in over 230,000 miles I have changed cap and rotor at 30,000 mi. intervals as well as a couple of times at closer to 60,000 miles and felt no perceptible difference in engine performance or mileage. I changed my plug wires out at about 145k miles, again, felt no real change. I think these ignition systems are really strong. Plugs I did at 60k intervals for the first say 180k mi. then more recently I have dropped this to about 30k miles intervals. Maybe I'm just getting a bit superstitious about a high mileage engine? I should mention that mine is a normally aspirated base 855 with manual tranny, '94 vintage.
Don;t know if that helps any.
PS one time I went to Napa for cap and rotor and was given a Bosch in a Napa box. Pretty sure these must all come form the same source. As for plugs, I have always run Bosch platinums, though there has been tons of discussion here and also on Saab board re: better results using copper on the turbo engines.
claude
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