HALOGEN bulbs: The H1157 halogen bulb is indeed very bright, but also overkill with the following problems.
a.) The running tail light low filament is nearly as bright as a normal brake light. This is easily seen in daylight conditions, but may be mistaken for a constantly on (stuck) brake light.
b.) The high filament brake light is also very bright, but may not appear to have sufficient contrast to the abnormally bright low intensity tail light. So a driver in a following car may not so quickly notice the brake light come on over the brightness of the tail light. This is the bit that bothers me the most, both as the driver and as the follower.
c.) A not often mentioned characteristic of halogen lamps is a slow warm up, meaning there is a very small delay while the light comes up gradually, not instant on. This may further aggravate the previous problem.
d.) The 50 watt high filament generates effectively 50 watts of heat, which is sufficient to melt the MGA tail light lens if it is on for extended periods of time, like sitting in stuck traffic with your foot on the brake. In fact the 15 watt low filament may be sufficient to cause some discoloration and hazing of the lens after extended usage. In case the tail light and brake light are on at the same time you have 65 watts of heat. The light output itself does not account for very much power, as may be seen with the very efficient and cool running LED lamps.
e.) The glass of a halogen bulb must be kept meticulously clean and dry at all times, as it runs at exceedingly high temperature. Do not touch the glass of a hot one for obvious reasons. Touching the glass of a cold one will leave oils from your fingers on the glass and cause uneven heating which may explode the bulb. A good regimen is to clean the glass with rubbing alcohol and not touch it afterward.
LED "bulbs": As of this writing in late 2004, LED "bulbs" are pretty much a lost cause for the MGA tail lights. The LED "bulbs" are built to project light, with the effective projection zone being a cone somewhere between 15 and 90 degrees wide, projecting a large bright spot on the back of your tail light lens, not seen well from the side. Even with the light directed in one general direction, these lamps are still (generally) not as bright as a standard incandescent brake light or turn signal bulb.
The first (and so far only) commercial LED plug-in replacement for the 1157 bulb that actually is as bright in the projected direction is 1.85 inches in diameter (and costs nearly $50). In the MGA tail light assembly the bulb mounting is in a downward direction, not pointing out the back. Radiantz does make an LED unit that could work in this application. It features 48 LEDs and a hanging base, allowing positioning of the LED unit at an add angle (if you supply the support). See photo and link. Also in hard wired version. The LEDs sure are cool though, being very efficient and generating useable light with very low power and almost no heat. Perhaps in the not too distant future technology will progress to allow brighter LED lamps to fit in small spaces and maybe not require the tight projection zone.
If you would use LEDs for the MGA tail lights, there is another problem. The turn signal flasher unit requires a specific current draw (two high intensity incandescent lamps) to operate properly and to flash at the correct rate. With the very low power draw the LED lamps would function almost like no load at all, and the flasher unit would not flash. One apparent fix for this would be to install a load resistor in parallel with the LED lamp to produce the original magnitude of current draw. This would totally offset the efficiency of the LED unit. It would also defeat the safety feature where the flasher stops flashing if one bulb should burn out or become disconnected. This is nearly equivalent to installing a heavy duty flasher unit which would flash at a constant rate regardless of the total load. Putting the shunt resistor in the LED "bulb" itself would also offset the efficiency of the LED unit, as well as generate the same heat as the original incandescent bulb. Radiantz makes a load equalizer that can be hard wired into the system to provide the current load necessary to make the flasher unit function.
Having spent some time thinking about why this is not good for the MGA, I finally thought of a very good use for the LED tail light. If I were to install these things in my trailer, it would have nice bright tail lights and turn signals, and it would not affect the flashing rate of the original thermal controlled turn signal flasher unit. Hmmmm. Would I spend $100 for that?
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