Author Topic: One heat range difference on spark plugs - how much will it impact an M117?  (Read 160 times)


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Folks, I need some expert advice on this. I get the idea of spark plug heat range and all that. But I don't have any practical data on what difference a single step-up or step-down will make to how the plug performs. I know that the 8.8:1 compression M117s need a colder plug than the 8:1, as they burn hotter and need to dissipate heat faster. But how much of an actual impact would a single step hotter plug have in the high-comp engine? Negligible, noticeable, critical?
1993 "Pearl Blue" W124 280TE
1979 "Icon Gold" W116 450SE (history, resto)
1977 "Milan Brown" W116 350SE (parted out)


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I am hardly an expert, actually not at all.

You have, IMO, a good perspective of what the 'heat' means.
If your engine is running rich(er), having a hotter plug could potentially eliminate the carbon deposits buildup stemming from BP6ES being cooled by too much fuel.

Perfect ignition is easily achievable and that helps a lot in terms of hte combustion. I recon a lot of ignition problems are projected to be fuel related.
Look up document 07.5-520 from the engine service manual to see how to test it and where it can be improved.

WRT the Bosch ignition module we reverse-engineered, well, the final diagramme may need a bit of tweaking and it can potentially be adapted to adjust the dwell angle. This would result in a better spark. Bosch did not have that as an adjustable feature originally and recommends replacement if the dwell angle is incorrect. But the world have moved on and this tech is easy to modify now. A hint, R3 may affect ti and also the C4;attach=3445
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