Bosch 7500 plug now WR7DC+ ???

[rogerh113]

Hello,

Just got a couple of sets of spark plugs, and it appears that the Bosch 7500 plug is now WR7DC+ rather than the W7DC.  When I questioned the vendor, they said that Bosch discontinued non-resistor spark plugs about 2 years ago.  I have seen several of the older postings as to the problems with resistor plugs in the 113, and wanted to find out if anyone has had success with the WR7DC+ plug, or if it has the same problems as the prior resistor plugs.  

I have a 'like new' set of plug wires in the car and it is running well, and so would prefer not to be shifting things around to try and accomodate a set of plugs.  I would guess that I could return these plugs if necessary, and try and locate a stock of W7DCs somewhere else.  I think I also read one of the posts where W8DCs or W9DCs were recommended??

Any input or recommendations would be appreciated.  Plug need is not urgent, as I just installed the last of my W7DC plugs.

Thanks and regards -- Roger

[mdsalemi]

Roger,

Have you considered NGK's?  My experience was better with those than with Bosch.  BP6ES.  My experience is that the NGK's are somewhat less prone to fouling.

Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America

[J. Huber]

Hi Roger. It is true that the non-resistors are all but extinct (from what I have heard). Michael's advice is good -- I have NGKs an they perform well. In my case, I have been running NGK BP5ES which is a hotter plug -- more equivelant to the WR9DC. For a match to the 7 Bosch -- try a 6 NGK. (readily available at local NAPA places)

James
63 230SL

[hauser]

Try Dr Dan Caron.  He seems to have no problem getting the right spark plugs.

1969 280sl 5 spd
Gainesville, Fl.

[Raymond]

My local NAPA store carries the W7DC.  Two of the clerks told me that resistor plugs had been discontinued, but a woman who works there told me they were full of it and proceeded to sell me a set at $1.19 each.

For some reason, Bosch seems to have named a group of their older plugs under 7500 in the software they supply stores.  So, you need a cooperative person at a non-busy time at the store to physically look at the plugs on the shelf.  



Ray
'68 280SL 4-spd Coupe

[rogerh113]

Hello,

Sounds like nobody has any experience with the WR7DC+, good, bad, or indifferent.......  

I have not had fouling  problems with the Bosch W7DC, but if no other alternatives exist I will take a look at the NGK.  Really surprised this has not been a problem for others, since even the local NAPA told me the Bosch were no longer available and all of the web suppliers that I normally use show out of stock on the W7DC.  Has the W9DC equivalent shown better performance??  Hoping to lite on a solution sooooooon......

Thanks -- Roger

[hands_aus]

My local auto store told me the W7DC plugs were no longer available.
What they should have said was that they no longer sell 6 packs of them however they still sell 4 packs and have them on their computer.

They sold their old stock 6 packs for about half price and I bought a few.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[rogerh113]

A technical question for those in the know....

I am running the 250 low compression engine (somewhat rare, I gather) - would the W9DC also be a good choice for this engine, or does the lower compression make a difference in plug selection??

Thanks -- Roger

[Benz Dr.]

On all but perfect compression engines I use W9DCo. The DCo plug is a slightly different design and is common in newer engines.
I find that the 9 plug works better than the 7 as it heats up more quickly and so there's less chance of fouling. The 7 is recomended in almost every book you will go to if you walk into a auto parts store. However, it's no longer available. I believe that they're replaced by W7DCo
They come 10 per box. I can get all WD9DCo you want and I have 24 WD7DC's left. Once they're gone I won't have any more available.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[zoegrlh]

Thanks for the info on the discontinuance of the W7DC plugs.  I went to our NAPA store yesterday and got the last box of W7DC plugs.  It pays to read the forum.

Robert Hyatt
Williamsburg, VA.

1970 280SL, Red on Silver,4-speed, Euro spec; 1999 C230 Kompresser Sport, Black on Black; 1990 Miata, Black on Red, 5 Speed

[ja17]

Hello Roger,
Resistor plugs add 5,000 ohms more resistance to circuit. The spark plug ends on the later W113s add another 5,000 ohms (1,000 ohms on the earlier plastic spark plug wire ends) , the coil will have some resistance also, the rotor another 5,000 ohms, the connector at the distributor 1,000 ohns, and the coil wire connector 1,000 ohms. Now if by chance you have mistankingly used the dreaded "carbon spark plug wires" on your car you can add at least another 5,000 ohms... way too much resistance!

The BBB suggests that you should have no more than 13,000 ohms resistance from the coil and all the way through the spark plug (early cars). A little more is allowed for the later cars with the factory transistorized ignitions (16,000 I believe).

This means if you will be out of specs as soon as you add resistor spark plugs. Chances are if everyting is in excellent condition with the rest of your ignition you should not have problems for a while.

As the system ages amd in colder temperatures, componants become less efficient and create more resistance. The bottom line is you ignition will be under more strain and fail or short-out sooner. The electric spark will find a path of "less resistance" when the resistance becomes too high.

Cold starts may be less intense, causing harder starting.

It just physics. Modern ignition systems use much higher voltage so resistor spark plugs have become the norm. Bosch has given in to the economics while companies like NGK who cater to the performance motocycle crowd, still supply non-resistor spark plugs.

Joe Alexander
Blacklick, Ohio

[rogerh113]

Thanks for all of the input.  I returned the Bosch R plugs a couple of days ago, and will be going with the NGKs.  Probably will get one set of the BP5ES and one set of the BP6ES and see if the hotter plug makes any difference in performance.  

Glad that there is an alternate source of correct plugs for the car......

Thanks and regards -- Roger

PS = HAPPY NEW YEAR!!

[jpressick]

For several months my 1967 250SL, with a newly rebuilt engine, was missing something terrible.  A 123 electronic ignition was added, new fuel pump, new fuel filter,new air filter plus the ignitors were checked.  None of this had any affect  After reading many of the comments on this site I decided to replace the Bosch WR7DC spark plugs with NGK6ES.  What a remarkable improvement.  The engine performs just like it did when I picked it up at the Mercedes factory in Germany in November 1967.  Thanks to all for the help.  Cheers Jim

[Jordan]

Just so I understand correctly, the WR7DC and WR9DC plugs would be the resistor equivalent to the W7DC and W9DC respectively?  I spoke with the local Bosch distributor here and was told Bosch, as with many manufacturers, are simplifying their part number system.  I was told the Bosch 7900 was the same plug as the W7DC (only the number has changed, nothing else) and the 7911 is the replacement for the Bosch W9DC.  Having said all that, are the non resistor NGK plugs better than the non resistor Bosch plugs? 

I currently have WR9DC+ plugs in the car (engine has not been rebuilt) and it has been running great, although these were just installed last month so I don't know how they will perform once the temperature starts to fall.

[jameshoward]

Jordan,

Hadn't heard the thing about changing the numbers. Sounds like they're making it more complicated. Also not sure whether every Bosch (or other parts) supplier is 100% accurate. The 'R' in wR9dc is indeed for 'RESISTOR.' You don't want a resistor plug in your car. I use NGK (the slightly hotter plugs equivalent to the Bosch W9DC) and the car runs superbly well. I changed from Bosch, but it was on a whim; can't tell you if I could really notice any difference now.

However, if your car runs brilliantly on a resistor plug, then it's hard to justify the expense in changing them. Stick to what works, perhaps until you think things aren't running as they should, then look to the plugs first.

JH

 

[ja17]

Hello,

I was told that Bosch is phasing out all non-resistor spark plug production in the future. Resistor plugs add an additional 5,000 ohm resistance to each plug wire circuit. Many cars had the 5,000 ohm plug wire resistor ends back in the day. In addition the distributor end of the wire had a 1,000 ohm resistor end, and the ignition rotor has another 5,000 ohm resistance.  Mercedes allows 13,000 ohms resistance on each wire lead.  So if you do the math you will find you are out of spec when using resistance spark plugs. Heaven forbid that you have a set of carbon wires on your car since this will add another 5,000 ohms resistance to each plug wire.

Now if your car has the 1,000 ohm spark plug resister ends you nearly make up for using resistor plugs. Also if you have changed to  a hotter coil or electronic ignition you may have improved the ignition spark. Resistor plugs may work in your car depending on your exact set-up.  In any case with resistor plugs you start off being 5,000 ohms closer to failure. (which is the point when the spark misfires to a different path).


NGK and some other manufacturers developed the copper core spark plug for very high RPM Japenese motorcyle engines. They were the choice for the performance minded in the day when manufactureres like Bosch were still making the steel core W175T30 spark plugs for the W113 cars. Champion Spark plugs soon followed with a decent copper core plug (7YC)and at some point Bosch also came out with a copper core spark plug"Supper" whcih had the new designation  W7DC , the "C" signifiing a copper core. I switched back to Bosch plugs at this pioint until they abandoned the non resitor market at which point I have returned to NGK which still manufacture a very good quality  non-resistor spark plug. Champion has also abandoned the non-resitor plug market.

Bosch spark plugs have  nice nickle plated threads which are a little more friendly to aluminum cylinder heads than the zinc plating on most other spark plugs.

The resistance of your  spark plug wire end should be  stamped on its metal schroud or check it with an ohm meter. .