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Old 09-05-2019, 12:33 AM   #1
Boscogn
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What's normal negative boost pressure?

Hi All,

My P3 vidi (odb2, no tap) reads my boost pressure as ~-20 is that normal? My buddy thought I might have a boost leak because he thought it should be around -10.

I'm 2013 gti DSG 4dr with Unitronic stage 1 ecu and tcu. I also have pflo cai.

When I'm goosing it I peak at only 12 psi and Unitronic says I should be at 16psi.

Thoughts?

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Old 09-05-2019, 01:47 AM   #2
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-20 is correct. -19 with the AC running.

Uni Stage 1+ should peak at 18 but as I have found out recently boost will be lower when the weather is cooler. On hot days I peak at 18 put on cool nights I am around 15. From my research it is because the computer is not set for boost target but for torque target. Colder dense air means less boost is needed for the same amount of power.

That is my understanding but I am still a noob :-/

Also you wouldn’t have a boost leak but a vacuum leak at -10 which is less vacuum then -20.
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Old 09-05-2019, 05:15 AM   #3
Boscogn
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Quote:
Originally Posted by GolNat View Post
-20 is correct. -19 with the AC running.

Uni Stage 1+ should peak at 18 but as I have found out recently boost will be lower when the weather is cooler. On hot days I peak at 18 put on cool nights I am around 15. From my research it is because the computer is not set for boost target but for torque target. Colder dense air means less boost is needed for the same amount of power.

That is my understanding but I am still a noob :-/

Also you wouldn’t have a boost leak but a vacuum leak at -10 which is less vacuum then -20.
Ty that is very informative!

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Old 09-06-2019, 11:05 PM   #4
Jiffyjetta
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Try loading the car up and see what kind of boost you hit. In 3rd gear at 2800rpms floor it and see what you hit. You should expect 15-18 psi between 3000-5000 rpms. Also, keep in mind that obd2 boost readings can be incorrect and respond slower that an actual boost gauge. Some stock diverter valves will also leak boost at those numbers too. There are quite a few different revisions of the OEM one. I'd recommend upgrading to the GFB dv+ I gained about 2-4 psi when I bought it.
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Old 09-15-2019, 04:06 PM   #5
bauhausVW
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I'm Unitronic Stage 1 with a NewSouth mechanical boost gauge.

On cold start it runs around -15in/hg. Once warm at idle around -22in/hg. Under load on 3rd gear pulls, I have no troubles hitting 16psi.

Hope that helps.
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Old 09-15-2019, 07:01 PM   #6
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~ -20-22 inHG
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Old 09-15-2019, 08:15 PM   #7
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I also have uni 1+ with vidi obd2 gauge and peak at 13 psi..
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