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Discussion Starter #1
Gents,
I've been logging some runs on the JB4. I'm running map 6 with a max boost of 3.5 in the 4000-5500 range. In that range, I'm only seeing the boost get up to 16.6, where I would expect it to hit over 18. Are any of you seeing the same? Wondering why its not boosting higher. (I'm in an RS so ECU boost should be hitting 14.7 in that range, which it is not....it's more like 13.1).
 

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Gents,
I've been logging some runs on the JB4. I'm running map 6 with a max boost of 3.5 in the 4000-5500 range. In that range, I'm only seeing the boost get up to 16.6, where I would expect it to hit over 18. Are any of you seeing the same? Wondering why its not boosting higher. (I'm in an RS so ECU boost should be hitting 14.7 in that range, which it is not....it's more like 13.1).
It's probably due to weather being cold (denser air). Less work for the turbos.
 

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It probably is most likely the cold weather or your altitude. I have also read from other JB4 users on this site/n54 forum that the sensors that are only in the RS's turbos could affect the boost output as well.
 

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You could honestly do 3.0 @ 3k, 3.5 @ 3.5k, 4.0 @ 4k, 4.0 @ 4.5k, and 3.5 @ 5k rpm.

Then again, I had FI LDPs & CBE when I first installed the JB4 on my '17 Q60 Red Sport. The stock exhaust and secondary cats might be the cause of back-pressure not allowing you to achieve the requested boost level, too.
 

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As previously stated, you should consider adding additional boost sooner than 4000 RPM. The stock torque curve comes to life starting at about 2500 RPM and you are missing out on the potential torque improvement if you don't add the additional boost until 4000 RPM.

Having used a JB4 on my RS for well over a year before adding a FI CBE and HKS LDPs, it was quite apparent to my butt dyno how much opening up the exhaust added to the performance the JB4 provides with the same Map 6 tapered boost profile. There's one advantage to living on the Oregon coast as far as my RS is concerned. The air temperature is usually cold (annual temperature average = 51.3 F) and I'm essentially at sea level, maximizing the available O2 being blown into the engine.
 

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Discussion Starter #7
My map is as follows:
2500 +0
3000 +1
3500 +3
4000 +3.5
4500 +3.5
5000 +3.5
5500 +3
6000 +2
6500 +1

Regardless, I'm looking at both ECU PSI and actual boost after my adders above.
ECU PSI was hitting 13.1 at 5000 (so 16.6 net boost).
ECU PSI hit 13.9 at 5500, but only 16.4 net boost, so not sure what happened there, as I would have expected 16.9.
Shouldn't "stock" boost of 14.7 be achieved with "stock" LDPs and "stock" exhaust? What's the definition of "stock"?
If I added LDP's and CBE, wouldn't I expect to get north of 14.7 psi boost even without the JB4? (or is it just faster achievement of the boost?)

Weather conditions were 48 degrees, clear day, (tough South Carolina winter).
 

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You're talking about fractions of a psi. There are a ton of variables including density altitude that will impact commanded vs actual boost. Just because a given psi is commanded doesn't mean it will be achieved. If you do downpipes and exhaust the motor will breathe easier and make more power at less boost. Since the car has electronic waste gate control if the boost drops due to an increase in efficiency, the ECU will see the underboost and keep the waste gate closed longer to try and achieve the target of 14.7.
 
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