re: Focus SE sparkplugs

In message , Hanne Brooks writes

It because the Focus uses a "wasted spark" ignition system where one of the two coils outputs a negative voltage. Ford fit the two negative firing cylinders with different plugs (I can only assume its for cost reasons) but when you replace them you fit them all with the same type of plugs as you have done.

In a wasted spark system the spark is triggered by a crankshaft sensor and therefore fires not only at the top of the compression stroke but at the top of the exhaust stroke, hence the "waste".

Thanks Paul,

A lot of systems do this but this is the first time I've seen different spark plugs. With a wasted system, where two cylinders share a single coil which I think is what your talking about, the cylinder/plug pairs will alternate between firing on power and then the exhaust stroke so both plugs need to work in the waste state. Do you know what the difference is between the 43E and 43EG designators?

Jim Brooks

Good explanation, but yet it doesn't quite fully explain why there are two different plugs used.

The reason is, in the wasted spark system (which is very common on new cars without distributors nowadays, and has been the standard for years on multi-cylinder motorcycles) the direction of the current is reversed in one of the plugs in each pair fired off a single secondary winding, because the two plugs are in series with the secondary winding.

The 'reversed' plug needs the platinum electrode on the opposite terminal to a normal plug. I believe this is because a spark will jump the gap easier and wear down the electrodes at a much slower rate if the negative terminal (or is it the positive one) is a precious metal, whereas the material the other terminal is made of doesn't matter nearly as much.

I believe the usual configuration of a 'single platinum' plug is a platinum center electrode. Presumably the OEM reverse connected plugs have a platinum ground (earth) electrode. The replacement 'dual platinum' plugs have platinum (or palladium or some other expensive element) on *both* the center and ground electrodes.

In message , Mark Olson writes

If that is the reason, you have to wonder how much money Ford actually save by fitting the two reversed plugs, bearing in mind that they will be a special order with the plug manufacturer.

Welcome to the world of cost reduction. The fact that Ford could justify going to the effort of making such a small cost savings should give you an appreciation of how tempting it is for them to make a decision that could result in what they consider a 'large' cost savings, like $5 per car.

As far as the special order goes, when you remember that Ford will be ordering hundreds of thousands of these plugs, it's worth the NRE charges that such an order will incur.

The spark plugs in my Zetec Focus are all the same. Is your son's name Hugh?

Thanks for any help shedding light on this. Jim