Re: What spark plug do i need for Johnson 25 hp outboard
Pulled out this answer from here:
http://www.wmi.org/bassfish/bassboard/boats_motors/message.html?message_id=188390
' types of plugs from seahorse (216.97.214.134) 12/3/2001 7:07:00 PM
The OMC ignition systems are made for a suppressor type plug, which is different than a resistor plug. A suppressor sparkplug has a small coil of wire that restricts high frequency AC current as the spark jumps the plug gap. It doesn't affect DC current. A resistor plug restricts both AC and DC current and weakens the spark energy. A regular plug without a resistor or suppressor, has a shorter spark duration and can feed larger amounts of AC current back to the pack thru the coil induction. This may cause ignition problems down the road. A 225 HO OMC needs a suppressor plug, Champion QL78YC. The letter Q in a Champion signifies a suppressor style, and R is a resistor type plug.
About a year ago, Bass and Walleye mag had a story on the different types of plugs and why they were needed with different motors.
Right off the NGK site.. from Thunder Runner 12/3/2001 8:07:00 PM
Inductive Resistor Spark Plugs Specified as original equipment in a variety of late-model outboard engines, spark plugs with an inductive resistor are utilized when a CDI (capacitive discharge ignition) system is used. The resistor plug is necessary to reduce radio frequency interference (RFI), to prevent interference with the sophisticated electronics onboard modern boats.
Most resistor spark plugs use a "monolithic" resistor, generally made of graphite and glass materials, to filter the electrical voltage as it passes through the center electrode. This "filter" reduces RFI to an acceptable level. Although this type of resistor develops from 1000 to 5000 ohms of resistance, there is no significant loss of voltage to affect ignitability at the electrodes in modern automotive ignition systems.
However, outboard marine engines utilizing a CDI system have a much lower voltage output, which would be compromised by a standard resistor plug. Therefore, an inductive resistor is used. In this method, a coiled nickel wire is inserted into the center stem, and held in place by a spring.
This coil basically acts as an electromagnet, inducing a magnetic field around the center stem, creating a natural "field" resistor, and reduces RFI to acceptable levels. This method creates only 40 ohms of physical resistance, compared to the 1000 to 5000 ohms of resistance in "monolithic" resistor plugs.
While NGK Inductive Resistor Spark Plugs cost more than standard resistor plugs, they provide major benefits in late-model marine outboard applications.
John M Richied