This is just another area where you really have to give the engineers at Toyota a lot of credit! The ignition system on the 83 - 86 MKII's is a well matched component system. The engineers who designed the ignition system probably calculated the electrical resistance values in such a way to include the core resistance of the Platinum plugs and also add the benefit of longer service life. Take the 82 for instance. The 82 5MGE seems to not like anything other than the conventional ND or NGK plugs. Swap the stock plugs for a set of Bosch +4's and the darn thing won't even fire or run. Please don't ask me how I know! :roll: (putting flame suit on). The aftermarket plugs would probably yield the most added benefit with an aftermarket ignition setup or modified engine with higher compression and/or cylinder pressures. Food for thought anyway as ususal. 8)
The "splitfire" design accomplishes basically the same thing as the U-groove or V-power design in that the design allows the spark to be positioned slightly away from center of the plug tip and also cause the spark to shift from side to side on the split or grooved ground electrode. This can be to some benefit on high performance engines that have very high compression and cylinder pressures, especially when combined with a hot ignition system like the capacitive discharge MSD 6A, Jacobs, Vertex magneto, etc.. Also, FWIW, in case some don't know this, MSD stands for Multiple Spark Discharge. Very useful on all out race engines!
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