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Can somebody smarter then me explain Vac/Boost numbers?

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Hello,

I just installed a VDO vac/Boost combo gauge in my gl10 turbo.

 

Measured at the plenum (after Throttle Body):

-20 inHg at idle (looks good)

~4psi at wide open. (low?)

 

Measured at turbo outlet

0 inHG at idle

~8psi at wide open.

 

So i have two questions, but they relate to one another.

I get with the throttle plate closed (idle) i shouldn't see almost any vacuum at the turbo outlet.

However how come i don't see vacuum when i press on the accel and i only see when boost starts to build? 

Shouldn't the (negative) pressure wave travel down the intake? Or is the flow rate too low to see a difference?

 

That would explain the lower boost numbers, however what number is more accurate when determining performance; why do most boost gauges measure from the wastegate if those numbers don't take vacuum into account?

 

And which is more accurate? Does my engine see "8 psi of boost" (the actual boost the turbo puts out) or "4 psi of boost" (the actual amount of boost minus the residual vacuum)

 

Or is my turbo not flowing enough compressed air?

 

Color me confused.

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The boost pressure should be the same. Sounds like possibly a boost leak somewhere. Should be an idle control check valve somewhere.... hoses, etc.

 

You don't usually see vacuum above the throttle plate. The throttle plate at partial throttle act's like a venturi and creates the vacuum. Once you get it open far enough to read anything above the throttle blade - well it's making boost at that point and the reading would be almost zero if it wasn't anyway.

 

GD

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The boost pressure should be the same. Sounds like possibly a boost leak somewhere. Should be an idle control check valve somewhere.... hoses, etc.

 

You don't usually see vacuum above the throttle plate. The throttle plate at partial throttle act's like a venturi and creates the vacuum. Once you get it open far enough to read anything above the throttle blade - well it's making boost at that point and the reading would be almost zero if it wasn't anyway.

 

GD

I figured once i started doing flow-rate calculations i was over-thinking it.

 

What is the canister that sits on the firewall on the passenger side. It's plastic with 2 lines coming off of it, one goes to the intake runner (i teed this like for my gauge) and the second goes across the centerline of the care and down into the drivers side interior. 

 

Is there any good way of tracing these leaks?

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The vacuum accumulator canister is for the HVAC controls.

 

A diagnostic smoke machine is pretty much a must have for troubleshooting turbocharged engines. It's very hard to find small leaks or leaks that only really show under boost without one. A good mechanic will probably charge a minimum 1 hour to do a hookup and show you where the leaks are. Otherwise you pretty much have to take it apart and do visual on every component.

 

GD

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The vacuum accumulator canister is for the HVAC controls.

 

A diagnostic smoke machine is pretty much a must have for troubleshooting turbocharged engines. It's very hard to find small leaks or leaks that only really show under boost without one. A good mechanic will probably charge a minimum 1 hour to do a hookup and show you where the leaks are. Otherwise you pretty much have to take it apart and do visual on every component.

 

GD

Probably not the worst idea to do a visual anyways; this engine is a mess. Is it safe to assume that since the vacuum is strong and steady that the leak is pre throttle body?

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