There are a lot of different types of spark plugs out there. Standard, single platinum, double platinum, twin tip, quad tip, side fire, iridium, etc. With all these choices, it can be difficult to know which one is right for our beloved Aerostar. Similar story in wires, you have the standard cheap store brands, higher priced brands, performance wires, mag core, etc. This choice gets compounded by the fact that some of the plugs on the Aerostar are difficult to get to, making it that much more important to get the right part so that you do not end up doing the job twice.

Ford engineers knew what they were doing when they engineered their engines. They have designed the original equipment spark plugs to have the proper heat dissipation, have certain durability features, and to be made from certain materials. They have designed them to take up no more than a certain volume in the chamber. So Let me proceed with what is wrong with many of the designs and why they should not be used.

Standard Plugs:
The only standard plug that should be used on an Aerostar is the Motorcraft part, or the Autolite equivalent. Autolite manufactures the original Motorcraft plugs. Even so, the standard plugs should only be used on engines that originally came with them. If in doubt, check the owners manual. The part number will contain a P is it is a platinum type plug, and a PP i it is double platinum plug. Otherwise it will be a standard plug.

Single Platinum:
Single platinum plugs contain platinum on either the tip or the electrode. Again, you should only use Motorcraft or Autolite plugs because they have been designed to have the proper heat range. The 4.0L engine uses a waste spark type system and comes from the factory with two different types of spark plugs, one one bank they are all platinum electrodes, and on the other bank they are platinum tips. Later 3.0L engines with coil pack ignition also use the same type setup. In these engines, you can, and are advised to use double platinum Motorcraft or Autolite plugs. The Autolite number to use on the distributor equipped models is #AP765

Double Platinum:
These plugs are recommended for models that use coil pack ignition. Because they are platinum plated on both the tip and the electrode, they can handle the flow of current in either direction. This way you do not need separate types of plugs for each bank. Use these plugs on all 4.0L engines, and later 3.0L engines as well. They can be used on distributor based ignition, but offer no benefits. Use only Motorcraft or Autolite brands, as others tend to have and incorrect heat range, which can cause misfiring, preignition, decreased plug life, and decreased performance. The Autolite number to use is APP765

Twin tip:
These plugs stray from the original design and are not recommended. The argument for these types of plugs is better spark presentation. However, no twin tip plugs on the market today have been engineered to meet Fords strict requirements for the engines used in the Aerostar. Also, they do not cause the spark to split or produce two sparks. Electricity will always follow the part of least resistance, which means that the spark will jump to one tip or the other. Another problem with this type of plug is that the extra electrode takes up more space in the combustion chamber. This causes a slight increase in compression and that in turn causes predetonation. Only use this type of plug in engines that are designed for twin tip plugs.

Quad Tip:
Read twin tip above. This has the same problems only more so. Only use this type of plug in an engine that was designed for them

Side Fire:
This type of tip claims to have better spark presentation. However, it moves the location of the spark. Ford engineered the spark to take place in a specific part of the combustion chamber so that the flame front spreads evenly, and delivers the best performance and efficiency. Also, this type of plug has the same issues as other non-OE plugs, in that the heat range is wrong, and the amount of volume they displace in the chamber may be different. To this date, they are no side firing plugs that have been designed to meet Fords requirements for the engines used in the Aerostar.

Iridium:
Iridium conducts heat and electricity better than platinum. It is also more durable that platinum. These factors do make them more desireable for use in a spark plug. Iridium plugs are designed to last far longer than platinum plugs, and are sometimes refered to as lifetime plugs. However, one much make sure that these plugs are truly equivalent to platinum plugs they are replacing. And other factors must be taken into consideration. For one, these plugs, like others, can seize in the plug holes if left in there too long. If a plug is designed to last over 200,000 miles, and it is left there for that long, replacing it may be very difficult. If the plug is the wrong heat range, or displaces too much chamber volume, it can cause detonation and pinging. The only Iridium plug that should be used is Autolite part# XP105

As for plug wires, use only Motorcraft or the Autolite OE equivalent. All others are a gamble at best. Ford engineered the plugs to have a specific resistance range, induction qualities, and suppression with the proper heat and vibration resistance. Most wires break down rapidly from the high temperatures found under the hood of the Aerostar. Most wires also deteriorate rapidly from the high voltage that Ford uses. Performance wires often do not have the proper suppression and induction properties, which can cause interference problems. Mag-core wires are the best of both worlds, but at a hefty price, and the low resistance they use can cause rapid plug wear, and can cause the plug tips to overheat, which in turn causes detonation and pinging. It can also put extra stress on the coil(s).

 

Excellent summary.

I would add that the split tip plugs usually cause the spark to occur at a slightly different position than stock design. This may or may not make a big difference in pass-car engines. The quad tip plugs I've seen behave almost like the side fire plugs; since only one spark jumps at a time, its position is much different than stock, and usually to the side where one of the tips are.

For very critical racing applications where the engine builder is trying to get the best combustion out of a plug, he may index it. This requires marking the position of the side electrode, and using shim washers so that it can be placed out of the way of the intake flow when it is tightened down. But the difference here is less than a percent in combustion efficiency.

 

Good summary. One other point, as noted above with EDIS system, two plugs fire concurrently & this "lost spark" ignition doubles the amount of energy the plugs wires & coils have to dissipate. Inferior parts will fail in half the time at best. I proved this out to my satisfaction when German Bosch plugs were installed. Although they worked fine for 50,000kms,they were not fouled at all but they "burnt away" on the driver side bank, due to this reverse polarity system & the fact that they were single platinum design.

 

Sticky Please (All new members should read this)

 

For anyone in Canada looking for the Autolite APP765's, "Bumper to Bumper" has them as a normal stock item. $9.99 each.