L31MaxExpress
I'm Awesome
Every 8.1 has a 5 blade steel fan, unless it was installed in a Kodiak. Internet lore is they are good for 11,000 peak CFM on that engine/that FEAD pulley ratio.
The trailblazers use a plastic fan with an outer ring that is the same diameter and pulls even more air than the stock 8.1 fan, but it is probably overkill for most cases. Not a lot of other 21.5" fans out there. GMT400 stuff, at least with small blocks, are 19.5" fans. The bolt pattern is different on the Trailblazer fan and most of them have electric fan clutches, but for 2008-2009 they had a standard mechanical fan clutch with the same thread pattern as all the other GM stuff of the era.
As far as I can tell, the Kodiaks actually use the trailblazer fan setup, or something visually very close to it. Depending on what you look at on Rockauto, some of the part numbers interchange.
For some reason, every aftermarket fan clutch I've had seems to engage earlier and cycle more than the OEM GM stuff, which is a bit of a bummer for fuel economy.
The Duramax fan is a bolt in for the 8.1L fan and moves noticeably more air especially around idle.
As for the clutch engaging eariler, it is great for ac and cooling in hotter climates. Idle and around town a/c output will suffer greatly without good airflow across the condenser and the excess head pressure will greatly shorten the life of the compressor. I am considering a couple of 14" pushers in front of my condenser on the van controlled by the PCM.
I suffer more fuel economy loss from the higher ac head pressure and higher engine operating temps making the PCM pull timing than I do from the fan pulling more air. I have seen my intake air hit 170-180°F on a hot day.