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Old 03-06-2007, 01:06 PM   #1
FenKa856
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Question New to the World of Boost: Couple of Questions!

Hey guys! New to the forum, looks great!
Bought an '07 STi Limited Edition, New with 93 miles on the clock!
Car came with boost gauge!
boost gauge reads in Psi, I think... (0, 0.05,0.1,0.15)

1. when i'm accelerating in second gear, the boost gauge SOMETIMES goes up to 0.05 then drops before i even take my foot off the gas to shift.
i say sometimes because on other times ( when im turning around a corner in second) the boost gauge does not go past -0.05) Question: does this sound normal???

2. How will i know if i have a leak in the piping while i'm driving??? sometimes (in second gear while accelerating) i would be hurled back in my seat, and other times it feels like being naturally aspirated.

Now theoretically speaking, if my foot is on the gas, the turbo should still be spooling correct??

being new to turbos coming from NA, I'm not even sure if i'm asking the right questions.

maybe if you guys ask Me the questions i can figure it out! thanks for reading!!
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Old 03-06-2007, 01:10 PM   #2
FenKa856
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Is there a way i can test my cars boost while driving???
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Old 03-06-2007, 01:20 PM   #3
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Mash on the pedal in 3rd,4th,5th or 6th gear at about 3-3500rpms and the needle on the boost gauge will (should) shoot up to max boost. I am not 100% familiar with the factory gauge so I don't know exactly where it should go to. Somewhere just short of 1bar is my guess? You will not see full boost in 1st or 2nd if these were the gears you were trying it in. Load also factors into how much boost you will see. Read the FAQs and UNABOMBERS manifesto.
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Old 03-06-2007, 01:21 PM   #4
conker69
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The boost gauge reads in MPa (megapascals) not psi. .1 MPa=14.5 psi.

As long as the engine is running your turbo is "spooling". That is the blades are moving. HOWEVER, this does not mean your are making positive boost (above 0 MPa).

As far as your boost question goes it all depends on engine load. You won't get much boost for example when you floor it going down a hill. Go up the same hill and the boost level will change. Yes you can have a passenger read the boost gauge while you are driving. In a higher gear (3rd or better) with a nice load on the engine you should be hitting .1 MPa. It is completely normal for the boost to drop while shifting gears because for a second or so (depending on how fast you shift) there is no load on the engine.
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Old 03-06-2007, 01:21 PM   #5
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Fill out your profile too please.
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Old 03-06-2007, 01:26 PM   #6
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Boost will taper towards redline as well.
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Old 03-06-2007, 01:44 PM   #7
mxboy15u
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Sounds like you could be running just spring pressure...
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Old 03-06-2007, 01:45 PM   #8
Schell
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Ok, I will take a stab at a few of your questions, although you can probably get better answers from some other folks on here. And since you have not stated otherwise, I'm assuming your car is stock. I also don't know where you're at in the cars break in, so I'm going to assume it's done. I will explain why this is important at the bottom of my post.

First of all your boost gauge reads in MPa not psi. the rough conversion is when the gauge reads 0.1 Mpa this equates to one atmosphere or 14.7psi if you're reading half of this (~7.4) then you're definitely under boosting at full throttle. Now you may not hit "full" boost in every gear. How much boost you are getting is dependent on engine load and not necessarily throttle position, although the two are correlated. Meaning in first and second gears, the short gearing means your engine is not under full load and therefore you may not hit target boost. In the higher gears accelerating puts more load on the engine (harder to turn larger gears) so the car compensates by using more boost. Therefore just because your foot is on the gas doesn't mean the turbo generating boost.

That being said, the stock boost gauge is fairly inaccurate (it's mechanical, as opposed to electrical). Electric gauges read boost pressures from the ECU and therefore are very accurate. Although it should not be that far off. Correct me if I'm wrong but this car should have a target boost of around 15psi (slightly over 0.1Mpa).

The car at idle, decelerating or at constant velocity (low engine load), the gauge reads in vacuum (-X.XX Mpa) when you give it throttle (accelerating or during periods of high engine load [very high speeds where large drag forces comes into effect]) the turbo will begin to spool and the gauge will show higher pressures, and eventually positive pressures, yes. The turbo will hit peak boost around 3500 RPM at full throttle (again correct me if I'm wrong, but this should be close enough for explanatory purposes, and will taper to redline).

You will know if you have a leak because the gauge will not hit target boost or full vacuum.

Additionally the 07's have a known throttle hesitation issue which sounds like it is being remedied once the car "learns" your driving style. Alternatively Subaru has a ECU reflash which it is offering to 07 STi owners, although you need to read up on this in the appropriate thread (also in this forum).

Now the big equalizer or neutralizer depending on where you're at in the car's break in....
If the car has not been fully broken in, you SHOULDN'T be at full throttle until the break in is complete at which case you shouldn't be hitting your target boost yet anyway. So if the gauge is reading 0.05 at partial throttle this is completely normal. If you are only hitting 0.05 at full throttle then something is wrong and you should take the car in to the dealer to have it looked at, you may have a small boost leak.

Oh, yeah. And while I have tried to answer your questions in a polite and through manner because I have nothing better to do at work right now, please remember there is a search function and may of these questions could be found by running a search for them. And fill out your profile, it will make answering questions easier in the future.

Shameless plug: Check out www.IWSTI.com It is specifically for STi owners, although they don't mind if lowly WRX owners hang out there too .

Last edited by Schell; 03-06-2007 at 01:50 PM.
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Old 03-06-2007, 01:58 PM   #9
FenKa856
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Thank you for the prompt replies and yes you guys ahve helped me a lot in understanding my car. ( althoug, i'm sure there is a lot to learn still).

and Schell, thanks for the insight. believe it or not i was just finishing registering at IWSTI.com before i even read this post, LOL.

sorry about the beginners post but i just didnt know what to search for, nor was i even sure of the questions.

thanks again folks.
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Old 03-06-2007, 02:08 PM   #10
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Quote:
Originally Posted by mxboy15u View Post
Sounds like you could be running just spring pressure...
What are you talking about?
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Old 03-06-2007, 02:27 PM   #11
Schell
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No worries. The search function can be tricky, and often it's hard to decipher the results, so I'm happy to help. See you over at IWSTI. Same screen name?
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Old 03-06-2007, 05:13 PM   #12
FenKa856
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yep! same name.
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Old 05-15-2009, 02:25 PM   #13
stijet
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thanks for the info

chasing low boost issue on my 06 also
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