The RC40 handles heat very well, gets warm to hot but never too hot to hold i have found.
Mass of course is key, the x65 has it in abundance……………but the xhp35 HI’s do not help! These i find run hot, or hotter than equivalent outputs with different LED’s with lower voltage.
The x65 can run around 5000lm till the 8 cells say no more………….gets toasty though compared to the rc40 at similar levels.
Not only is the battery tube very far away from the heat source (the LEDs) and therefore reducing the effectiveness of the surface area, but the cross-sectional area of the battery tube is very little, causing bad heat conductivity, AND 220sq/cm is nothing compared to the 1000+ from 10 heat fins 100mm in diameter.
And yes, the X6 has more than 10 heat fins.
No, no it is not.
Mass does literally nothing for heat dissipation, it just stores heat.
Surface area is what matters.
Adding mass just makes it heat up slower, over time it will still reach a high temperature.
If you care about long continuous runtime at a lower temperature, then surface area, metal heat conductivity (copper is best), and airflow speed/temperature is what matters.
Mass,size , metals used etc. All helps take heat away. When I hold an rc40 it has more mass, it can run longer than a light with less mass/size at similar outputs. Yes mass matters to me , larger the light with more heat sink works better than one with less mass/heat sinking.
So mass is important to me 
So having more surface area creates more mass, maybe my figure of speech is the issue.
Maybe if all you use are the typical normal-shaped flashlights that would apply.
But no, depending on how you shape the metal you can have something with the same amount of mass and 100x the surface area.
But we are on a Flashlight forum, I am simply saying larger lights handle heat better (generally speaking) than smaller ones at same ouput levels. More mass/size/ heat fins to soak up heat then dissipate. Which I have found enough to sustain fairly high output levels for decent amounts of time.
Tk75 and K60 do not handle heat as well as a larger light like the rc40 at similar outputs.
lux-rc FB1? Active cooling makes me suspect it may be no. 1
Any regulated HID will surely work nicely!
Off the top of my head, I’d say either the Trustfire TR-S700 or the Trustfire TR-J20. Love them both and can be used on HIGH mode as much as required.
J20. Only light I can use in summer on high without overheating :heart_eyes:
Flashlights, spotlights, searchlights, we talk about all of them here.
OP didn’t specify just tube flashlights or EDC or anything like that.
You don’t seem to understand that there are lights with different designs that aren’t your typical “battery tube with big head” and actually have better heat dissipation, higher output, higher price, etc.
Oh well, the op’s main beef is “what light can sustain the turbo or its highest level without stepping down and will only notch down due to battery depletion”.
How it can do it is not the issue and perhaps deserves another thread.
I fully understand thank you enderman…
The Trustfire T90-2 second from the left, and the Stock Lumintop SD75 third from the left on top.
The Q8 can’t do 4000 lumen continuously, it has not enough surface area for that. I have half an hour tonight and will try 3000 lumen, from cold start, and measure how hot it gets.
Edit:
I did a wee test at 3000 lumen (on GA cells), at 12 minutes the head temperature of the Q8 is 63 degrees Celsius, so still far from dangerous but too hot to hold.
May do a proper test later. My guess is that the sustainable output (=can hold the flashlight at room temperature with no active airflow?) is less than 2000 lumen.
The Olight X6 is 5k lumens, the PH50 is 4-5k lumens as well, the WiseLED Xtruder ST is 10k lumens, The XeRay LX70 does 7500lm, and there are probably a few more HID lights I’m missing.
Thanks for for sharing those KawiBoy1428.
Isn’t the Trustfire T90-2 a 2500-ish lumen light? (not nearly 4000lm )
The Lumintop SD75 is a very good light, and has some of the better heat dissipation fins, but has been tested by a few people, as only putting OTF lumen numbers of about 3150lm. Then even if it was a 4000lm OTF flashlight, its run time chart looks like a ski slope.
Thank you Enderman. 
Do you happen to know where we can go to see some reviews of any of these lights, that may also incorporate run time and lumen tests? I am having a hard time finding much of anything.