Re: Champion QL77JC4's only for 1989 70hp Johnson??
It has to do with how OMC calibrated the fuel delivery system and combustion temps. They designed a specific motor to burn a specific ammount of fuel at a specific combustion temp range. The heat range of the plug has a lot to do with combustion temps. Basically its design parameters.<br /><br />Assuming we're talking about a motor thats set up correctly theres nothing saying that another brand cannot be used, the system just wasnt designed for that. If - a big if - another brand plug is used and its exactly the same heat range or cooler, the motor should be just fine. Its possible, and likely, that different brands are slightly off in heat range from each other. <br /><br />The range of the combustion temp has a cushion built in. Using a different plug may take away that cushion. The motor may still operate properly but if anything goes slightly wrong to increase the combustion temp and that "safe zone" cushion is gone and damage may happen.<br /><br />Some motors are more prone to this than others. On older motors that used a lot of fuel (therefore cooler combustion temps) this was not as much of a concern as newer, leaner burning motors. IE - the range was much wider and the motor more forgiving.<br /><br />Its not that NGK's are not a good plug. They are. Look at Mercs for example. The motor was designed with NGK's as the design parameter. Many times someone will install Champions in a Merc or Yamaha and complain about fouling the plugs. Does this mean Champions are junk? No, the motor just wasnt being used as intended.<br /><br />We use hotter plugs in performance and race motors all the time but we flow a lot of fuel. We make more power but still keep the combustion temps in check.
