Tech Tip: Split Testing BPF Headlight Bulbs (Halogen vs LED vs Incandescent)

Tech Tip: Split Testing BPF Headlight Bulbs (Halogen vs LED vs Incandescent)

Today’s tech tip is jammed packed with some awesome information that I can’t wait to share with you.

In this post I will cover not 1 but 2 topics all in reference to BPF headlight bulbs.

With the new 446LED bulb making its way into the market today we have found it very necessary to explain and test why LED's are the best all around bulbs for vintage British motorcycles.

Before we begin our tech tip post let’s go over some points that I will cover.


 Points Covering today


  • Common bulb problems
  • Introduction to the new British Pre-Focus LED bulbs
  • Explanation of BPF bulbs
  • Calculating Watts
  • Split testing
  • Split testing bulb review
  • Bulb notes
  • Final thoughts
  • Selecting your bulb

Common problems using incorrect bulbs

It’s 2018 and about 90% (educated guess) of vintage British motorcycles on the road today are still running incandescent headlight bulbs.

The other 10% is split between sealed beams and halogen bulbs

All are great bulbs that will provide you with as.much light as possible but there are some problems you may face when you choose to install some of these bulbs.

Often times sealed beams and halogen bulbs put a heavy load on your charging system that most stock EMGO or Lucas alternators struggle with. Especially for later models with turn signals and other high-load accessories.

Today's mentality is "I want more" out of an old system that just "works".
Back firing, batterys going flat and electronic ignition problems are all leading issues that one may face when you have fitted a bulb whether it is a halogen, incandescent or sealed beam and your stator couldn’t keep up with the power it was consuming.

If you happen to run a high out-put (200W / 20Amp) stator or a three-phase stator most likely these problems wouldn't effect you.

With all the modern technology today why not have something more for less?



In February of this year (2018) a new type of bulb was developed to cope with all the charging problems many experience today.

This type of bulb is called an LED. Not only are LED bulbs smarter, brighter, cooler, and convenient they also emit a brighter light with more than half the load from a traditional bulb.

What makes the new 446 LED bulbs “smarter” is the fact that they have an IC chip built in to allow this same LED bulb to work with 6V, 12V and 24V applications.

Still not convinced?

Further below we will put the new BPF LED bulb to the test so you can see exactly how much you are saving and also gaining.


What is a British Pre-Focus bulb?

Original Lucas bulbs and headlight units from the 1940's though the 1970's accepted what is called a “British Pre-Focus” bulb.

The British Pre-Focus bulb or better known as "BPF" bulbs are unique to what is now known as a classic automobiles and also classic motorcycles.



British Pre-Focus bulbs typically have a round base flange with a small half-moon shape locating slot to prevent the bulb from rotating and to keep the bulb holder lead contacts in the same position.

BPF bulbs rely on the base as the ground where modern bulbs use 1 of the contacts or prongs as the ground.

For example, an H4 bulb with a 3-prong adapter uses 1 of the 3 prongs as a ground or common wire.


Split test (Halogen vs LED vs Incandescent)

If you have seen our product listings on the new LED bulbs some may get the vibe that it’s all fluff..

It’s not..

Below I will explain to exactly why we recommend using an LED bulb by performing a few bench tests.

The 3 types of bulbs I’ll be using for this split test is a Lucas 446 bulb, an EMGO 446 halogen bulb (446H) and the new LED bulb (446LED).

I’ll be performing this test using a 12 volt sealed battery, positive and negative leads and (2) multimeters. The first multimeter is to measure the battery under load and and the second multimeter is to measure amps.

The goal of this split test is to record each bulbs amps to calculate the wattage and also recognize the draw that each bulb puts on ones charging system by performing a few simple tests.


Obtaining Watts

Watts = Amps X Volts Formula

Above is a simple formula which will help you obtain wattage.

To obtain wattage I must obtain the correct battery voltage, measure amps then multiply the battery's voltage to obtain watts.

Using the formula above is key to obtaining the correct data.


Test #1 - 446 conventional bulb


Testing A 446 Lucas Headlight Bulb With Multimeter

The first test I’ll be performing is on a standard Lucas 12V incandescent headlight bulb. This is the same bulb that was fitted to most British motorcycles during the mid-1960s through the late 1970s.

Test Bulb Rating Voltage Part Number
BPF Incandescent Bulb (Dual Filament) 50W/40W 12V 446 / 414

Using the formula to calculate watts I will take measurements and record what I find for this bulb.

This is what I have found…

Battery Voltage (under load) Amps Wattage
12.7V 3.02A (Low Beam) 38W (Low Beam)
12.7V 3.39A (High Beam) 43W (High Beam)


Test #2 – British Pre-Focus halogen bulb


446 Halogen Bulb Amp / Wattage Test

The second headlight bulb I will be testing today is a British pre-focus base with a halogen bulb.

This specific bulb was produced by EMGO a few years ago and fits inside any headlight housing that can except a British pre-focus bulb.

Test Bulb Rating Voltage Part Number
BPF Halogen Bulb (Dual Filament) 60W/55W 12V 446H


Using the same formula I will take measurements and record what I find for this bulb.


Battery Voltage (under load) Amps Wattage
12.63V 4.74A (Low Beam) 59W (Low Beam)
12.62V 5.20A (High Beam) 65W (High Beam)


Test #3 – British pre-focus bulb LED bulb


 LED Bulb 446LED Testing For Amps / Wattage


The British pre-focus LED bulbs are the newest types of bulbs on the market today.


They offer a bright and clean white emitting light along with less load on your charging system.


Test Bulb Rating Voltage Part Number
BPF LED Bulb (Dual Filament) 970lm / 510lm 12V 446LED


Using the formula further above I will take measurements and record what I find for this bulb.


Battery Voltage (under load) Amps Wattage
13.6V 0.22A (Low Beam) 2.992W (Low Beam)
13.7V 0.49A (High Beam) 6.713W (High Beam)


Split testing bulb review

All the bulbs tested today performed as they should - no surprises here.

Right off the bat I noticed that just putting a load on a fully charged sealed battery I saw the voltage drop especially when testing the 446H halogen bulb followed by the incandescent bulb then the LED bulb.


446 halogen bulb notes

I was very surprised that the EMGO 446H halogen bulb required much more draw than the incandescent bulb.

Granted the halogen bulb is the brightest of the 3 that we tested followed by the LED and incandescent bulb. The halogen bulb also has the highest load one could put on your charging system.

I found that if you where choosing the halogen bulb you would indeed want to consider upgrading your stock stator as opposed to the LED and incandescent bulb.


446 / 414 incandescent bulb notes

The 446 / 414 incandescent bulb (stock bulb) is ideal for those who are happy with a traditional bulb. There is nothing negative to say other than most would think the bulb is too dim?


446 LED bulb

The LED bulb is the ideal choice for everyday riders. Although the bulb is much brighter than a traditional bulb however it is not the brightest in the arsenal... but a big increase non the less is visually noticeable.

The 2 upsides to the LED bulb is the light improvement and the lower amps that it requires. Between the two this is the best all around bulb that should satisfy any vintage British motorcycle enthusiast seeking more light.

The 446LED bulb comes in positive or negative variations


Chart Comparisons

Here is a final chart showcasing each bulb together, I only listed the high beam for reference as I believe that is the most important.


Lowest to highest amps / watts draw chart

Test Bulb Amps( High Beam) Watts (High Beam) Part Number
BPF LED Bulb 0.49A 6.713W 446LED
BPF Incandescent Bulb 3.39A 43W 446 / 414
BPF Halogen Bulb 5.20A 65W 446H



Final thoughts



LED bulbs are definitely the way of the future. It's really nice seeing LED bulbs suited for vintage British motorcycles because it has less load in regards to the charging system but it also keeps us riders safe.

Before LED bulbs where introduced in our industry we had to settle with incandescent bulbs, sealed beams and halogen bulbs. Although all are great alternatives there are many factors in which take place when replacing or choosing headlight bulbs.

Not all bulbs are created equal. Those that have installed headlights that resulted in dead battery's or ignition issues know exactly what I'm referring to.

I personally believe the introduction to high put stators had a lot to do with the bulbs users where installing on there motorcycles.


Select your bulb

Now that you have seen all 3 bulbs in action, now is a great opportunity to select a bulb for your application.

Whether it is a BPF halogen bulb, BPF LED bulb or a BPF incandescent bulb - the choice is yours

Below I compiled a few links for you if you would like to order your own headlight bulb, just click on each title

446LED Headlight Bulb (6V / 12V) (choose positive + or negative - in listing)
446H Halogen Headlight Bulb
446 / 414 Incandescent Headlight Bulb


Thanks for reading today

I hope I was able to shed some light (no pun intended) on which headlight bulb you should install in your vintage British motorcycle.

We all can agree that the traditional headlight bulbs that where fitted to these old machines work but just don't "cut it" for today's world.

Having stronger, brighter and safer bulbs make your vintage Triumph, BSA or Norton that much better from the day it left the factory.

Should you have any questions please feel free to drop us a comment in the section below. We look forward to all comments and feedback.

Ride safe!


  • Everett Sticken

    Sorry that should say 64 benly. Auto correct got me.

  • Everett Sticken

    I tried these on my 71 bonne, 72 thunderbolt and 61 T110. The 61 is 6 volt and they all work great. When are you going to get some neg ground bulbs so I can put on my 64 belly and my 75 ST90 ?

  • Classic British Spares

    Sluggo – absolutely! I’m sure there is a better way (or more accurate way) to perform this test but for average Joe (including me and many others) this test gets to the point. Thanks for reading buddy!

  • Sluggo

    Spot on and good info on power consumption. Several good books out on testing with electrical tools.
    However would be helpful in also showing data on actual light OUTPUT, which is typically measured in Lumens, although might be able to show with a camera light meter. Awesome we have useful upgrades to our old iron isnt it?

  • Rod Smith

    What an interesting test. Never realised how much less electricity the led used than the other bulbs . I have a single led bulb for my rear and brake light on my ‘72 T100R. Thanks for doing the test !

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