Choosing the Right Plug for Your
by Michael Lee
One of the most overlooked and less tended to part of keeping a performance car in tune is the spark plugs. If they are fouled, out of gap, are the wrong heat range for your application, or style problems may arise. Sometimes these problems could be simple: like hard starts in the morning or loss of horsepower (ahhh! We don't want that!). Other times it can be detrimental like the all feared-DETONATION (which can scrap a perfect motor within seconds).
The step is choosing the correct heat range. Spark plugs come in many heat ranges so the plug can be matched to the type of use an engine receives. For most low speed driving or short grocery runs (we do all have daily drivers, right?), the plug electrode must stay hot enough to burn away carbon deposits, so a hot plug is needed. On the other hand, for high speed driving at high RPMs, like at Holtville Raceway last month, a hot plug can glow like a diesel glowplug causing pre-ignition and detonation. To prevent this, a cold heat range plug is needed. But the evil to this is, at low speed driving, a cold plug can foul with carbon and become useless. You are the ultimate judge of what kind of driving you do most.
How do I choose a hotter or colder plug? Generally speaking, I don't recommend anything but NGK or NGK platinum plugs for Nissan and Datsuns. Don't get sucked into the "Splitfire" or any other style plugs that the current magazine ads hype up. When you use an aftermarket plug, its specifications can be different than your stock one (even with the same heat range). An example would be a shallower plug or an extended plug. Yes, these manufacturer's claim X-amount of gain or self cleaning plugs in their test labs not in your engine designed by NISSAN/DATSUN.
One theory is an extended electrode puts the plug in the more turbulent section of the combustion chamber, which helps to prevent fouling. In theory this is great, but in your Z this can sometimes put the electrode too close to the piston, screwing up the flame propagation and losing power (as much as 4%). NGK plugs were designed for our Z and generally speaking the higher the number the colder the plug (i.e. PFR5B-11 is hotter than PFR6B11).
Also, when you are adding a turbo, increasing boost, and/or adding Nitrous Oxide you should go to a colder plug. For example, a Z32 running more than Stage 4 should consider going to a colder plug, say PFR6B11 (stock for the Z32 is PFR5B11). The rule of "higher number is colder" only works with NGK. Check the plug manufacturer for their method of denoting hotter or colder plugs.
Finally, don't let the infamous "maintenance free" type plugs fool you. You will usually see this in platinum tipped spark plugs. I was even fooled by my favorite plug manufacturer NGK. You figure spending $15.40 a plug anything was possible, right? Wrong, strapping my car to the dyno with plugs only 10,000 miles old (both NGK and Nissan stated that the Platinum tipped plugs on the Z32s were good for 60,000 miles maintenance-free) I learned that I lost almost 27 hp. Upon inspection my maintenance-free plugs only needed to be regapped since they were out of spec. And there you have it, 27 horsepower, I don't know about you, but that's a lot of power to lose from some fairly new plugs that were just not tended after. If you do a lot of spirited driving and want your Z running at its peak-take care of your plugs, they are cheap for the returns they give.