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Discussion Starter · #1 ·
Dumb question, My 2018 WRX is my first turbo car so I’m not a pro with knowing how negative boost works yet. But I’m still breaking my car in and have tried to stay away from boost. Everyone says it’s bad to boost your car when it’s cold and all that. Is that only apply to when my boost gauge is readig above 0? Or does that also apply when it’s in negative boost readings.
 

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Negative boost readings are vacuum. That's the amount of air the engine is pulling out of the intake system.

Boost is the amount of positive pressure above atmospheric conditions, vacuum is the amount of negative pressure below atmospheric conditions.
 

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Discussion Starter · #3 ·
So it’s okay to be anywhere in the negatives when the car is cold and stuff? Just don’t boost the car past like 1 psi technically right ?
 

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Your car is always in "negative boost" unless it's making boost.

Let's think of it this way. I'll try to kind of explain how it works and what it is. It'll be short and choppy because I'm on my phone.

Your altitude and location the absolute atmospheric pressure is 14psi. So if your boost gauge reads 0 you are filling the intake at the rate the Pistons are drawing the air out. If it's a positive number your turbo is pushing more air than the engine is pulling from the manifold.

Now, with the negative boost pressure. As the Piston moves down for the intake stroke it pulls a volume of air out of the intake. It does this by vacuum. The piston moving down creates a vacuum in the cylinder, the higher pressure air in the intake rushes to that area and a creates a loss of pressure in the manifold (negative boost) which in turn draws air through the intake. So at idle your car will read something like -11psi of boost. That's just the vacuum created by the engine drawing air out of the intake system.

The only time I've been told to be concerned about the number is if it is close to 0. It could mean the cylinder is pulling air from elsewhere. Either bad piston, rings, cylinder wall, head gasket, or failed manifold gasket.

Just the raw atmosphere is 14.7 psi. Or 1 bar. The manifold has a map sensor or manifold absolute pressure. At 0 bar it is a perfect vacuum. Your boost gauge will read -14.7 psi or -1 bar. At 29.4 psi it will show 14.7 psi of boost or 1 bar.

If the space in the cylinder, the manifold, and the air around it is all 14.7 psi air doesn't move.
 

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Hey op, yes you wanna stay out of boost (keep your boost guage less than zero) while your car is cold, you can adjust your top center dash screen to monitor old temps, you wanna make sure your oil temps are about 190 or above before you can push the car and make boost. The reason being is the oil in your motor isn't adequately lubricating the motors internals untill it reaches a certain temp. While breaking in your car you want to avoid reving your motor beyond 4k, above 4k every once in awhile is fine just not too often. Once you reach 1k on the odometer you can consider the motor broken in. Also while breaking in your motor you may want to avoid driving in the same gear at the same rpm while on the highway for too long, try switching gears and changing the speeds and rpms. You're trying to expose the motor and transmission to different speeds and rpms so things break in more evenly.
 

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When the car is cold, as long as you aren't nailing the throttle to the floor, it really isn't hard on the engine. Just drive it as if you have grandma in the car, but don't be all that concerned about staying out of boost. Once its warm, let 'er rip. (assuming engine is broken in).
 

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Your car is always in "negative boost" unless it's making boost.

The only time I've been told to be concerned about the number is if it is close to 0. It could mean the cylinder is pulling air from elsewhere. Either bad piston, rings, cylinder wall, head gasket, or failed manifold gasket.
When I'm highway driving in 6th gear around 70-75 mph I notice the PSI Gauge reads 0 (Zero) psi. is that normal?,
Also using about 15-20% acceleration so my foot is on the gas while it reads zero and finally changes to positive pressure I accelerate more. Just want to make sure there isn't something wrong with the car as this is my first WRX and first turbo car.
 

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It's normal.

Sent from my SM-G960U using Tapatalk
 

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Your car is always in "negative boost" unless it's making boost.

Let's think of it this way. I'll try to kind of explain how it works and what it is. It'll be short and choppy because I'm on my phone.

Your altitude and location the absolute atmospheric pressure is 14psi. So if your boost gauge reads 0 you are filling the intake at the rate the Pistons are drawing the air out. If it's a positive number your turbo is pushing more air than the engine is pulling from the manifold.

Now, with the negative boost pressure. As the Piston moves down for the intake stroke it pulls a volume of air out of the intake. It does this by vacuum. The piston moving down creates a vacuum in the cylinder, the higher pressure air in the intake rushes to that area and a creates a loss of pressure in the manifold (negative boost) which in turn draws air through the intake. So at idle your car will read something like -11psi of boost. That's just the vacuum created by the engine drawing air out of the intake system.

The only time I've been told to be concerned about the number is if it is close to 0. It could mean the cylinder is pulling air from elsewhere. Either bad piston, rings, cylinder wall, head gasket, or failed manifold gasket.

Just the raw atmosphere is 14.7 psi. Or 1 bar. The manifold has a map sensor or manifold absolute pressure. At 0 bar it is a perfect vacuum. Your boost gauge will read -14.7 psi or -1 bar. At 29.4 psi it will show 14.7 psi of boost or 1 bar.

If the space in the cylinder, the manifold, and the air around it is all 14.7 psi air doesn't move.
Not to be nitpicker but 1 bar = 14.5 psi and not 14.7 psi. Common mistake everyone, well not everyone thinks it 14.7. 15 years in the turbo industry and I've seen this mistake hundreds of times.
 

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That's fine, it was close enough for the point
 

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Not to be nitpicker but 1 bar = 14.5 psi and not 14.7 psi. Common mistake everyone, well not everyone thinks it 14.7. 15 years in the turbo industry and I've seen this mistake hundreds of times.
Hell I even make that mistake sometimes, I admit it.

The number is so close to Gasoline's stoichiometric ratio that I think even industry vets get the numbers mixed up on occasion.
 
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