They are completely different... JB+ only controls boost based on Manifold pressure..
The JB4 controls boost based on many other factors that I am not entirely aware of but probably more than 30 or 40 data points.
I have a JB4 and my friend has a JB+. He has odd shift behavior (sometimes) and I dont (mostly).
The car no matter what will not drive nearly as smooth but I guess it depends on how much you can tolerate. I cant tolerate much thus the JB4 I am very happy with.
To add to above - The JB4 has the ability to monitor timing - which is a key component of making power. The JB+ isnt even aware of timing. The DME in theory should be mindful of timing related to boost but all it will do is retard the timing to save the motor from a knock - thus you will lose power no matter how much boost you add without the proper level of Octane. With the JB4 you have the ability to see this in the logs and make a conscious decision to scale back the boost (even at a specific RPM) to gain more timing - Low Octane and High Boost is the culprit of timing retard.
Boost is not the end all of performance... A good balance between advancing of timing and boost together yields performance gains. You can run 19PSI but if the motor is retarding timing (not advanced) you in fact could be 1. Putting your motor in a position for knock and 2. Significantly lower performance. In fact my tests have shown for my fuel quality (93 octane in NY) lowering boost yielded some higher timing with improved performance.
Lastly in light of the above, I will for a stock B58 on 93 octane 4lbs of boost will yield some nice gains.. Both with the JB+ and the JB4.
You cant go wrong with either.. - but just know there is a significant difference between the two; Logging isnt the only benefit of the JB4
If you have the MPPSK - My advice would be completely different - I honestly could only recommend the JB4.
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Originally Posted by jmb2027
What's the reason though? Doesn't JB+ add boost like a typical piggyback, and similarly for JB4 (but with logging capabilities)?
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