Subaru and Bosch Plugs

Howdy - -

I had my 2005 Forester at a dealership the other day for a factory recall. I mentioned that my gas mileage wasn’t so good lately. I went on to proclaim that I had splurged on some expensive Bosch “4+” platinum plugs.

The service folks were horrified that I had put Bosch plugs in a Subaru engine. They were adamant that I would experience extended cranking at start up, poor gas mileage, or both as a direct result of having used Bosch plugs.

At first, I thought that they were simply talking about the “4+” design with the four ground electrodes. Then, I realized that they had been referring, categorically, to ALL Bosch plugs. Their position was NGK only !! This is where my skepticism kicks in. If the heat range, gap, and what-have-you are all to spec, then why should the brand matter?

Does anyone have any light that they can shed on this as to Bosch, or specifically the “4+” plugs, being incompatible with Subaru engines?

The funny part is that when I spoke with the parts manager at this same dealership, he claimed to use nothing but Bosch plugs in his own Subaru! Go figure.

- - Herb

Many people have reported problems in this forum after using Bosch spark plugs–particularly those gimmicky 4+ plugs.

As with virtually any other make of car, you should use what the vehicle manufacturer specifies–in this case, NGK plugs.

There is no LOGICAL reason the BRAND of spark plugs should make any difference…But I have learned the HARD WAY enough times that it CERTAINLY does. I use NGK’s in everything that I can and I don’t have any spark plug problems period. It’s a superior product. I DO use the Autolite platinum’s in my Crown Vics, which makes them happy…

2-stroke engines in particular benefit from NGK’s

I second VDCDriver’s post. I tried Bosch plugs when I first got my used Subaru, but quickly reverted to the standard NGK’s when I noticed poorer fuel economy.

I’ve heard various theories, one of which is that the ECU is programmed to expect certain behaviors from the spark plugs, and when you change that, you’re dependent on the ECU programming to adapt. Some are better than others at adapting. If the ECU is optimized for a certain type / heat range of plug, why change?

NGK V-powers are very good plugs, and they’re cheap. Not many other things related to Subarus are …

It doesn’t SEEM logical, but some cars a finicky to the brand and type of plug used. For instance, I noticed a major power loss and fuel economy drop in my Mazda when I tried Autolights. The NGKs fixed that problem. Mitsubishi engines, like those used in Dodge Caravans HATE Bosch plugs. Supra turbo engines don’t like NGK iridiums, but NGK platinums are just fine. I could go on.

Today, since plugs last over 60,000 miles, some to 100,000 miles, I just use OEM plugs. I know they will work, and I don’t need to worry about it for at least 5 years or more. Just not worth saving the pennies and rolling the dice.

Interesting thread, guys.

I have already bought NGK plugs to replace the Bosch “4+” plugs. At the parts store, they had two NGK plugs listed: standard and Platinum. Should this matter?

    • Herb

Best bet for plugs is OEM brand. Lots of reasons why. I also like OEM wires, or at least OEM quality. No need for the fancy fat colored wires, all so and no go.

I does. Some cars do not like platinums. Did your car have OEM platinums or copper-core? How much are the OEM replacements at the dealer? Are you really saving any money with all these different plugs?

The platinums were only 50? more per plug.
I don’t know what was in there originally. I’ll have to refer to Subaru’s spec.

How often have you changed the plugs in this car? OEM Platinums last 60,000 to 100,000 miles. Copper-core last 12,000 to 24,000 miles. Iridiums last 150,000 to 300,000.

This is simple.

Don’t try to out engineer something that somebody already spent millions of dollars to engineer to find out what makes it work right. This goes from spark plugs to brakes. Because when you go out of the specifications set by the manufacturer, you delve into unknown territory. And sometimes the results can be expensive.


The original spec is NGK FR5AP-11 (Double Platinum). Subaru wants these plugs changed every 30k.

Tester - -

It’s not a matter of “out-engineering” anything. I would never in blind faith accept anything that a manufacturer tells me.

It is reasonable to ask why it is that a part from one manufacturer should be preferable to a part from another manufacturer when both bear the same specs.

    • Herb

And, those are the plugs you should use. Is this a turbo engine? I’m just trying to determine why the shortness of duration, especially with platinum on both the anode and cathode of the plug. 30,000 miles seems so short compared to other applications.

I would second that to OEM plugs, as I did not know the gm manufacturer but followed the wisdom of the Auto parts guy as he had seen the stories and strongly recommended them. 8 bucks apiece, but I think the money was well spent. Auto zone does sell oem spark pugs.

Subaru for whatever recommends changes plugs out every 30k on non-turbo engines. 60k on turbo cars. Often…

This is not a turbo engine.
The turbo version calls for NGK ILFR6B (iridium).