1989 300TE M103 Spark Plugs
 

G'Day, i hope you can help. I am stuck between using NGK BP5EFS or the cooler BP6EFS. I currently have the latter but after researching the web it seems the decision is split. Can anyone advise me correctly on what plug to use?

Hello Ned. I'm not sure it's a matter of "correct" so much as it is what's better suited to your driving conditions. It is generally regarded that a hotter plug is better suited for lots of in-town driving, while the cooler plug does better when the car is operated mostly at freeway speeds. My driving tends to be more in-town driving, and I use the BP5EFS plugs in my M103, and they do very well under those driving conditions. That's what my experience has been; someone else can chime in with a more technical explanation of why that is. Have you checked the NGK website for an explanation and a recommendation?

Most of my driving is also in-town. What is the downside when using the hotter plug should i get out on the freeway?