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2017 Black Raven A8 V Sedan
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219 Posts
Discussion Starter · #1 ·
With these cars being out for years now has anyone figured out what causes these cars to make such inconsistent power on the stock tune ? With the engine and oil completely up to temp I can do back to back runs where a few of them will be consistent then one will randomly be 2mph faster both in 1/8 and 1/4 mile for no apparent reason.Sometimes this is the 2nd or 3rd run regardless of the weather but the difference is always considerable right at 2mph faster
 

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2017 ATS-V 6MT CWT Sedan, Tuned 2016 Explorer TT
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716 Posts
Air in the intercooler system?
But generally, I don't think this is atypical to turbocharged cars. The ECU picks up a bit of knock (real or perceived) and pulls timing. Or doesn't pull timing where it did previously.
 

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2017 Black Raven A8 V Sedan
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219 Posts
Discussion Starter · #3 ·
Those where the first things I checked and its definitely not air in the intercooler nor oil temp related.I have seen the same phenomenon in other stock V's where they can do 3 runs and out of no where have the middle run go 2 tenths faster with a worse 60ft time.Whatever the ecu does makes the cars power output completely random
 

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LF4 Platform Master Tuner
2018 ATS-V M6
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637 Posts
With these cars being out for years now has anyone figured out what causes these cars to make such inconsistent power on the stock tune ? With the engine and oil completely up to temp I can do back to back runs where a few of them will be consistent then one will randomly be 2mph faster both in 1/8 and 1/4 mile for no apparent reason.Sometimes this is the 2nd or 3rd run regardless of the weather but the difference is always considerable right at 2mph faster
If you want to send a VCM Scanner data log over I'll be happy to review the file and help you figure out the cause. My direct email address is [email protected].
 

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'93 SedanDeville 60 Special
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614 Posts
GM cannot control what gas owners put in their vehicles so they are forced to protect the engine within their calibrations
Add the use of E10 and now E15 gas and end results is more induced knock

So GM adds lots of conditions on how to react to knock in those tunes

In a simple case the PCM has two timing tables which are called LOW and High timing tables

IF the engine is running well and no issues then the PCM will command timing values from the HIGH octane
timing tables

BUT if their is knock reported then the PCM will look at the low Octane timing table for the same point as to amount of aircharge to cylinders and at what RPM.
It begins to start pulling timing down more towards the low timing values and if there is enough constant knock the PCM can only assume bad gas and jumps solely into using the low timing table values

That of course kills off torque, hence less HP.
PCM might then stay in low timing table until it sees gas was added to tank and jump back to using HIGH octane timing table.

In any case why good tuning helps reduce knock ( which reduces performance) and I suggest to all if running like boost is to assure the engine has injector sizes that help reduce knock and works well is installing a water/Methanol kit
that will reduce the airmass by good 30-40 degrees making a better charge to the cylinders.


Compare the low and HIGH octane timing table below to see how much timing can be reduced (retarded)
and that there is many otther factors the PCM looks at as to reacting to knock.

598841


598840


598839
 

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16 ATS-V Sedan
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9 Posts
If you have HPtuners or another ability to scan/log your V. What Team ZR-1 is referring to is called knock learn factor. If it is 0 your 100% on high octane timing table, 1 is 100% on low timing table.

Honestly what you're describing is an oddity. The stock programming with my car had it running the same et weather it pulled 114 or 116mph. I have determined if my V isn't gaining at least 24mph (8 speed) in the second 1/8, I might be running high on MAT temps. I bleed the intercooler system for at least 45 minutes.
 

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2017 ATS-V
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114 Posts
Have you thought about traction control pulling power in response to tire spin? That was a factor for me. There are so many factors at play here; TC, DA, slope, temps, etc.
 
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