I am a newbie in this field. I bought a Cree XT-E recently from fasttech. After that only i realize that it has a lower vf (2.85 - 3.4). Seem like 3.7v from li-ion 18650 bit too much to drive this xt-e. I have a question here, can i drive this xte using my current Nanjg 101-AK-A1?. Or i just go for direct drive? I need help to drive my xt-e? Any advice is appreciated. Thanks 
the principle behind all regulated led drivers is the same - they will automatically adjust the voltage to the emitter to achieve the rated current. if your xt-e can handle 1400mA (ak-a1?) it will be fine.
direct drive is another issue but you may already understand the principles, unless you have seen tests otherwise direct drive from li-ions will be too much for this emitter 0:)
From the xte datasheet, it max DC forward current is 1500mA, means it can withstand 1400mA from my driver for long time? Thanks 
i guess, attach it to the heatsink properly and only time will tell
Can XT-E be overdriven like other new CREEs?
yes you can drive xt-e with Nanjg 101-AK-A1, assuming its current is not over 1,4A or 4 amc chips.
i drive red leds with amc chip drivers, and reds only 2-2,2v
> drive red leds
How long can you drive them, and do they start to dim after a while?
I’m poking at doing something similar with various amber LEDs (aka “yellow” from China, sometimes, though not reliably described)
I have two XP-G’s on direct heat path copper sinkpads (in my UltraFire 602b custom builds), at 1400mA they make ALOT of heat, in fact I use that exact same 101-AK-A1 (flashed with custom firmware) but they work very well. The XT-E is rated at the same current as the XP-G but different Vf so it should work, on stock aluminum star’s pushing them that hard might generate alot of heat resulting in shortened life span however, you can reflow them to the XP-E/G stars.
Thanks for explaining and advising all! I got my xte working perfectly with 101-AK-A1 . Using stock 20mm star attach with a small heatsink, driving in HIGH mode (1400mA) get hot within minutes, but for my daily use MID mode is enough for me. Maybe in some situation i will use HIGH mode, but not for too long.
XT-E comes on XP stars, same footprint.
But my question kinda remains:
Can you, if they’re on direct-heat-path-star, push some 3 A through them, as you can with XP-G2?