There is too much confusion and misinformation out there, so I'm going to simplify everything.
First off Here's a quick do and don't use
OEM Plugs from the dealer
Anything made by Bosch
Secondly, I'm not going to touch Forced Induction, Nitrous, or 30+ hp gains
I'm going to base this off my 2006 D20 (vin N) non p-zev.
The Focus comes with "Motorcraft" Spark Plug made by NGK.
Yours may have come with a DOUBLE PLATNIUM plug.
The reason your Focus came with this type of plug is for 1 reason only. THE BEST SPARK OVER THE LIFE OF THE PLUG
it is a tricky play on words but just read it it seems simple enough. They're not making any claims beyond long life. You can expect 80-100,000 miles out of factory plugs.
offers the OE replacement for $18.19
There are a few different options available out there. Other manufactures who make a factory replacement, there are cheaper options, and then there are designer plugs.
I'm not going to touch on the designer plugs because if it's too good to be true...
I will say that the Bosch +2 and +4 work wonders, on BMWs from the late 1990's to the early 2000's. BMW contracted Bosch to come up with a plug to help clean up emissions. Bosch, like any smart company decided well heck, we can make a fortune off this. However, BMW stopped using these around 2005 when they released their "N" generations engines. Now BMW only uses Bosch spark plugs in a 1 or 2 engines.
The bottom line is Bosch +2 and +4 plugs will never perform the way they were intended on any car other than what they were designed for.
To begin with there are many companies making a good double platinum spark plug, but I will say that with the double platinum spark plugs that modern cars are designed so tight that you really need to stick with the original manufacture of the plug to have the car running right. There is no government control over the spark plug industry and the thickness of the platinum from manufacture to manufacture is going to vary.
Now onto replacement options.
You're first replacement option is to replace the spark plugs with the ones it came with. In my previous rambling, as mentioned you really want to go with the plugs from the Dealer, yes I know it sucks. But when playing with platinum you're just that, playing. You can try the replacement plug from NGK in the NGK box, but you really don't know if you're getting their 100% quality. If you go to the dealer and buy them in a Motorcraft box you know for 100% fact you are getting the spark plug designed by NGK for Ford. Unless your parts guy is an idiot and there are a lot of them out there, so do your research and ALWAYS ALWAYS ALWAYS go into the parts counter with your vin#.
You're second replacement option is to go with an iridium plug. Iridium plugs have not been around that long, and were introduced to make the longevity of the plug last to a true 100,000 miles. I think we all know someone with 80,000 miles on there vehicle with double platinum plugs and it started running like crap, and the plugs were just worn out.
In my opinion, iridium plugs push the line of designer spark plugs, and only reason I included them is becasue, there are a few manufactures using them now.
Now onto the confusion. Copper spark plugs.
To begin I will only be covering Autolites, however NGK makes a quality "copper" plug as well.
Copper conducts electricity much better than anything else on the market.
Why not copper?
It's life is about 1/10 that of a platinum plug HOWEVER
at 1/10th the price, you do the math, if you're time and labor are free you're not really losing anything in the end.
All copper plugs from Autolite, are part #'s A###
That's it, if it says AP it's not, if it doesn't have an "A" or an "AR" infront of the part number it's not copper.
We have 3 choices when it comes to copper
The Autolite A104
The Autolite A103
The Autolite AR103
I will breifely touch on gapping after this next rambling.
I always gap my D20 non pzevs at .055"
The A104 is of the factory Heat range, it is considered a replacement plug. You may or may not feel a gain with this, but you will feel your vehicle idleing smoother and possibly a decrease in fuel consumption.
The A103 and AR103 are a heat range lower.
A lower heat range works by allowing the heat to dissipate faster across the spark plug. This is benificial to us.
The advantages of a lower heat range:
lowered combustion temps
lowers chance of pre-detonation
burns fuel more completly, this is the one that makes us power and decreases fuel consumption
There is a higher chance of the spark plugs to become fouled because it is cooler and if you have OTHER ISSUES SUCH AS LEAKING INJECTOR, it's going to cause the electrode to get wet and it will never get hot enough to burn that wet fuel away. If you're fouling a 1 step colder range on your focus, you have other issues going on.
First the A103, a good plug you will notice results from this. Your car wil be quicker, it will idle better and you're fuel conspumtion will decrease.
The AR103 is identicle tot he A103, but it has a groove cut in the electrode and the electrode is nickle plated. It is also way more electro magnetic interferance resitant than any plug out there, which is not benificial to us using coil on plug, but still nice to know if there is ever an EMP attack our spark plugs will still be ok. lol The AR103 will be less prone to fouling and carbon build up, which may results in a little longer life. But the big thing for us is the groove cut in the electrode, the idea behind this is to make a wider flame front and it works well and will make 5hp over stock plugs on a stock car.
On to life and gapping.
If you gap the spark plug to the factory recomendation of .050" you should see 30,000 miles from them, however reducing the gap will reduce your hp gains, as you want the spark to jump as far as possible without extinguishing it's self to create a better flame front.
If you gap the spark plug to .055" you should see 15,000 miles. This is the best compromise in my opinion. The gap make the spark huge and will give you a much improved gain over stock.
You can also gap to .060" however I do not recommend this unless you keep a set of hand tools in your car. This will make a very nice gain, however life until the gap becomes to large is very short. When I ran .060" I swear it felt so much better and was getting around 38mpg highway. However, after 8,000 miles started to misfire. and when I checked the gap they had grown to .062" and i guess .062" is just to much for the coil on plug to handle.
If you want longeveity, go to your ford dealer and suck it up and buy the OE plugs
If you want more power and better fuel consumpion, go with the A103 (or AR103 if you can find them)
Hope you enjoyed. Any questions feel free to PM me.