Should I use old style copper or platinum spark plugs in a truck that came with copper plugs

I have a 1991 Ford F150 with the 4.9L straight six engine, it originally came with copper spark plugs, but recently a mechanic recommended I switch to platinum or iridium plugs. I know of people who’ve had problems with putting platinum or iridium plugs in vehicles that had copper plugs as OE parts. Anyone else have experience with this.

All it means is the platinum plug will last longer under the same operating conditions.


Well it depends, yes platinums “last” longer but they also have tiny pin point spark electrodes, even though the spark is hotter, the spark profile is narrower, I’ve changed over to platinums in an older ford 3.0, ford 300 I6, gm 4.3 v6, and a dodge 318…all of them stated standard copper plugs, I noticed a negatuve change in power output and fuel milage in all 3 engines…the 318 specifically using a dyno did lose 15hp and 20ft lbs of torque

So, in a “modern” engine your probably okay to swap at will but in older school steel I’d stay copper

Also have 2 dodge trucks both with 4.7 magnums that dislike Bosh or autolite plats, they “feel” like they run better on some good ole ngks

In the end, the dyno said worse performance in the 318 and I’ll trust the dyno over “feel”

My mechanic recommended that I stick with copper plugs in my 2005 4Runner rather than try to switch. The reasoning was that platinum or iridium would not perform as well. Experiences of others?

My car came with copper. Removing manifold to reach plugs is pita. Cars website recommends GM +4 copper plugs and I don’t recall why. The +4 are not platinum. Been a long time since I went thru details. I just put in dbl platinum type recently and have no issues.
If your car came with platinum, use them.
If it came with copper, also use them.

I use to own a 05 4runner (4.0 V6). Only used Standard Copper plugs (NGK). Replaced about every other year (60k miles). Never an issue for the 300k+ miles I owned it.

Some people think you get better performance with Platinum or Iridium…you don’t.

They are designed for longevity…NOT performance.

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I have used NGK G-power (fine wire platinum) plugs in several vehicles which originally came with copper plugs. I did not notice any difference in performance or fuel economy. Also, these spark plugs lasted no longer than the OEM type would have, but this could be due to the performance of an old engine with a lot of miles.

You may be better off sticking with the OEM plugs. Good news is that the plugs are easy to change on your engine.

Platinum and Iridium tips are much harder and do not erode near as much over time as copper/iron does, but is only a benefit if both tips are platinum or iridium. Because they are much harder, they are made with a finer point on the center tip.

A finer point allows a more concentrated corona to build up and will arc at a lower voltage across a given gap. This is why the specified gap for these plugs is usually larger so that it builds up more voltage before arcing.

Usually the double tipped plugs come pre-gapped and it is not recommended that you make any adjustments as the tips are very brittle. Anyway, the gap recommended by Ford for your vehicle is for the copper/iron plug. A double platinum or iridium/platinum plug will arc at a lower voltage and that could cause lower performance.

You could increase the gap to match the performance, but how much. I don’t have the charts and graphs or formulas that would give the optimum gap for your engine if you use a double platinum or iridium/platinum plug. You or your mechanic could make a guess, but if you’re wrong, you lose.


Motorcraft are the OEM plugs for that engine. Those plugs can be found almost anywhere…including WalMart.

Anybody else notice that some people on this board are like this guy?

They answer questions that nobody asked!


I’d use the plugs recommended by the carmaker. Anything else is an experiment, and one that is not likely to yield anything good other than, possibly, longer life - but at what cost?

The owners manual or under-hood sticker should be your guide.

My vote is for sticking with what the vehicle was born with at the factory and especially since the plugs are fairly easy to change. Some very difficult engines it might be best to go with platinum.

With copper core plugs there are a few dyno proven tricks to gain a bit more horsepower and torque if you choose to spend a bit of extra time. Round the squared ends off on the side electrode. Mark on the ceramic with a Sharpie where the gap is at. Screw the plug in to each cylinder until the mark is pointed at the intake valve in the head. Sometimes one has to swap a plug from cylinder to cylinder until the mark lines up or even buy a few extra plugs to make this work.

I’ve done this in my personal cars for years although for a mechanic in the shop this is something that a customer may not want to pay for as it makes it a bit economically unfeasible.

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This looks like your answer, OP. There’s some real reasoning behind this guys post rather than “copper plugs work better because they work better”.

So if you want some fine tip plugs, it’s probably safe to assume that a 25% or 30% increase in spark plug gap over the stock gap will yield better results.