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Old 01-02-2001, 05:34 PM
300E Russ
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We have a 92 300E sporting Bosch F 9 DCO plugs gapped @ 0.032 in. Can I safely replace those with Bosch Platinum HR 9 DPX plugs? I was told they are a hotter plug. Will the hotter plug burn the piston top or am I off base with that notion? When I pulled the old plugs to inspect them, the F 9 DCO's were gapped at .050 in. and thought there must be something wrong. I've never seen such a large gap on any plug. I regapped them at .032. Right, wrong? What can the result be if the gap is way off? Consume oil? Misfire? Run rich, lean?

Thanks in advance for your input.

Russ 300E
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Old 01-02-2001, 05:53 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
For 99.9 % of the 300E out there the F9DC0 plug is the best bet for the $$$. By opening the gap, the engine will have a better idle. Most electronic ignition systems like a wider gap & at dealer we set plugs at 1mm(.039"). Platinum plugs are for 1998 & later cars. The OE plugs are copper core & will last as long as the platinums on your car.
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Old 01-02-2001, 09:09 PM
Registered User
Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Run for your engine's life!!!

Just kidding. I would stay away from the Plat's. I have seen more than 10 different problems with engine performance when installed in an engine not originally designed for them.

Donnie Drummonds
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Old 02-02-2001, 11:42 AM
Josie's Dad
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I gap my plugs at .039 as suggested on this thread. As a follow up to Russ300E question, does this larger gap chage the lamda setting etc. Will it run lean or rich by doing so? I have the equipment, Fluke etc. to adjust the settings if necessary. Appreciate any input. Thank you, Josie's Dad
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Old 02-02-2001, 12:20 PM
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"Hot" and "Cold" spark plugs defined.

A "hotter" spark plug will not burn a piston. The heat range of a particular plug is determined by the volume of metal mass in the shell vs. the volume of ceramic around the core. A hotter plug, having more metal mass in the shell, will hold more heat and transfer less into the surrounding area of the cylinder and thus less to the coolant in the water jacket.

In lower performance engines, hotter plugs are typically found. These engines typically have less heat in the combustion chamber.

If the heat range is correct, the plug will burn off deposits at the correct rate and keep the plug clean. With the exception of a heavily modified engine, it is typically not only unnecessary, but also undesirable to stray away from the specified spark plug heat range.

My limited experience with the M103 engine has shown best results with the factory specified Bosch copper core plug, gapped at .038" or so. I now stay away from the platinums. The platinum has a very thin center electrode which generates a thin spark which contributes to poor idle quality.

Good luck,
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Old 02-02-2001, 08:33 PM
Benzo 300
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larry how would the .38 setting affect performance and mileage?
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Old 02-02-2001, 10:31 PM
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I don't advise using platinums or else you'll have to most likely put new plugs in again soon. I use the copper core Bosch Super also. They last longer and should burn just efficiently and last longer. Don't advise moving up in the heat range unless, you have a lot of carbon bukld-up. Stay within the parameters of your stock specs. If you run too hot of a plug, you could lose the engine, but if you run too colder plug. The only thing you could lose is the plug itself. You may also have carbon build-up but, it will burn off as soon as you get the correct heat range and replaced. You might not burn a hole in the piston but, I heard from a friend of a friend lose a whole electrode from the plug in the piston chamber b/c the plug was too hot.
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Old 02-09-2001, 06:52 AM
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From what I've been able to tell, copper core with .038" gap is optimum for idle AND performance/fuel consumption.

Good luck,
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Old 02-09-2001, 10:07 AM
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My 1987 W124030, spark ignition has a Jacobs Omni Magnum Ignition System.
I requires Copper Cored Spark Plugs to complete the resistance of the individual plug circuit.
The air gap for this system that performs best is 0.054 inches.
Is there a special reason for using these Plat, multi electrode, resistor or whatever is being presented to the auto owner?
All of these new snake oil plugs available to day were here 30 years ago, just in a different box.
So the best bet is what is in your Owners Manual.
Happy Trails Beep Beep from Houston.
Donald, El Cheapo(copper cored plugs cost $0.99 each. They are so cheap you could change them with your oil change schedule)

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