Sparkplug question

[lloyd]

I recall having read that someone on this board was "happily" using WR9LS Bosch plugs in both his high compression coupe and his 02. These plugs are obviously extended tips. I have a few leftovers from the 6 series days and am considering using them on a coupe but the extended tips give me pause. Can someone confirm that the sparkplug electrodes will not kiss the coupe's pistons.


Thanks in advance.

PS because of the new format I was unable to fully search this topic.

PS Thank you for the new format!

 

[velocewest]

I use WR9LS in my relatively high compression 2800CS, and in my even higher compression 3.5 liter in my e12. They work great in both cars. The difference in length is about 1.5mm, and it's just the electrodes, not the threaded part of the plug body.

 

[lloyd]

re Sparkplug question

Velocewest, thank you for the response. What gap are you running? And, if you dont mind my curiousity, what prompted you to run these plugs in the first place? Do you also run resistor wires/connectors?

I am aware these plugs are listed for use in the later 6 cyl models, but I wonder why they are not listed in any x reference charts or why the plug manufacturers do not list this plug as an expensive alternative. Could it be the later models run lower compression ratios or more "open" combustion chambers?


I experimented with a jacobs ignition setup on a 2002 and when the gaps of the stock W7DC plugs were enlarged from .028 to .050., it was just enough to interfere with the pistons. In fact, I'm certain I heard the contact at speeds slightly above idle. (The pistons regapped the plugs to about .040!) Cant recall if it was an e-12 or 121 combustion chamber, but as you can imagine, I am not interested in repeating that experiment.

 

[velocewest]

In the e9, I run stock gap. That engine is stock, the ignition has Pertronix, BMP Performance wires and a high output MSD coil. The wires are OEM configuration, so they have resistors. BMW uses resistor plugs and resistor wires to get the appropriate resistance for the ignition and good electrical interference levels.

In the e12, I also run stock gap, but the engine is from an e28 so the gap is appropriate to the engine. That engine is modified, and the ignition has BMP wires but is otherwise stock.

You're correct that later cars use lower compression, but it's through the use of flat top or dished pistons. Prior to the M30B35 variant, the specs of all M30 heads are the same for combustion chamber shape and size, regardless of engine displacement.

I decided to run Bosch Silbers in the e9 at the urging of a local independent BMW shop that I trust. I was assured that there would be no piston contact even in the higher compression 2800CS engine, and there has been none. I run Silbers in the e12 engine because that's BMW spec, and even with the piano top 10:1 pistons there's no problem. If you have a 3.0CS the pistons are flat tops, so they come nowhere close to the spark plug.



A couple of refs for general knowledge: