I have an '85 190E-16, and I've been running Bosch H9DC plugs, rather than the H5DC or H6DC. I'm not a real aggressive driver, and so perhaps these hotter plugs I've been using are more appropriate for my car. Is there much of a difference? What changes might I see if I went with the H6DC? Thanks very much. Randy

I believe the US model 16V were equipped with H8DC as were the SOHC 8-valve fours. The M103 sixes got one range hotter, H9DC.

For normal US driving conditions I think heat range "9" is fine, especially if you are light footed. My Cosworth Vega uses the same geometry plug, but I run the NGK TR5, which is about the same heat range as Bosch 8 or 9. The CV recommendation is for a resistor plug. For race track hot lapping events I run Denso T22EP-U, which is a non-resistor plug equivalant to the Bosch Super H6DC.

Since your car is a European model, I imagine that the OE plug was colder than US models since they probably assume the car will be driven much harder on unlimited speed Autobahns.

Too cold a plug will build up deposits faster than a hotter plug, but a hotter plug can overheat and cause preignition, which can lead to detonation if the car is driven at sustained high throttle settings, such as you might see in race track hot lapping or Autobahn driving.

A hot plug should should a clean insulator - white to tan, maybe with some little reddish brown specs. If the insulator has a lot of deposit buildup, it is either too cold or the engine has high oil consumption. If the insulator is glazed or has little broken bubbles, it's getting too hot and the deposits are melting.

Duke