Hotter Spark plug debate

[Olazz]

On Saturday I went to my local motor factors to buy some NGK BP6ES plugs.

There was a chap behind the desk who saw me pull up in the SL. He was in his mid 30's.
I asked for  the plugs and he produced BPR7ES. I looked at him and said no, I think you mis-heard me. I wanted BP6ES...

He said no, you car runs BPR7ES as its a colder plug and if you put in BP6 non resistor plugs you will crack the head. "I've been working in this shop and as a mechanic since I was 16 and I know what I'm talking about" says he getting animated.  He just wouldn't accept that I was not interested in his advice.

I turned to his colleague and asked for the plugs and he promptly gave them to me, whilst his irritated colleague continued to force his opinion on all in the shop. His parting salvo as I walked out was "don't blame me if the head goes pop mate!"

Complete B/S IMHO,  or does someone know better on here?

[mdsalemi]

I'm not sure one range hotter or colder will cause a head to crack; usually people fine tune the running by doing this.  I've had the BP6ES in my car for many years--at least 6 years and 6,000 miles.  I don't think my cylinder head is cracked.  If it is the car still runs well...

But the R?  No way.  The resistance is supposed to be in the plug connectors on our cars, not the plugs, and not the wires.

[Olazz]

Mike,

Yep I didn't think so either. Good to Know yours has done 6k with no problems..

Lazz

[J. Huber]

I use the BP5ES and have so for almost 10 years and about 20K miles. I have had people (behind counters) tell me the Bosch Resistor Plugs WDRs are recommended for our cars but we have all learned here otherwise... I think your initial suspicion is correct.

[Benz Dr.]

Cracked, warped heads? Yeah, that sounds about right.

  OH! We were talking about engines? Sorry, I missed that part.

  The best plug to use is the BP5ES. This plug crosses over to the old W9DC that's no longer available. The recomended plug is 6 for NGK and a 7 for Bosch but I find them to be a bit too cold in all but engines with perfect compression. I'm out of 9's but used them for almost 20 years without any problems.

  There might be some sort of crack in the pipe he's been smoking but you won't find it in your cylinder head. It just won't happen. 

[cth350]

Just finished reading a short article on plug "hotness" in John Olson's latest "SL Market Letter".  The article, authored by Michael McKinney (www.blackforestllc.com).

Basically, plugs "too hot" can lead to pinging or worse and plugs "too cold" can foul.  A critical factor in making the decision is knowing the compression ratio (your 9.5:1 is higher than the 8.0 present in the US spec sedans). 

Clearly on needs to pay attention to what the car is trying to tell you and you should check your plugs for deposits, color, etc. to decide which ones are "just right" for your motor.

As for resistance. I think it's been stated enough times that the overall resistance in the system is part of the spec and one has to pay attention to the plugs, boots, wires, rotor and coil to get it right.

-CTH

[Olazz]

Basically, plugs "too hot" can lead to pinging or worse and plugs "too cold" can foul.  A critical factor in making the decision is knowing the compression ratio (your 9.5:1 is higher than the 8.0 present in the US spec sedans). 

Ok, I'm sort of there on how engine compression ratios are calculated, but how does compression ratio translate to readings on a compression gauge?.
Is there a direct correlation between higher CR and higher gauge reading and if so what is the correlation?

[Olazz]

In answering my own question I found this interesting article


THIS


Hope its useful to some!

[DaveB]

Yep I read something similar stating Compression = (CR-1)exp1.2 x atm. press.

This checks out pretty well giving 180 psi for 9.3:1 compression (Factory 230SL specs are I think 11-12 bar or 156-170 psi).

Other sources say the cam can have a big effect on compression which I guess is understandable if there's significant valve overlap. I presume valve overlap is the reason the factory 230SL compression specs are slightly lower than the ideal 180 psi.

[Ulf]

I usually order a complete service kit from SLS each year, the package just arrived at my office and now that I came across this thread I opened it  to check - and yes, they've given me the same NKG BP7ES as the hot-head in the shop mentioned. I don't want to join the debate (as I'm really clueless on this matter), but should I be worried that these plugs will cause problems?

Best

Ulf

[Olazz]

Ulf,

You'll be fine with the BP7ES plugs as they are the factory recommendation.

I'm running a hotter BP6, and some are running even hotter BP5 plugs.

The buffoon in the shop wanted to sell me BPR7ES which is a resistor plug.

[Ulf]

Ahhh - failed to see that extra letter in my feverish attempt to rip open the package and write here simultaneously :-)

Ulf

[Naj ✝︎]

NGK numbering system is bass ackwards to the Bosch system.

BP7ES is a colder plug than a BP6ES; BP5ES is hotter.

I think you need to look at the plugs that you take out to decide whether to use hotter or colder plugs.

I am happy with BP6ES but use the car for longer runs. The weekend SL Day Club Rally was 250 miles round trip $$$

Naj