noob question: Straight swap of Q5 for a XM-L T6

Hi - I've been lurking here over the weekend trying to learn some electronics (I'm a mechanical engineer, so electronics is still in the "magic" category for me at the moment....), any you guys look like you know your stuff

I've got 2 old (2008) SpiderFire torches [http://www.tactical4u.com/en/product/item/238/product_detail.html]
that I'd like to boost the output on. I've been having a good rummage around the forums and I think I know what to do, but I wanted to run it past some people who really know what they are doing first!

I dug out my multimeter, so I'll start with some measurements: Batteries are 2x Ansmann 2800mAh NiMH AA's, giving 2.6V when fully charged.

Emitter is a Cree Q5 (XP-G?). Measuring across the LED (which I assume is ok across the red/black leads from the driver), I get about 2.9-2.95V when lit.

Measuring "at the tail cap" (I assume where the cap will contact - across rear of battery to housing) I get 0.47A when lit. Its just a basic on/off cap, so the driver must be a 1 mode thing.

They normally run for 3.5 hrs on full and then get dimmer until about 5 hrs. So I think if the battery power is (2 x 2800mAh x 1.2V) = 6.72W, then that matches the LED requirement; 6.72W / (2.95V x 470mA) = 4.84 hrs.

So these were my questions:
1. Is the driver regulating the voltage (batteries give 2.6V, but its 2.95V across the LED)?
2. Can I swap the Q5 directly for a XM-L T6 and get something like a 280% increase in brightness? If the driver is voltage regulated, and both LED's seem to have the same forward voltage, then it should pull the same current. So that would be nearly 3x the brightness for the same battery life. Sounds a bit to good to be true would it work?

I know I could by a T6 light off the web for no money, but thats not the point is it? Thanks in advance for the help!

Most drivers are regulating the current and the voltage will adjust automatically to the Vf of the LED at this current.
The LED of the light You linked looks like a CREE XR-E. A XM-L T6 will give You roundabout a 125% increase in brightness, maybe a little bit more. It should fit in the same reflector but it will be very floody (less throw).
The biggest challenge when modding a light is opening the head and removing the old LED without damaging it. If the LED is mounted on a PCB, get a new LED mounted on the same kind of PCB (same diameter, shape and thickness) and the LED swap should be very easy. Use thermal compound to glue the new PCB to the heatsink of the light.

It's hard to tell if the driver is regulated or not. NiMh batteries have a relatively steady voltage until they run out of power, so the driver could be a simple boost circuit.

If it is a XP-x (C,E, or G) LED, then a XM-L should be a simple swap as long as the LED fits. the larger die of the XM-L will give you a larger hotspot then the previous LED. If the LED is an XR-E, you might need to adjust the reflector a bit to get the LED at the right focal point.

:welcome:

the light is actually a Romisen RC-N3.....it si easy to mod...the LED sits on a 16mm PCB. PCB thickness is vital for proper focusing, but they are all of the same type if yuo buy them for the chinese stores that sell flashlight parts.
GO for it...it is easy and it will be a good introduction to flashlight modding