BOSCH WR9LS alternatives?

[AdamHam]

It would seem in my research that the Bosch WR9LS plug has become hard to come by, and a bit more expensive than it used to be. I see that the NGK ZGR5A plug is considered the NGK equivalent to the WR9LS, and is alot more plentiful and cheaper. Is there any reason why I shouldn't run the NGKs in my M30B34?

[RobertRO]

Adam, the NGK ZGR5A is an excellent spark plug for any M30B34 or M30B35 engine. Except for the Euro/dirty B34 with 10:1 compression.

You can get it in standard (p/n 5839) or in V-Line flavor (p/n 2941, V-Line #15). Both are fine, but my preference goes to the V-Line one.
The V-Line one has a V shaped groove in the center electrode. It forces the spark to occur at the outer edge of the center electrode, and NGK claims that this makes it easier to ignite the air-fuel mixture, and requires lower voltage to produce a spark.
When you install a new set, you may want to gap the spark plugs a bit narrower, at 0.7mm instead of 0.8mm recommended by NGK. Replace them every 15k to 17k miles.

Link to Niterra / NGK part finder

[harrypalmer]

I knew the NGK ref no. replacement but the rest of your info, I’ve found very educating. Thank you RobertRO!

[Ju@n]

Just my 2 cents, I'm running BPR5ES, granted I don't have the original ignition system anymore, but can't get the WR9LS nor the ZGR5A locally.
Those same BPR5ES are very similar to what friends use on their E30s.
From looking at the decoder and pictures, it looks like the ZGR5A is a bit more projected than the BPR5ES

[Blue Shadow]

Adam, the NGK ZGR5A is an excellent spark plug for any M30B34 or M30B35 engine. Except for the Euro/dirty B34 with 10:1 compression.

When you install a new set, you may want to gap the spark plugs a bit narrower, at 0.7mm instead of 0.8mm recommended by NGK. Replace them every 15k to 17k miles.

Interesting.

Can you talk a bit more about the correct plug for the Dirty B34?

Back in the day, 1988 or so, at a BMWCCA GGC Tech Day the recommendation was to gap the WR9LS at .032 (0.81) but you are talking about the NGK plug. This was performance, no other reason. The BMW number is 0.7+0.1mm.

And now after almost 38 years of ownership, I learn the spark plug interval is fairly short on these motors. I thought it was more than double the recommended which is Inspection I. On my car that is about 35000 based on the SII but I know that thing has been lying to me since I bought the car. Car always ran better with new plugs but only for a month or two.

[RobertRO]

Almost four years ago, I opened a topic on the subject of spark plug gap:
https://www.mye28.com/viewtopic.php?t=155207

For the dirty B34, I like the NGK BP5E (V-Line #8), gapped 0.7. Note that it is a non-resistor spark plug. There's already enough resistance built in the M30 motronic ignition wiring, and I consider it detrimental to use resistor plugs - unless there's no option. I never had interference issues with non-resistor spark plugs.
Unfortunately, the ZGR5A does not have a resistorless equivalent.

[AdamHam]

Adam, the NGK ZGR5A is an excellent spark plug for any M30B34 or M30B35 engine. Except for the Euro/dirty B34 with 10:1 compression.

Awesome thanks for the advice!

Link to Niterra / NGK part finder