bosch spark plug

[ptmar]

hi i have a 1967 w110 with m121.940 engine im using w8dc bosch spark plug and im re measuring it and dont know what is the correct gap measurement of my spark plug. And do u recommend using platinum plugs or ngk plugs ?

[Douglas Broome]

There is a spark plug article on the ponton page: www.mbzponton.org.

Absent technical data to the contrary, I would gap the plugs at 0.032 inch. How many cylinders does your car have? What does your manual say?

The choice of plug brand is mainly one of choice and personal experience. Some people sing the praises of NGK, others like modern Bosch. The heat range is perhaps more important. Why are you using Bosch W8 heat range? It may be the correct one, but how have you determined this? What was the orioginal Bosch plug specified in your owners manual? I have concluded, from hard experience, that there are too many errors in the technical data contained in the Glenn book to place heavy reliance on it. This is particularly true of its pages on spark plugs.
Spark plug conversion tables are notoriously inaccurate.

I sense that, generally, the consensus re older Mercedes is not to use Bosch platinum plugs, just regular plugs. The exception may be fuel-injected Adenauers, but I am not certain.

[psfred]

pt:

Gap is 0.6 mm or about 0.028", normal for "low" voltage coils (as used without electronic ignition).

Bosch plugs are fine, but if you want to use platinums you must use non-resistor wire. I'd stay with "plain" non resistor plugs if you do not have transistorized ignition. Bosch plugs willnot recover from carbon fouling, they have to be mechanically cleaned. NGK do not have this problem, otherwise they are very similar

Chances are you will have to order non-platinum plugs these days, seems that only platinums are readily available. Stands to reason, since they are MUCH better plugs, but only if they work well in your engine!

Peter

[ptmar]

i have a 4 cylinder engine with two solex carburetor .my manual recommend using w200 T27, or radio shielded w200 rt 27 or d200 14/3, or radio shielded ed 200/14/3 with electrode gap of .0275-.0315. i installed a pertronix kit so i guess will go to a larger .032 gap . is a w8dc plug radio shielded ??? . is it better to use cooler plug. what is the difference between cooler and hotter plugs? im from florida so what should i use?

[Douglas Broome]

See the ponton spark plug article I preparted awhile back re spark plugs. I obtained the modern equivalents from Bosch Technical Assistance, so I am cautiously confident of their accuracy. I am planning a revision, but the changes are unlikely to affect your interest as expressed. Be very careful of conversion/crossing charts, including the one I provide. Errors long ago by others all too often get repeated and thus become--incorrectly--accepted as truth. I believe there may be errors in crossing the modern Bosch to some--but not most--NGK and/or Champion numbersin the chart I provided. The heat ranges do not directly/entirely overlap. The Glenn book has many spark plug errors and also confuses because Champion changed its numbering system in the mid-1960s. I have found modern Bosch regular plugs easily locally at my auto McParts stores. Cheap, so you can experiment to find the optimum one for your car.

http://www.mbzponton.org/valueadded/technical/sparkplug.htm

The Pertronix situation may alter your requirements. I have no information. What does Pertronix recommend re changing heat ranges?

[psfred]

The Pertronix setup will not change plug performace if you only replace the points and use the original coil. If you use the replacement high voltage coil, you can open up the plug gap to whatever they recommend. You should also replace the plug wires if you change the coil as the old ones may not hold up well to the increased voltage.

Heat range should be as specified by Benz unless you find you need a higher or lower range to prevent fouling or plug overheating. The exact performace of your carbs will have an effect, too, as it's very difficult to get them to give exact mixture control all the time.

If you have problems with carbon fouling and you KNOW you aren't running rich, use a hotter plug. If the insulators blister on the tips while staying brown at the base, you need a cooler plug.

Peter

[Litton]

FYI, pertronix recommended that I increase the gap to .45 with the high voltage coil on my 280 sel 4.5. I won't tell anyone that I am getting 12 mpg in city driving and 17 hwy.

[ptmar]

if i have a very good condition engine and use hotter plugs instead of cooler ones will there be any bad effects. will my engine temp increases? or will it damage parts of my engine?

[psfred]

pt:

The temperature range determines the actual operating temperature of the tip of the spark plug. Hotter plugs have longer, thinner insulators, colder plugs have shorter, fatter ones. This determines how much heat gets transferred away from the tip.

Hotter plugs than necessary in an engine will lead to pre-ignition and possibly engine damage if they get so hot the porcelain insulator cracks off!

Set mixture correctly before changing heat range. Then, if you still get fouling problems, move up one heat range. Check the plugs for signs of overheating, too -- insulator should be light gray or tan (although platinum plugs sometimes stay bright white). If the plug is too hot (or the mixture too lean), the insulator will be bright white and may have small blisters where it is starting to melt in small spots. If you see this, richen the mixture and/or use colder plugs.

If, however, the plugs stay dark brown and collect black soot or crusty brownish stuff, and the mixture is right, use plugs one heat range hotter.

The vast majority of the time, the factory recommended range is fine. However, if you do ALL stop and go city driving, no expressway at all, they may foul. MBs prefer some hard driving regularly to keep the engines working correctly!

Peter

[j9fd3s]

http://www.ngksparkplugs.com/techinfo/spark_plugs/techtips.asp?nav=31000&country=US

kinda long but its a good read