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Thread: Pressure drop across throttle body at idle

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    Pressure drop across throttle body at idle

    My car, like probably most of them, idles with a manifold pressure of -10psi (20inhg). Obviously the pressure on the other side of the throttle plate is something higher. I always assumed that the exact pressure depended on a number of factors (like the specific compressor, engine displacement, timing maybe?).

    Well today I decided to check to see at what 'looseness' I could get my BOV to start leaking, and to my surprise I found that I couldn't make it happen at all. So then I decided to measure the pressure just in front of the throttle plate and found that is was exactly 0 psi. I'm trying to figure out if there is a reason why its 0 psi, or if its just coincidence for this particular 13t car.


    (BTW, I made a rig to test my GReddy type RS and found that it holds an 11-15 psi pressure differential across its adjustment range. So a pressure differential across the throttle body of of just 10psi is not going to open the value at idle. Well, it probably opens it a bit, but not much.)

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    You get no pressure until you start spooling the turbos up. You don't have enough exhaust energy to spin the turbo fast enough to start pressurizing the air at or near idle. Just a wild ass guess but I don't think you'll start making positive pressure until the turbos are spinning over 20K RPM. Not a typo.

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    I don't disagree, but I'm surprised it's exactly 0psi and not some negative number. Made me think there might be a reason why its 0 as opposed to being vehcile/turbo specific.

    Edit - Now that I think about it.... It makes perfect sense to be 0psi... It could only be negative if there was some intake restrictions, and there better not be any restrictions bad enough to cause negative pressure... And, as you said, it's unlikely to be positive at such low spool speed.
    Last edited by q2418130103p; 07-15-2019 at 01:26 PM.

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    Thermal Engineer- Mmbr Since:8/2005 verified Feedback Score 0 Alex3000gt's Avatar
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    Whatcha measuring pressure with?

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    And old mechanical boost gauge. I couldn't imagine the accuracy is more than 2psi or so.

    In the case of this measurement though, the gauge literally didn't move when I connected it.

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    And in case anyone cares, I connected a pressure source to the pressure pipe side of a GReddy Type RS, and left the manifold side unconnected. I then checked how much differential pressure the valve wood hold before it started to leak as well as max pressure I was able to see on the gauge during a blow off. I did this for all adjustment screw positions.

    I'd say the leak pressures (marked 'hold' in the image) are probably fairly accurate, although the number is the pressure the valve would hold and not necessarily where it started leaking. The blow off pressures are only as good as my ability to open the valve quickly, my ability to read the gauge quickly, and the speed the gauge response.

    My takeaway was that this car, given a 10psi differential pressure at idle, won't leak through the BOV at any screw position.

    20190714_163329.jpg

    Heres a video, its not very interesting:
    https://i.imgur.com/dFcfgyp.gifv
    Last edited by q2418130103p; 07-18-2019 at 07:53 PM.

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