Try the bigger better basket mod and throw a full chimney in it. It will hold above 500 easy for a good amount of time
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I haven't gotten scientific about it, but I've tried that and in my experience the 26 tops out below 500 no matter how much lit charcoal I use and with all vents open. Just not enough air flow. As soon as I crack the lid, temps will climb to 500+. So on my 26 I fairly certain it's not a charcoal issue regardless the arrangement.
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