I have a 6637 and 4 in exhaust all the way back on my early 99 and the most I see without a chip of any sort is 20 psi. I know that I have a few little leaks in my boots, so a few PSI higher on a stock truck would be normal. Most will tell you that running more than 27 ish PSI through the stock charger will significantly reduce its time of service.
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