Hi Roofman,
First decide if you want fog or driving lights. The other posts are mixing the terminology and it's important to decide what you want.
Fog lights are designed to improve your vision in (tah dah!) fog. They generally have fluted lenses that spread light more than driving lights. Some have yellow lenses or use yellow bulbs. As a rule, they throw a very low and wide light pattern.
Driving lights generally have clear lenses and are designed for long range work in clear weather. The beam they throw is narrow and intense, to reach well out into the distance.
Many LBCs and Rally cars will have both types of lamp installed. Usually the driving lamp is in front of the driver to help him see well down the road, and the fog lamp is in front of the passenger, to best illuminate the side of the road.
When both types of lamps are installed, sometimes they will operate at the same time or are indivivually switched, but often they will be linked via relay to the high and low beams. On high, the driving lamp will also illuminate. On low, the fog lamp will be used.
You'll want to look into added wiring and relays to support high output lights of either type. You can't go wrong if you buy a sub-harness kit from Dan Masters and follow his instructions.
Depending upon the year of car, the lights might be a bit much for the charging system. Certainly, TR4A and earlier, with their 22 amp generators, don't have the ooomph needed for extended use of auxiliary lights. So, many of these earlier cars were fitted with higher output alternators for just this purpose, including rally cars and the "police special" TR4s bought by Belgium.
Later TR6 have better alternators, but still might need a higher powered unit if using the lights a lot.
There are many types of lights to choose from. Lucas SFT576 (fog) and SLR576 (driving) are the stem mount approx. 6" round ones that are still being produced (or reproduced). These are what you most often see installed on older LBCs. These were also used on the factory rally TR4s.
A rear-mount version, WFT576, was used on the trunk (boot) lid as a reversing light.
Larger 7" stem mount Lucas SFT700 (fog) and SLR700 (driving, aka "Flamethrowers) were used on the LeMans and Sebring cars. Also sometimes on the rally cars.
I am pretty sure there are also smaller versions, about 5" in diameter, but am not sure of the designation.
One drawback to the Lucas lamps listed above is that they use a special proprietary bulb. There are adapters to use more common Halogen H3 (I think) in them, but these are pretty rare.
There were less expensive Lucas lights also offered, such as Fog Rangers. I'm not familiar with all the the types, but know that many of them have special sealed beam bulbs that might be hard to replace.
In later years, Lucas offered rectangular fog and driving lamps. These were used on the Shelby cars, for one. I think a more recent model is the "20/20". Not sure, but these might use more modern bulbs. However, in particular the models used on Shelby cars can get quite pricey.
There are lots of other good lamps out there, Hellas and Cibie were mentioned and I've used both, in round and rectangular designs.
My Land Rover has a set of large, 7" driving lamps (actually larger, that's the bulb size, which is probably made by Hella) as well as rectangular fog lamps. Even without them, it's got excellent lighting. But with the auxiliary lamps fired up, visibility is great, and these are all just 55W halogens.
As mentioned above, the driving lamps are keyed to the high beams, and the fogs to the low beams. There is just a single switch in the car for both, and so they are not all on at the same time except for that brief moment when switching from low to high or high to low. (Heh, heh... I did have a guy stop his car directly in front of me in a parking lot one night, and start flashing his brights. I had just the lows & fogs on, which are all well adjusted and nowhere near as obnoxious as some of that HID lighting.... Let him have it momentarily with all 6 lamps and he stopped his complaining, probably too busy waiting for the blue spots to clear from his vision.)
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