Measuring boost back pressure

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Test 5:
New VS Racing Gen2.5 7875 with the 1.25 exhaust AR and new VS Racing 50mm wastegate.
Running on spring pressure only. Has a 4 lb spring in it at the moment.
4" down pipe that dumps right before torsion bar cross member. Everything else is the same.
This is from a quick shake down test, rolling into boost in 3rd gear.
Boost PSI: 6.4
Back Pressure: -1.5 (negative)
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IMG_0647.jpeg

6.4psi_Negative_1.5_BackPress.png
 
Are these results even possible? I thought the best you could hope for was slightly less back pressure than boost pressure. Could the bung be experiencing a vacuum effect due to its proximity to the wastegate? Or maybe the calibration is off since it hit -9psi? I just don’t understand lol

Either way, there must be a significant difference vs your first test. Thank you for posting this!
 
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turbovan said:
Are these results even possible? I thought the best you could hope for was slightly less back pressure than boost pressure. Could the bung be experiencing a vacuum effect due to its proximity to the wastegate? Or maybe the calibration is off since it hit -9psi? I just don’t understand lol

Either way, there must be a significant difference vs your first text. Thank you for posting this!
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I agree. It doesn’t make sense and there has to be something else at play. I logged about 5 runs and all but one showed negative numbers like this BUT one did show like 2-3psi of back pressure. The calibration of the sensor hasn’t changed. Next steps are to enable boost controller and add a little more boost and see what back pressure readings are.
 
Let's say you eliminated back pressure altogether. Isn't back pressure needed to drive the turbo?
 
RustyRatRod said:
Let's say you eliminated back pressure altogether. Isn't back pressure needed to drive the turbo?
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Theoretically no but in reality there will always be some present. You just need a pressure difference on either side of the turbine. Heat also imparts energy to the turbine, as it cools going across it.
 
What sensor is it? Is post 1 you said 100 psi but that’s all the info you gave. If it’s a 0-100 psi sensor it can’t measure vacuum so the scaling is wrong somewhere.
 
TT5.9mag said:
What sensor is it? Is post 1 you said 100 psi but that’s all the info you gave. If it’s a 0-100 psi sensor it can’t measure vacuum so the scaling is wrong somewhere.
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It’s a Low Doller 0-100psi. If it’s calibrated wrong than all my previous readings are bunk also
 
So calibration and scaling did change between firmware 1.5 and 1.6 on the MS but I think I'm just dealing with a faulty sensor. I use the same sensor for oil pressure and fuel pressure and they are all calibrated the same. I did update them to the new 1.6 settings and oil and fuel give the same outputs. I took the Valiant for another spin after recalibrating and get positive boost back pressure (1.5 back pressure at 5.5psi) but then the back pressure readings get stuck at 0.3 out of boost.
**Firmware 1.6: Select the “Linear 0.5v-4.5v” Option for an output range of 0.5V to 4.5V, corresponding to a pressure range values of 0 to 100 PSI.
**Firmware 1.5 or Older: Select the “Linear” Option for an output range of 0.0v to 5.0V, corresponding to a pressure range values of -12.5 PSI to 112.5 PSI.
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Screen Shot 2025-01-04 at 2.06.19 PM.png
 
Try validating the cal by hooking compressed air to the BP canister, you can validate against a regulator and analog gauge at various pressures.
FWIW I’m also using Low Doller sensors and I have a couple I needed to offset a few tenths.
 
I just hooked my back pressure lead to my oil pressure sensor and it works as it should. I’m swapping out this sensor for a spare I have on hand.
 
This is getting comical now. Apparently I’ve introduced an electrical gremlin cause now my oil pressure lead isn’t working. Swapped out the back pressure sensor and it looks better.
 
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