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Old 11-14-2012, 05:53 AM   #1
WRBlueJ
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Question Underboost/fluctuating boost (datalogs incl.)

Hey guys,

So first of all I have an 05 WRX with the following:
-Invidia V2 catless DP
-HKS midpipe/flowmaster axelback
-Catless UP (from 06 WRX)
-Cobb AP flashed to stage 2 ACN91
-Large ebay intercooler
-Cosworth high flow air filter in otherwise stock intake

According to AP monitors boost levels are significantly under target; at idle boost hovers around -8 (target is -1) and is usually 3 psi + under target at highway speeds. Under acceleration it's not as significant but still outside the ideal range. The following datalog shows a pull in 3rd gear:

https://docs.google.com/spreadsheet/...GQ3SXZsSDkybUE

I've also had an intermittent problem where the boost will fluctuate under throttle. For whatever reason the above datalog was during a period when the car decided not to have the issue I only have a datalog of the fluctuating boost from when the car was stage 1 but I think it will show necessary info. The following is in 4th gear under moderate throttle:

https://docs.google.com/spreadsheet/...UZXQUUxaXRtN1E

I've had a boost leak test done and nothing was found, but since the fluctuating boost is intermittent I can't say for sure that that's not the problem...

I know I could use a HWG map to counter underboosting but I want to know what's causing it. I was expecting to have to use a LWG map to counter boost creep from the catless DP!

So... what's causing these problems? Could they be related? Any help is appreciated
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Last edited by WRBlueJ; 11-21-2012 at 01:25 AM.
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Old 11-14-2012, 09:45 AM   #2
jthatch12
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Your boost looks like it's just tapering near redline. Most tunes have this happen
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Old 11-14-2012, 12:07 PM   #3
IA Performance
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Vacuum at idle should be close to 16-20 inHg (inches of mercury). Keep in mind, boost cannot be a negative value, hence inHg. 8 psi = 16 inHG.

Boost Pressure Oscillation
This is often a result of over boosting, where the car over shoots the ECU's goal of PSI for that given Throttle Position and RPM. Example using arbitrary numbers:
- ECU only wants to see 8 PSI at 40% throttle and 3500 RPM
- Car makes 10 PSI at 40% and 3500 RPM
- Resulting, ECU will decrease the waste gate authority, and often decreases too much, making a oscillating pressure curve (fluctuation)

Car is Not Achieving Target Boost
From the data log, assuming in 3rd gear, the car is not achieving target boost. You should run the HWG map or shorten the wastegate arm by 1.5-2 turns. The larger TMIC may be contributing to this depending on the pressure loss it has compared to the stock TMIC.

Taper In Boost
During a full throttle pull the boost pressure will gradually decrease. This is not in the tuning, it is a limitation of the stock waste gate. No matter what a tuner tells the ECU to do, it will always decrease in the higher RPMs as the higher exhaust velocity/volumn forces the waste gate door open.

From Cobb's website:
Stage2 ACN91 v310
o Fuel Requirements: 91 octane fuel found in Arizona, California, and Nevada or better
o Intake Requirements: Stock airbox or COBB Tuning SF Intake
o Exhaust Requirements: Upgraded turbo-back exhaust
o Boost Targets: ~16.0psi peak boost pressure tapering down to ~12.0psi by the 7250RPM redline, +/- 1.0 psi

All Stage1 and Stage2 performance maps include a “HWG” and “LWG”:
HWG: Higher wastegate duty cycles (HWG) for use when normal wastegate calibration produces lower than targeted boost (i.e. under boost).
LWG: Lower wastegate duty cycles (LWG) for use when normal wastegate calibration produces higher than targeted boost (i.e. over boost).

Stephen@iaperformance.com
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Old 11-14-2012, 04:58 PM   #4
WRBlueJ
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Thanks for the good info, much appreciated. It cleared a few things up for me but I still have a few questions...

To clear things up on my end I understand that tapering boost is normal, what I meant by under boosting was boost levels under target at specific rpms/throttle.

Quote:
Originally Posted by IA Performance View Post
Vacuum at idle should be close to 16-20 inHg (inches of mercury). Keep in mind, boost cannot be a negative value, hence inHg. 8 psi = 16 inHG.
The AP boost monitor reads in psi, at idle it reads -8 and says target should be -1. Under light throttle it could read (for example) -5 and says target should be -1. What should I make of this? Are you saying the reading is wrong? The WG is closed in both examples so changing it's duty wouldn't affect boost (or vacuum) at this level.

Also, what I've gathered from your post is that my car is experiencing symptoms of both over boosting and under boosting. As you can imagine this is mighty confusing haha. I will try first shortening the WG arm by the amount you said then running the HWG map. But... wouldn't either of these induce over boosting and thus induce the fluctuating boost problem?
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Old 11-14-2012, 05:25 PM   #5
IA Performance
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Are the -1 target values coming from the TD Boost section? It may be best to ignore this section as Target and Actual are 2 different things and it can get confusing since Target does not mean the car will actually achieve it.

At idle the negative 8 reading is correct as that corresponds to 16 inches of mercury.

Next step for boost pressure settings
If I read the post correctly, the car only fluctuated with stage 1 and has not done so with stage 2. If this is the case then stage 1 was over boosting, whereas Stage 2 needs more waste gate duty cycle. I'd try running the HWG map and see if:
1- the car achieves close to target boost (+/- 0.5 peak psi is acceptable)
2- the PSI fluctuates up and down

If the car only achieves 15psi then I would shorten the arm by 1.5 turns. That should net ~16.0 PSI.

Keep us updated as you find out more

Stephen
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Old 11-15-2012, 04:54 AM   #6
WRBlueJ
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The AP live data monitor list I look at has 'boost' and then 'boost target' right under it, TD boost is further down the list. I'll take your word that -8 (16 inHG) is the correct reading at idle but I previously assumed that the AP's boost reading should match its boost target reading at specific rpms/throttle.

The boost has fluctuated with stage 2, it just didn't happen when I took the above datalog. I'm not sure yet what conditions induce the fluctuation; I'm pretty sure it does it in hot and cold weather.

I'll have some time to mess with this tomorrow, will do as you advised then report back
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Old 11-15-2012, 12:53 PM   #7
IA Performance
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Target versus Actual after often not the same. The target serves as a general goal for the car, but it cannot always achieve that. When you go full throttle at say 2000 RPM the wgdc should be 99% and the boost pressure goal around 13psi, although the car is actually producing much less boost pressure. This is a limitation of the motor, not a problem. Don't worry about it.

you may also want to look at, and replace, the vacuum T that goes between the turbo's compressor housing and the wastegate. These T's get fragile over time and a slight crack in it can create issues. The same applies to teh vacuum lines going to the wastegate, the compressor cover, and to the factory electronic boost control solenoid. If the lines are hard as a rock by the turbo, I would replace them. The only KEY thing is that you remove the restriction pill from the compressor side line and put it in the new vacuum line. Otherwise, the turbo will not make much boost over 8psi. More details may be found in Cobb's article here:

http://www.cobbtuning.com/Technical-...es-s/70674.htm




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Old 11-16-2012, 03:35 PM   #8
WRBlueJ
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Ok, like I said I had some time to mess with this stuff yesterday and mess with it I did; got a bunch of good data.

So first I loaded the HWG map and did a pull in 3rd here's the datalog:
https://docs.google.com/spreadsheet/...U1acGVvbWQ2OUE
Still under boosting, not much change besides the higher WG duty and slightly later peak boost

Then I shortened the WG arm by 1.5 turns while still running the HWG map and soon realized that it was way over boosting. I flashed back to regular stage 2 and recorded this pull in 3rd:
https://docs.google.com/spreadsheet/...m5kcEpQQ3ZHNVE

It peaks just under the target of 16 psi which I was happy with until I put down throttle in 4th and noticed spikes to almost 18 psi and fluctuations as a result shown in this datalog:
https://docs.google.com/spreadsheet/...1IwSnhJaGhVX3c

To counter the over boosting I flashed it to the LWG map (still has the shorter WG arm) and did a pull in 3rd:
https://docs.google.com/spreadsheet/...HR0QzVuTWZBbUE
The boost spikes to a little over 16psi early and there is some light fluctuation that I didn't really notice while driving.

I'm thinking I should lengthen the WG arm by .5 turn and continue to run the LWG map? Or is there a problem here that can't be fixed by just messing with the WG?
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Old 11-16-2012, 03:43 PM   #9
IA Performance
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I second your vote for lengthening it by 0.5 turns.

Stephen
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Old 11-19-2012, 09:04 PM   #10
WRBlueJ
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After a lot of messing around the WG arm is back to the original length, and I'm running the LWG map. I'm pretty sure the problem this whole time was over boosting, and the under boosting that I previously experienced was a result of ECU compensation for over boost.

I also realized that I have an exhaust leak at the DP to mid pipe connection point. I'm going to replace the gasket that's currently there with a thicker crush gasket to seal it up, then record another data log to see where boost levels are.
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