On all Honda Engines, it is extremely important to always use Resistor Spark Plugs (Spark Plugs with built in Resistors). Using non-resistor plugs can cause significant electrical interference to on-board electronics: ie: your ECU. (This is one of the common causes of troubles with on-board datalogging on properly equipped ECU's)
For Honda engines we recommend only using NGK/Denso spark plugs. Using other spark plugs like Bosch, Autolite, etc has been the cause of countless ignition misfires that we've seen which could have easily been eliminated by using NGK/Denso plugs. Why? Honda engines were designed for the flame front to develop and travel in a specific way that corresponds to the way NGK/Denso spark plugs ignite. Switching to another brand spark plug causes this flame front to travel irregularly causing misfires, detonation, and poor fuel economy.
Don't believe us? Here's info taken directly from NGK's website:
"NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.
They are also recommended on any vehicle that has other on-board electronic systems such as engine-management computers, two-way radios, GPS systems, depth finders or whenever recommended by the manufacturer.
In fact, using a non-resistor plug in certain applications can actually cause the engine to suffer undesirable side effects such as an erratic idle, high-rpm misfire, engine run-on, power drop off at certain rpm levels and abnormal combustion."
Spark Plug Heat Range Selection
The simplist way to determine what heat range spark plug you should be using is this: If you want your combustion temps a little hotter, use a hotter plug (lower number plug). If you want to lower your combustion temps, use a colder plug (higher number plug).
On performance engines with higher compression, forced induction, etc, the combustion chamber temperature will increase from the additional power being released. To combat detonation and excessive combustion temperatures, we recommend moving up one number cooler plug for every 100 horsepower created. (This is just as a guideline and not to be used as a rule) In example; If your stock engine produced 150hp, and you have increased power to 250hp, you should be using one step colder plug than factory.
Spark Plug Gap
Spark plug gap is directly related to the plug's tip temperature and the voltage necessary to "bridge" the plug gap and ignite the air/fuel mixture. Generally speaking, the higher the compression ratio and/or boost pressure, the smaller the gap will need to be. (to be able to successfully ignite the mixture) The factory Honda ignition system does an excellent job of delivering enough voltage to the spark plugs, but if you find yourself running an extremely small gap and having trouble achieving proper ignition, you may want to invest in a higher output aftermarket ignition coil. This will allow you to run a larger spark plug gap and still have enough voltage to ignite the air/fuel mixture.