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Boost gauge reading, whats normal?

 
Old 07-09-2019, 12:51 PM
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Default Boost gauge reading, whats normal?

I have noticed that my boost gage is reading more negative lately during normal driving conditions. I just crossed 15k miles and it seems that under normal driving conditions the gage reads ~ -10

I seem to recall that it typically stayed in the range of 0 to -5 until recently.

What would be expected in normal driving conditions? Other than the typical hesitation of the M7 from a start the car seems to be running fine.
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Old 07-09-2019, 02:13 PM
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dar02081961
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The negative side as you know indicates vacuum or any condition in which the throttle is not wide open or you are not in a boost condition.
So depending on the throttle opening and gear your vacuum could be running between -20 to 0 anytime you are cruising and not in boost.
If I recall correctly I see -10 to -15 cruising at 70 mph or so at about 17% throttle opening.

If I get some time this evening I will check some old data logs and post the exact numbers.
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Old 07-09-2019, 04:39 PM
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small changes in pedal can make large changes on the gauge depending on what gear you are in. accelerate moderately in 1st and with the same amount of pedal in 7th and watch how much more the gauge swings up.
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Old 07-09-2019, 06:47 PM
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The PDR vehicle health page shows my boost at 30psi but my spring mountain car read 9.5-9.86psi any ideas?
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Old 07-09-2019, 06:53 PM
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Originally Posted by Pacembellum View Post
The PDR vehicle health page shows my boost at 30psi but my spring mountain car read 9.5-9.86psi any ideas?
Mine does that too. but i put it into Pi toolbox and its reading 30 psi only exactly when I get on or off throttle. reads 20-21 off throttle.

However for me, PI toolbox is only showing 4.5 in 1st, 5.25 in 2nd and 3rd and 5.5 in 4th for me. 85 degree ambient. Stock car with haltech intake and corsa X. figured i should be seeing 9.5 to 10.0. I only lost to a friend with intake/pulley/tune by 1.5 cars on a half mile run so its definitely making power.

well ****. my 60-130 was 8.8 seconds on 2 runs (pi toolbox). It should be in the 7.5 to 8.0 second range. wtf. Can texas heat really affect it that much?

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Old 07-09-2019, 11:32 PM
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I tried the 3" Corsa which slowed me down at the drag strip compared to the stock X-pipe with the factory cats. I re-installed the stock X-pipe and got the lost performance back.
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Old 07-10-2019, 11:35 AM
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Originally Posted by BrunoTheMellow View Post
Can texas heat really affect it that much?
doesn't really matter where the heat is from, it is a HP killer.

On these cars, it's even more pronounced because all kinds of power is pulled preemptively when it gets hot so not only do you lose power from the temp, you lose power from the ECM being scared.

Going from 50 degrees to 100 degrees can easily cost 50 HP, on the C7Z, even 100 would not be surprising.
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Old 07-10-2019, 11:39 AM
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Originally Posted by Higgs Boson View Post
doesn't really matter where the heat is from, it is a HP killer.

On these cars, it's even more pronounced because all kinds of power is pulled preemptively when it gets hot so not only do you lose power from the temp, you lose power from the ECM being scared.

Going from 50 degrees to 100 degrees can easily cost 50 HP, on the C7Z, even 100 would not be surprising.
Really? And you think it would cost me 5 psi of boost? I mean. Coolant is at <200, IAT at <130 .

Last edited by BrunoTheMellow; 07-10-2019 at 11:39 AM.
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Old 07-10-2019, 10:01 PM
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Originally Posted by BrunoTheMellow View Post
Really? And you think it would cost me 5 psi of boost? I mean. Coolant is at <200, IAT at <130 .
no it won't cost you boost, I think there is an error in your readings or interpretation of your readings. or your throttle wasn't opening all the way due to torque management.
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Old 07-11-2019, 08:05 AM
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Originally Posted by Higgs Boson View Post
no it won't cost you boost, I think there is an error in your readings or interpretation of your readings. or your throttle wasn't opening all the way due to torque management.
Yep
Error with PDR/pi toolbox data.

I used torque app last night with intake manifold pressure pid, it reads 23 ish psi at WOT. Makes sense if you add atmospheric pressure.

Then I used their "boost" pid and it read 9.4 psi at a quick 3rd gear pull. All normal.

I'll play around with Torque some more and try to find the PID that matches PDR so that I can add a multiplier to PDR data to create an accurate pi toolbox boost graph.
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Old 07-12-2019, 09:49 PM
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Figured out what was going on today. Went to get the oil changed today and I noticed a ticking coming from the engine bay when I pulled up. Service advisor heard it as well. Long story short had them take a look since my warranty runs out soon. Ended up finding that a plug boot was barely on the plug. Plug wire was securely seated and they cleared codes (I never got a check engine light so not sure that there were codes to clear). When I left I immediately realized the car I had been missing. The acceleration returned, the car started easily (less cranks), was smoother riding and I swear the clunkiness I was getting with shifts was improved...oh and the typical M7 stumble was much improved, it had gotten bad.

Not at all what I would have expected but glad it was caught and fixed.

Tech said the plug wire was basically arching onto the plug. They did not replace the plug, should it have been? Any long term internal damage that could have been done?
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