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2010 Subaru WRX Limited Hatchback 5MT
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Discussion Starter #1
This is driving me nuts. I usually shift at 3k driving around the city with about 20% throttle and never fully get out of vacuum. Is this the case with everyone? Looking at my logs the closest I get is -0.9. Seems like I should be at least seeing 0 no?
 

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BooSTIng
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The only time you should see a 0 in vacuum is when the engine isnt running, or that very brief moment before increased manifold pressure goes above atmospheric pressure and you build boost. (or press harder on that skinny pedal to the right). When not in boost or above atmospheric pressure the car is in "vacuum". You will notice as you accelerate that the vacuum decreases until it goes into positive boost pressure, thus creating boost.

So being in vacuum is normal and will always be present in all forced induction engines.
 
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Discussion Starter #3
Thanks for the reply.

Sorry I should clarify. I’m just wondering when I should be out of vacuum, not stay at 0 vacuum. I know it’s not a one answer fits all deal but just curious, for example, at what RPM people are starting to build boost when puttering around town with light/normal acceleration.
 

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BooSTIng
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It all depends on your right foot. I can run my rpms all the way to redline and not hit boost if I am light on the throttle. It also depends on the set up. The vf52 turbo on your car is a quick spooling turbo. So applying heavy throttle you should begin hitting boost around 2500rpm give or take if I remember correctly. On the opposite side of the spectrum my STi has a much bigger turbo (gtx3582) and doesnt start building boost until around 4000rpms.
 
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BooSTIng
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There are literally numerous variables as to which a car will begin building boost though. For example in 4th gear on a flat straight road and applying approximately 20% throttle as you mentioned you will more than likely not get into boost. Use those same driving conditions on a different road though.... Say for instance going up a mountain or steep upgrade in 4th gear, 3k rpms, and 20% throttle and your car will start building boost. The reason for this is that the car is under a much higher load trying to climb the hill. Thus building boost to maintain the speed desired as per your throttle input.
 
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BooSTIng
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There is no designated rpm or throttle input that make a car reach positive boost pressure.
 
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2010 Subaru WRX Limited Hatchback 5MT
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Discussion Starter #7
That’s what I was looking for. Exactly, the VF52 isn’t a big turbo so I’m expecting it to get into boost sooner. Something just seems off. Even under hard acceleration and well into 3k it’ll see only a couple pounds of boost.

The other day - 2nd gear, 3500 RPM, 40% throttle, 0.43 psi. 4000 RPM ~6 psi.
 

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BooSTIng
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Have you tried a WOT pull in 3rd gear? If so, how much boost did you make?
 

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Discussion Starter #9
Only in 2nd, and I hit 15.5 psi during the split second I touched the floor as I was increasing throttle and let off.
 

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Moderator
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Your boost levels will be directly tied to engine load. If I'm not mistaken the 2010 should have per gear boost targets as well. So lower gears are going to see lower boost as the load to the engine is low due to the mechanical advantage of the low gearing. So you SHOULD see less boost in 1st and second. In fact you shouldn't see much at all in first
 

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Æternum
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I'm sorry, what's your question?

Are you saying that you aren't hitting target boost?

Your boost levels will be directly tied to engine load. If I'm not mistaken the 2010 should have per gear boost targets as well. So lower gears are going to see lower boost as the load to the engine is low due to the mechanical advantage of the low gearing. So you SHOULD see less boost in 1st and second. In fact you shouldn't see much at all in first
Yes, but boost control is only able to control manifold pressure when operating above wastegate pressure. Below wastegate pressure, MAP is purely mechanical and 100% a feedback loop between engine load, turbo, and exhaust design.
 

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Discussion Starter #13
Thanks for the replies and explanations everyone. I’m bettering my understanding of how this all works.

I just wanted to know how hard I have to accelerate to start building boost. Was expecting it sooner based on the way I drive.

From my research stock boost target ‘should’ be 14.2 psi but I hit 15.5 psi in 2nd. I’ve read how some cars boost more than they should, but will do some logging and see what happens.
 

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Æternum
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What's with the obsession with "boost?"

People seem to get fixed on a number when it's somewhat meaningless.

I can make a lot more power with less "boost" depending on a lot of other supporting factors.

The best way to think of this is exactly how Heywood decribes engines in his book "Internal Combustion Engine Fundamentals"

Engines are just air pumps. Generally the more air that they pump, the more power that they make (this is a generalization and reality is obviously more nuanced). Superchargers (Turbos are superchargers, FYR) increase the airflow. "Boost" is a measure of the Supercharger working against the engine's ability to pump air and therefore increase the overall airflow by raising the pressure in the combustion chamber. If you listen to the old muscle car guys, they'll tell you that boost is a measure of engine restriction and while there's some truth in that, the restriction is what allows additional oxygen to be packed into the combustion chamber on each cycle.

Turbocharging is a positive feedback loop (as is other forms of supercharging to an extent). A turbocharger reacts to exhaust gas volume and velocity which is proportional to engine load. The more load the engine sees, the more air that a turbocharger flows allowing the engine to achiever higher load -- up until you exceed the efficiency range of the turbo.
 

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What's with the obsession with "boost?"
In some instances it's a really good indicator of a problem. If a known target is being missed it's an indication of a problem. That's how I look at it for the most part.

I always heard old guys describe turbos as displacement modifiers. By increasing the amount of air going into the intake you increase the amount of air the engine is displacing outside of it's fundamental capability. So if you were to double an engines manifold pressure at a given rpm it would be moving air at a roughly equivalent rate of a vehicles natural displacement.
 
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