-12-2013, 10:21 PM   #8
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I'm still confused. You're complaining that you have negative boost sitting at idle? So you're car is fine because you don't boost at idle?
this..
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Old 03-12-2013, 10:23 PM   #9
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No I'm concerned because idle vacuum is suppose to be around -20psi and I'm at -10psi that is kind of a big difference that I should worry about...
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Old 03-12-2013, 10:29 PM   #10
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Quote:
Originally Posted by 2011boostdwrx View Post
What is idle boost? Check your AP, what does it say or is this off the AP?
There is NO Boost at idle, during idle you have vacuum. Please do not confuse people with bad terminology.

Quote:
Originally Posted by 424wrx View Post
cobb intake issues i bet. do a search. there is a reason soo many are for sale here.
No. The OTS tune for Cobb+SF Intake is just fine. Plus a protune will eliminate any "issue" you might think of. MAF Scaling is the issue that intakes cause, no one intake is that much harder to fix for it.

Quote:
Originally Posted by steve2788 View Post
My AP says idle is at -10 psi, ill do my research on the Cobb intake, I'm taking it to a car shop and if they can't find the leak I might try a new intake
"New" intake won't change a thing. Plus a different intake will require a protune.

Quote:
Originally Posted by steve2788 View Post
No I'm concerned because idle vacuum is suppose to be around -20psi and I'm at -10psi that is kind of a big difference that I should worry about...
Says who? Nothing in any of your posts say any problem EXCEPT an overly worried owner. If you are that worried, do a DATA LOG and send it to Cobb or post it in the Engine Management section.
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Old 03-12-2013, 10:29 PM   #11
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You don't read vacuum in psi, you read it in hg/inch^2. And although I don't personally have an ap, I have read on here from other threads with almost same concern, that it reads only in psi. So if you converted it I bet you would come up with being perfectly normal.
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Old 03-12-2013, 10:32 PM   #12
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Originally Posted by steve2788 View Post
No I'm concerned because idle vacuum is suppose to be around -20psi and I'm at -10psi that is kind of a big difference that I should worry about...
Where r u getting your info about -20 cuz mine has always sat around -7 to -9.7 with or without a cai.
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Old 03-12-2013, 11:10 PM   #13
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There is no possible way to have -20 psi of vacuum on this planet. Maybe if you drove to Saturn you could see the levels you want. Vacuum on the AP is read in psi, but vacuum is usually measured in Torrs for Torricelli an early Italian physicist (1608 - 1647). A Torr is equal to the displacement of a millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684 Pascals above absolute zero pressure. Vacuum is often also measured using inches of mercury on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is often measured in inches of mercury (inHg), millimeters of mercury (mmHg) or kiloPascals (kPa) below atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure in inches of mercury (e.i. 29.92 inHg) minus the vacuum pressure in inches of mercury. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg - 26 inHg).

So in layman's terms. Your idle vacuum is just fine. You're just confused because the AP reads it in psi instead of inHg like a normal boost gauge.

Last edited by superdonkey23; 03-12-2013 at 11:21 PM.
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Old 03-12-2013, 11:25 PM   #14
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Yea i derped pretty hard there. I even took out my intake checking that everything was air tight. Well sorry for this stupid thread but at least I know that my car is ok now. Thanks a lot for the input everyone!
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Old 03-12-2013, 11:41 PM   #15
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Quote:
Originally Posted by superdonkey23 View Post
There is no possible way to have -20 psi of vacuum on this planet. Maybe if you drove to Saturn you could see the levels you want. Vacuum on the AP is read in psi, but vacuum is usually measured in Torrs for Torricelli an early Italian physicist (1608 - 1647). A Torr is equal to the displacement of a millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684 Pascals above absolute zero pressure. Vacuum is often also measured using inches of mercury on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is often measured in inches of mercury (inHg), millimeters of mercury (mmHg) or kiloPascals (kPa) below atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure in inches of mercury (e.i. 29.92 inHg) minus the vacuum pressure in inches of mercury. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg - 26 inHg).

So in layman's terms. Your idle vacuum is just fine. You're just confused because the AP reads it in psi instead of inHg like a normal boost gauge.
My boost gauge reads -20 at idle.. I drive a 9.5:1, 2.0L though.
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Old 03-13-2013, 12:15 AM   #16
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Let me preface this by saying I had a 2003 Wrx that idled at -20 something psi and have had a 2011 Wrx and my current 2013 sti idle at -11 or so. Read on to find out why.

In my 2003 Wrx I had a prosport digital boost gauge. When looking at the gauge at idle it would read -23 to -24 psi but my access port reading in the car AT THE SAME TIME was at -10 to -11 psi. In both my GV subarus I used/use an access port to read boost levels and the levels read around the -10 to -11 psi.

The reason is the boost gauge runs off of an actual vacuum line off the turbo. The access port is based off of the actual sensor from the cars ECU. While in boost each read the same. From reading the post about 3 before mine the different scales of measure makes a ton of sense too. I'm going to believe his because it sounds smart haha.

I'm guessing your reading your boost around -10 psi off an access port? If so your fine.
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Old 03-13-2013, 12:54 AM   #17
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Mine sits at 24 at idle
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Old 03-13-2013, 12:58 AM   #18
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Mine sits about -22psi at idle with turbosmart boot gauge tee'd to the recirc line on stock bpv.
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Old 03-13-2013, 01:14 AM   #19
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Quote:
Originally Posted by Harbs View Post
My boost gauge reads -20 at idle.. I drive a 9.5:1, 2.0L though.


See how it says psi when it goes above 0 and then inHg when you are below 0? You are reading your gauge wrong if you think you are 20 psi in vacuum.
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Old 03-13-2013, 10:18 AM   #20
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This thread is such an enormous facepalm. Vacuum is not measured in PSI.
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Old 03-13-2013, 10:41 AM   #21
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1psi = ~2.04inHg. Do the math...
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Old 03-13-2013, 12:14 PM   #22
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Originally Posted by mechatricity View Post
This thread is such an enormous facepalm. Vacuum is not measured in PSI.
^ This.
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Old 03-13-2013, 12:29 PM   #23
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This thread contains so much fail.

Your accessport reads vacuum in PSI.
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Old 03-14-2013, 03:04 PM   #24
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Cobb read PSI in vacuum. Most Boost gauges show Hg in vacuum.

10psi = approximately 20Hg. There is nothing wrong with your car, just its owners ignorance.
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Old 12-05-2013, 03:38 PM   #25
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Originally Posted by mechatricity View Post
This thread is such an enormous facepalm. Vacuum is not measured in PSI.
Well, not usually. While it’s standard in most industries to measure vacuum in inHg or mmHg, this is pretty much for user-friendliness. Take a look at the gauge:

Quote:
Originally Posted by superdonkey23 View Post


See how it says psi when it goes above 0 and then inHg when you are below 0? You are reading your gauge wrong if you think you are 20 psi in vacuum.
I used to do analysis and design for a company that manufactured medical suction devices. Our high-vacuum regulators would start at 0 mmHg while off, and go down to -760 mmHg at full vacuum (not a level you should typically apply to a human, FYI). The gradations between 0 and -760 mmHg are much easier to see than 0 and -14.7 PSI.

I think superdonkey hit it on the head with his post:
Quote:
Originally Posted by superdonkey23 View Post

"Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure in inches of mercury (e.i. 29.92 inHg) minus the vacuum pressure in inches of mercury. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg - 26 inHg).
The way I look at pressure is that you can go MUCH higher, but only so low. Remember what pressure is: it’s particles that exert force over a surface. The elements and molecules that make up the atmosphere have instantaneous velocities of hundreds of feet per second (depending on temperature).

At sea level, we’re at about 14.7 PSI of pressure. If you have a boost gauge that is reading 0 PSI when the car is off, that means it is reading GAUGE pressure, not ABSOLUTE pressure. For our purposes, going below 0 PSI gauge pressure (14.7 PSI absolute) means reducing the number of particles in a volume. That being said, there should be NO measurement on ANY gauge below -14.7 PSI. This would mean that you have less than nothing in that volume because -14.7 PSI on the gauge is 0 PSI absolute.

If you start your car and idle, it should go down to about -10 PSI (4.7 absolute) if your engine is drawing air in and the turbo isn’t forcing anything, which leads to Hinshaw’s comment:

Quote:
Originally Posted by HinshawWRX View Post
There is NO Boost at idle, during idle you have vacuum.
When your engine is under load, your turbo will build boost pressures, bringing your gauge pressure above 0 PSI.

This is where my confusion begins. I have a logging tool that measured my idle boost pressure at around -24 PSI. After changing a setting, it measured it at -9 to -10 PSI, not unlike heyitsjeff’s experience:
Quote:
Originally Posted by heyitsjeff View Post

In my 2003 Wrx I had a prosport digital boost gauge. When looking at the gauge at idle it would read -23 to -24 psi but my access port reading in the car AT THE SAME TIME was at -10 to -11 psi. In both my GV subarus I used/use an access port to read boost levels and the levels read around the -10 to -11 psi.

The reason is the boost gauge runs off of an actual vacuum line off the turbo. The access port is based off of the actual sensor from the cars ECU. While in boost each read the same. From reading the post about 3 before mine the different scales of measure makes a ton of sense too. I'm going to believe his because it sounds smart haha.
...
Assuming anyone has read this far, I would like people to note that both heyitsjeff’s numbers and my numbers made shifts of about 14 to 15 PSI. Remember what the difference is between gauge and absolute pressure? 14.7 PSI. The setting I changed in my logger reset the base point for my pressure, so I may have to disagree with heyitsjeff about it being a scale issue (assuming he’s talking about inHg vs. PSI).

What the hell are these gauges supposed to be measuring and where are they measuring from to get readings of -23 PSI?
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