Resistor spark plug debate - Bosch assistance - PeachParts Mercedes-Benz Forum

Graham · #1 08-23-2010, 08:04 PM

There is a thread on this subject under the SLs. It may be of interest to sedan owners. Bosch are trying to be helpful on the never ending resistor spark plug discussion

All you could ever want to know about Bosch W9DC vs W9RDC

Mark DiSilvestro · #2 08-23-2010, 11:50 PM

Your link keeps sending me to an encrypted Yahoo page!

Tomguy · #3 08-24-2010, 08:40 AM

The link works for me, here's the thread itself:
http://www.peachparts.com/shopforum/mercedes-benz-sl-discussion-forum/282781-all-you-could-ever-want-know-about-bosch-w9dc-vs-w9rdc.html#post2524419

Graham · #4 08-24-2010, 11:35 AM

Quote:

Originally Posted by Tomguy

The link works for me, here's the thread itself:
http://www.peachparts.com/shopforum/showthread.php?p=2524419

Thanks Tom - I guess I just linked to message 1

I had another email from Bosch. I guess my question would be - What IS the optimum spark plug gap for the current plugs when used in place of the OE plugs? (Bosch W 175 T 30, Beru D 175/14/3 A, Champion UN 12 Y for my '72 SL) I have not actually found the recommended gap for the original plugs in the shop or owner's manual - It must be provided somewhere - We use 0.030 +/- 0.002".

Quote:

Graham,

I looked at some of the threads. "MB-Dude" and "300holst" seem to have a pretty good grasp on the theory / concepts. "300holst" even mentioned doing some checking with an oscilloscope. This is probably the best tool to use in looking at ignition systems / components. Unfortunately, many shops do not have them as standard equipment for whatever reason.

The one thing I would add to some of their commentary, which is in one of my earlier emails, is to keep in mind that when we are talking about a 3-5 kOhm resistor, this only translates into a very small incremental demand voltage, <1 kV, as current is measured in milliamps. Do not make the mistake of thinking that demand voltage increases on a 1:1 basis based on the kOhm value of the resistor. Adding the RF resistor really only increases the demand voltage by a very small % of the overall capacity of the system, which is typically built to operate at less than 75% of max output in the harshest ignition environments. It really is mainly additional RF interference protection we are adding.

One thing that stood out in their commentary is the effect of spark plug gap. MB-Dude noted,

Quote:

"Thus in your beautiful explanation, as the secondary voltage rises – regardless of in-series resistance – the exact same voltage potential is felt at the spark gap – until the instant the spark fires. So again, the secondary coil cannot rise higher than what the spark gap is set for. Therefore, spark gap has a significant impact on how high the secondary voltage rises. But just the spark gap. This is why adding the old school ‘RF interference spark suppressors’ would have no impact on the secondary – as long as the spark gap remains constant."

They clearly and correctly identified spark plug gap as a key issue.

A very small mistake with a slightly wider gap can have a great effect on performance. The biggest single issue we see when it comes to ignition and sparking events, assuming all components are in good operating condition, is spark plug gap. We recommend that if you are having issues, start here. We DO NOT recommend using a ring type gapping tool (round tool with various thicknesses of U-shaped wires extending out). We have seen significant gapping errors when using this type of spark plug gapping tool We would recommend a pin style gapping tool. See below:

Mit freundlichen Grüßen / Best regards,

Mark Wilkinson
Robert Bosch LLC
2800 South 25th Avenue
Broadview, IL 60155 USA
www.boschusa.com

230/8 · #5 08-24-2010, 06:17 PM

Check Job #771 in the service manual. It shows a gap of 0.8mm which equals 0.032".

Graham · #6 08-24-2010, 08:34 PM

Quote:

Originally Posted by 230/8

Check Job #771 in the service manual. It shows a gap of 0.8mm which equals 0.032".

I am sure the 0.032" is correct, but I have to admit defeat in searching for it

I have searched through the owner's manual, the maintenance manual and the M117 service manual and have found nothing about plug gap or any reference to a Job #771.

Perhaps there is another manual I am missing?

ADDED: OK found it in W116 on-line maint manual. But for some reason not in 107 MM.

230/8 · #7 08-25-2010, 12:22 PM

Sorry...the CD Rom factory service manual section on maintenance items has job# 771 which relates to checking out the plugs. The 0.08mm reference is located there. I use this handy indexing system:

http://web.me.com/dakota/Mercedes/Indexes.html

It make the CD ROM very easy to use.

GGR · #8 08-26-2010, 09:05 PM

Hum, I don't remember where, but I read in different places the gap should be 0.6 or 0.7 mm (both values read in different sources) on V8s, at least on older models. I don't know for later cars such as W116s