So I installed my center mount boost gauge today and something doesnt seem right. when i'm going down the highway at 75mph at 3300rpms the gauge is leaning toward the vacuum side about 10 in hg. I know the turbo spools up at 2750, so shouldnt the gauge be reading boost at 3300rpms not vacuum? The car is stock it does boost to 8psi, but it seems like maybe its not holding for some reason.

 

Guess you dont know how turbos work. They only build boost when you accelerate and create load. When you are crusing, you dont make boost. That is why turbo cars get good gas milage, when you are crusing, you are just a plain 1.8L motor.

 

ok thanks

 

Theturbo stil helps a little bit when you're cruising. You're right that the turbo isn't pushing (positive) boost during cruising, so 98A4QT, there is nothing wrong with yourgauge or turbo in that respect.

 

Im sorry, but i seem lost, where did i say anything incorrect? I said the turbo doesnt make boost. The wastegate is open and their isnt enough load to produce boost. The turbo will always be spinning, that why you get fast spool when you get on the gas on the highway and when you shift gears. Im sorry if you thought i should of went further in depth. Didnt think the question warrented that kind of response.

 

ok so the turbo is still spinning at highway speeds, but the boost is not built up because the wastegate is open. Learn somthing new everyday.

 

Ok.

 

Actualy, turbos do add back pressure.
They do not spool when cruising, cincy is correct. It is spinning, but not creating boost pressure. At WOT (wide open throttle) there is no vacuum, it decreases as the throttle plate in the TB opens.
There is no boost until your foot reaches far enough into the RPMS to allow the turbo to spool, and the wastegate to allow boost.

 

So are you saying, that when you are acclerating from 25 in Hg of vacuum all the way up to 0 in Hg vacuum (or 0 psi), that the turbo's wastegate is open the whole time, and the turbo is contributing nothing towardsstarting to build any manifold pressure whatsoever?My bad before, removed it so no one gets it backwards reading this.

 

Boogie, you are backwards my friend.
At idle, vacuum is the highest because the throttle blade is closed.
At WOT when the blade is open, vacuum is at its lowest.
The higher the RPMs, and the harder the throttle, the lower the vacuum. No exceptions.
Think of why diesels need external vacuum sources. They have no throttle blade.