Alrighty then! Since my MGA is driven a lot it makes a good vehicle for long term testing. Occasionally I can shed a little light on the reality of reliability and cost vs. value, so here is another one of those nifty long term test reports.

I have used lots of different types and brands of spark plugs in my MGA over the years, and they all work pretty much the same for function, performance, fuel economy, etc. So for many years I was using whatever was cheap and conveniently available, which was usually Autolite 63 non-resistor type from the local NAPA store for about dollar each. A high compression race engine might need a little different heat range, but they're still cheap. Being cheap and easy to change in the MGA it's hard to beat the price/value.

I would avoid Champion resistor plugs like the plague. It has been my experience that about 10% of them may be bad right out of the box with far too much resistance, and others may fail in short service for the same reason.

I tried Platinum plugs for a while, purchased at a local discount department store for about $4 each. I don't recall the brand name, but they kept cracking the ceramic insulator off the tip in #1 cylinder, so I gave up on those. They might have been too high in heat range. Excessive spark advance is not an issue in my car with low compression.

Several years ago a club friend gave me a set of Halo spark plugs, model number H-9A. He was selling them, and he wanted me to give them a long test run in my MGA. I suppose he was looking for a rosy review to help sell the things, but I told him up front I don't play favorites so don't expect much. My original expectation was that the things might have a very long life, but otherwise no (noticeable) difference in function vs. any other plugs.

50,000 mile Halo spark plugs

They have been in my car for just over 50,000 miles now. I have never touched them except for occasional inspection during routine service. They have been working fine, running that nice sandy tan color for me, and they will stay in the car for as long as they continue to work.

Subsequent research has shown no upper limit for useful life of the Halo spark plugs, some having run over 100,000 miles, so maybe they will last forever. You can check out the following link for detailed information about Halo spark plugs. Keep in mind it is from the manufacturing company, so as nifty as it sounds it may not be revealing any opposing commentary.

From the WayBackMachine archives: http://www.halo-sparkplugs.com/halohistory.htm
New web site address: http://www.halosparkplugs.com

Addendum, November 2010:
Now at 61,000 miles and still doing well.

Addendum, May 2012:
Now at 68,000 miles on the Halo plugs.
New web site address for Halo: http://lsgbrisk.com

Prior to April 2002, I had driven my MGA 175,000 miles in 15-1/2 years using various brands of spark plugs. While fuel mileage varies considerably under various circumstances, the overall average on an annual basis was about 26-27 mpg. After installing Halo plugs in April 2002, I ran an additional 11,000 miles in 18 months with average fuel mileage was 26-27 mpg (no change). In October 2004 I changed the final drive ratio from 4.3:1 to 3.9:1. Near as I can tell, fuel economy then improved about 1 mpg to 27-28 mpg average (about 4% improvement due to 10% reduction of engine speed while cruising).

In the last 6 months (through April 2012) I have been noticing a hard starting issue. After the car sits for a few days, it may require 7-10 seconds of cranking to start (with full choke), where it previously would start with 3-5 seconds of cranking (with full choke). I'm pretty sure I didn't do anything to change the choke function, so this begs for some investigation. I will first check compression, then verify and/or adjust tune-up settings as required, including points gap, spark timing, valve lash and fuel mixture. If after a few weeks of driving the slow start issue has not improved, I will then install new Autolite spark plugs. I have been holding this new set of Autolite plugs in reserve for 10 years for this special occasion, as they are same vintage (perhaps even the same production batch) as the prior Autolite plugs I was using before the Halo plugs. If that alone (without touching anything else) makes some significant improvement, then I would declare the Halo plugs as deteriorated, and call it end of life for the Halo plugs. Check back later for results.