Nightbird - good to see your COURUI came out so well. For the K40, oh boy, could be several things. First suspect would be the LED itself - maybe a poor reflow to the MCPCB, maybe just a poor performing LED. Definitely check out the reflector and lens carefully, but if that looks ok, the LED would be my first suspect - sorry, can be pricey to sub it out though, just to verify. I'd assume you have it focused, and the LED situated without a blocking centering/alignment piece, etc. From the beam pic, it looks well focused.
Thanks Tom E! I think I found the problem.
I had a feeling that the problem is with the K40 reflector so I tried to compare the performance of the K40 and the D01’s reflectors using the D01 body (and emitter). Here’s the set-up:
First I used the D01 reflector set-up like above and took the readings at 6 meters. The highest reading I got after all focus adjustments was 6030 lux or 217,080 cd.
Next, I used the K40 reflector set-up like above and repeated the whole process at 6 meters. The highest reading I got after all focus adjustments was only 3180 lux or 114,480 cd. 
So my I guess my theory is true. The K40 reflector is NOT as good as the cheap D01 reflector. My K40 is an old model originally with an XM-L.
Here’s a closer look at the two reflectors:
The K40 reflector at the left is deeper than the D01 reflector at the right and it is just a bit narrower so all other things equal, it should perform better if not at par with the D01.
Here are the front ends of the two reflectors. What I noticed is that the K40 reflector is dark where the D01 reflector is bright colored and this could be the culprit. Much of the light from the LED could just be absorbed instead of reflected forward in one direction. Also, not obvious is that the K40 reflector is like a cross between an SMO and an OP reflector while the D01 reflector is 100% SMO.
For comparison purposes, the Vostro BK-FA01 reflector is also bright colered like the D01 reflector and this could be the reason of the Vostro’s and the D01’s more than 200 kcd compared to my K40’s 182 kcd max.
To further confirm that the K40 LED is not the problem, I reassembled the K40 and took the readings at 6 meters, and I got 4650 lux or 167,400 cd. Compared to the 3180 lux using the D01 body (and LED), the 6.1 amp current of the K40 has indeed driven its emitter (an XM-L2 T6 1A) farther than the 4.4 amp current of the D01 (XM-L2 also).
Are the reflectors of your K40s similar to mine? :quest:
Yes I have a foggy reflector, seems standard on the gearbest couruis because I have a second one with exact same foggyness…
I tried to wash and or polish it away but all I got was a foggy and scratchy reflector…
Can't check mine now (@work), but I know my K40 performs well - got mine in the group buy. That dark look is bazaar 
.... Don't think I ever seen anything like that. The depth of the reflector explains the tighter beam pattern - a tighter beam pattern doesn't always mean better throw, I've seen several times, as is in this case.
But with the almost similar specs of the two reflectors, their outputs using the same LED source should have been close. I agree that deeper doesn’t always mean better. The dedomed Vostro gives 223 kcd and a very tight hot spot at measurements taken at 11 meters. The dedomed D01 on the other hand gives 233 kcd and a wide hot spot.
My guess (if its not as is from the factory) is that the dark reflector of my K40 is a result of tarnishing. I’m working in a Geothermal Field and metal parts of our equipment and tools here like brass, copper, aluminum and stainless steel tarnish easily. I think its because of the H2S in some parts of the field. My K40 is the most opened flashlight I have because I learned to mod using my K40 and many times during my learning phase I continue modding during breaks at the office. :bigsmile: From now on I think I should limit modding at home.
Pure copper pill for this Courui available: ( by NikolaS )
Thanks tatasal for sharing my offer ………
Guys, take a look at this: !!!
Courui "Even Bigger Head" D01 Projector: 113mm Lens , Luminus SBT-70 @ 13.5Amps *Build Stage* (by LinusHofman)
Perhaps going to be the ultimate `Big Head`….(this thread, and that of NikolaS, now added in OP)
I am sorry if I missed this mentioned,
I want to know if there is enough room to add a “blank” PCB between the batteries and the head (converting from 3P to 3S).
2 of the cells will need to be insulated from the head by the PCB. I am wondering if there is room?
thanks
This has been done before:
I think CK is also working on this
I ‘shamelessly’ copied FmC’s mod last night and got 3010 lux at 11 meters which can be computed to 362,210 cd or 1207 meters throw.
Also got 3.62 Amps reading at the tail cap with 4.13 volt batteries. 
This is now my strongest thrower so far and I’m really loving this light.
Quick question guys, if I piggyback two R100's on top the existing R200 what should that compute to for current?
-Garry
Assuming fully charged batteries & resistance mods, I’m guessing around ~3.5 Amps. Could be a little more.
Well my Courui mod didn't go so well (: . I had a short at the reflector due to a sharp peak of solder (which I couldn't see) poking through the kapton tape. I smelled a little bit of a burning smell but I didn't see any bad components. I found and fixed the solder peak then tried it again. It worked, but output looked really weak. I do have the "moonlight" mode when off, then outputs look like Low then High instead of High then Low. Highest output looks around +/- 200 lumens or so. Tried different batteries (first started with a single weak cell) but still weak output. I ran out of time to work on it and put it away all disgusted.
So do you think my driver is ok since it's still giving me modes? Could my triple stack of resistors be an issue? (I don't see how it would be any worse than a direct short across the stock one.) Would this be typical behavior of an LED after a short? (Still works but output way low.) This was a dedomed XM-L2 on copper.
And by the way, I had an awful time trying to get the reflector centered! The butterfly insulators don't "snap" to the reflector opening to hold it centered.
-Garry
FmC that seems awful low. I read some other mod posts and came to a conclusion that adding one R100 should have been sufficient. My objective was to have nearly direct-drive performance by way of ultra low sense resistence.
And you responded moments before I posted my new issue.
-Garry
It’s soooo easy to short out the driver with this reflector 
I’d say that you have burnt out one of the Fet’s.
I guess it depends on what type of battery you are using. Honestly, I’m just guessing on the figure here. With high drain cells you may well see over 4 amps. …well, you may have, before it was damaged…
Time to get a BLFDD driver in there!
- edit - I looked back through my own thread, as I was sure I had taken some current readings. With the resistor completely bridged, I saw just over 4.5A using a fully charged 25R.
Hmm, ok. So you think the LED is still fine most likely? I plan (when I get time again) to try and connect an 18650 direct to the emitter wires and see if it has the output level I expect. I kind of already did that during my first investigation but the light was face down and I was only interested to find out if it lit up.
-Garry
I have (3) 4.35v cells for this light, but haven't uses them yet as I wanted to make sure it was working properly first.
-Garry