The Magna Headlamp FAQ:

[spud100]

If you have a good look at the wire sizes going into the headlamp main bulb connectors you will find that they are really quite thin.
The reason for this is that for cost and weight reasons the car manufacturers started squeezing the conductor cross sectional area down as simultaneously they reduced the thickness of the insulation.

Headlamp relays were added to vehicles in the 70's because of increasing problems with headlamp switch burnouts.
For example the change from the MKII Cortina to the MKIII in Ford of Europe in 1973.
This way the headlamp could be become reliable because it was only switching a signal voltage to the relays.
The relay then was usually placed in the engine compartment to achieve shorter wiring lengths that were carrying the heavier current.

I had a Capri 2.8i in the 80's and changed the 4 headlamp setup to Cibie's from Hellas, 100 w bulbs, outer lamps with high power H4s and changed the relay to switch the outer lights from the dip element to the high beam element as well as switching the inner high beam lights on.
I then wired this lot up with 4mm2 wire.
Flash someone hogging the overtaking lane on a motorway at night and apart from illuminating their brain through the back of their head, also got them to actually react and pull into the middle lane that was empty.


The FAQ advice to install relays and decent thicker wiring to reduce the voltage drop is dead correct. Even more so if you are running higher power globes.

I will get the multimeter out over the weekend and report on the actual voltage being delivered at the bulb connector.
Gerry

[spud100]

Update :-
Voltage drop is 1.45V.

Test 1.
Engine OFF.
Voltage at battery under load = 12.03 V
Voltage at LH High Beam bulb holder = 10.57 V
Difference = 1.46 V

Test 2.
Engine IDLING.
Voltage at battery under load = 14.07 V
Voltage at LH High Beam bulb holder = 12.61 V
Difference = 1.46 V

Had a look at the wiring to remind myself just how thin it is!!
Looks like a trip to get 4 relays, 4 bulb connectors and some nice thick wire!

Here is the table from the Daniel Stern site.

10.5V : 510 lumens
11.0V : 597 lumens
11.5V : 695 lumens
12.0V : 803 lumens
12.5V : 923 lumens
12.8V : 1000 lumens ←Rated output voltage
13.0V : 1054 lumens
13.5V : 1198 lumens
14.0V : 1356 lumens ←Rated life voltage
14.5V : 1528 lumens


The Europeans take a slightly more realistic with their voltage ratings; they consider output at 13.2v to be "100%".
The loss curve is the same, though.
When operating voltage drops to 95 percent (12.54v), headlamp bulbs produce only 83 percent of their rated light output.
When voltage drops to 90 percent (11.88v), bulb output is only 67 percent of what it should be.
And when voltage drops to 85 percent (11.22v), bulb output is a paltry 53 percent of normal!
It is much more common than you might think for factory headlamp wiring/switch setups to produce this kind of voltage drop, especially once they're no longer brand new and the connections have accumulated some corrosion and dirt.

See the effect of the voltage drop when the engine is running from 14.07 to 12.61 Volts. This reduces the potential light output from 1356 lumens to 803 lumens, a 553 lumens drop.

Gerry