Bosch spark plugs: WR7DC vs W7DC

[GGR]

The other day I ordered some spark plugs for my 3.5 with transistorized ignition. I got some Bosch WR7DC instead of the original W7DC. What does that "R" stand for? Is this what they call integrated resistor plugs? Is it OK with the transistorized ignition or do I risk to fry something? The car works OK, even if it seems to be a bit less responsive at mid and higher ranges. But I don't know if this is the cause as I worked on several things at the same time.

[Mike D]

Yup, the "R" is for resistor. I don't think you'll fry anything but you are not getting a "hot" spark. Could definitely be causing the lag under load.

Try the NGK's, BP6ES's or whatever number they've changed to for a better spark.

[::matthew]

I thought the resistors were to suppress ignition "noise" in the electrical system. Keep the radio from sounding funny. How does it affect the performance of the spark?

[Mike D]

The resistance in the ignition plug circuit is indeed designed to eliminate the electrical noise for electronics. Kind of like using a nozzle on a garden hose to restrict water flow.

The wires already have a resistor built into the ends. You are doubling the resistance in the circuit. It's like using two nozzles in a row with the first one already at the "half closed" position.

Here's a simplified version of DC electricity as explained to me years ago.

Electrical current is a river. It flows from high to low. When you put a dam in the river (resistance) it decreases the flow. The holding pond behind the dam creates a potential force (capacitor) which is released through the flood gates (transistors) to power whatever it is you're powering.

When you are using your starter motor think of the ignition key as being a mechanical transistor to create the flow.

[wolf_walker]

It's getting hard to find some flavors of non-resistor plug. I think the mentioned NGK line is still available, seems like I had trouble finding the Bosch non-R's for my M110 in the heat range I needed awhile back. Had some way old stock at work luckily.

The more spendy alternative is not having resistors elsewhere in the ignition circuit, assuming someone makes, or one can make themselves, plug wires as such. They you can freely use more common resistor plugs.

[JMela]

Yeah, do you guys have sources for the Bosch W7's? I switched over to NGK's but used Bosch until the supply dried up.

[Graham]

Originally Posted by GGR

The other day I ordered some spark plugs for my 3.5 with transistorized ignition. I got some Bosch WR7DC instead of the original W7DC. What does that "R" stand for? Is this what they call integrated resistor plugs? Is it OK with the transistorized ignition or do I risk to fry something? The car works OK, even if it seems to be a bit less responsive at mid and higher ranges. But I don't know if this is the cause as I worked on several things at the same time.

The R means resistor plugs. It is getting hard to find non-resistor plugs these days. Bosch have apparently discontinued them and perhaps NGK too.

But Bosch have also changed the plug wires. They used to have resistors embedded in the ends of the plug wires. Now they do not.

I have been running with Bosch or NGK resistor plugs and new style Bosch wires for years now with no problems and good performance. I have a Pertronix installed, but still use the original MB coil and switchgear.

My old original wires with the internal resistor measured 8 kohms. The new wires measured 1.6 kohms (Bosch Premium opti-layer mag, part 09027). Old plugs were W7DC and had low resistance - about 5 ohms. I don't have resistance of R plugs handy.

There has been a lot of discussion on this subject in the past! Many opinions. But in end, it seems to me:
- if you have OE style wires with built in resistors, then use non resistor plugs.
- if you have new style Bosch wires without resistors, then use resistor plugs.

Those who have had problems with resistor plugs may have been using them with the high resistance plug wires.

[280SE Guy]

Originally Posted by JMela

Yeah, do you guys have sources for the Bosch W7's? I switched over to NGK's but used Bosch until the supply dried up.

Bought 6 from my local Mercedes Benz Dealer last summer part number 003-159-12-03 for $4.20 each with my MBCA discount. Thought I should order a spare set but they said there were over 25,000 in stock in the USA.

[GGR]

OK, I checked my plug wires and they are written "Bosch 7 mm opyi-layer copper core ultra premium" on them. The ends on both sides are screwed inside the wire. I put my multimeter on the "Rx100" position and I read "20" when I measured resistance on two different wires, including the ends. I guess this means 2 kohms and from what Graham wrote they are more on the low resistance side. So, I should be OK with the resistor plugs I put in?

(Worth to note I put that spare set of wires I had very little before I changed the plugs because the car was misfiring and the other wires looked old and cracked. Didn't cure the problem so I went on changing the plugs and contacts which cured the problem. So it may well be that my other wires were high resistance with no resistor plugs)

[Graham]

Originally Posted by GGR

OK, I checked my plug wires and they are written "Bosch 7 mm opyi-layer copper core ultra premium" on them. The ends on both sides are screwed inside the wire. I put my multimeter on the "Rx100" position and I read "20" when I measured resistance on two different wires, including the ends. I guess this means 2 kohms and from what Graham wrote they are more on the low resistance side. So, I should be OK with the resistor plugs I put in?

(Worth to note I put that spare set of wires I had very little before I changed the plugs because the car was misfiring and the other wires looked old and cracked. Didn't cure the problem so I went on changing the plugs and contacts which cured the problem. So it may well be that my other wires were high resistance with no resistor plugs)

Yes - those are the same wires I have and use with the resistor plugs. I have used Bosch & NGK resistor plugs with them.

Another thought about plugs & wires. If we switch to a Pertronix Flamethrower or other high voltage coil, how would that affect the car? Could it perhaps burn rotor/distributor cap contacts quicker?

[Mike D]

Originally Posted by Graham

Yes - those are the same wires I have and use with the resistor plugs. I have used Bosch & NGK resistor plugs with them.

Another thought about plugs & wires. If we switch to a Pertronix Flamethrower or other high voltage coil, how would that affect the car? Could it perhaps burn rotor/distributor cap contacts quicker?

I suppose with both resistor wires and plugs it could. I imagine the difference would be fairly minor since we're dealing with voltage as opposed to amperage. I'd bet running a set of fouled plugs would create more damage to the cap/rotor contacts.

[mak]

I have been using Beru 4 electrode plugs in my 103 euro engine and they deliver a brisk performance .The original recommended plugs are good but with them #2 always fouls up ,maybe a little worn seal or guide .
The Beru's simply burn through maybe due to the 4 electrodes, resistance is 2.5K ,chrome plated and euro specs.,Made in France.
mak
300se 103
300sel 3.5 109

[GGR]

I drove a few days with both low resistance wires and non resistor plugs and the radio was not doing any funny noises. Is no resistance anywhere supposed to give a better spark and improve performance?

[winecountryone]

I don't have the crossover numbers from bosch to NGK. If you know which ones you need try motorcycle repair shops.A lot of the 70's-80's models used stranded copper plug wires with resistor end caps like our vintage benz and non resistor plugs.
We have a chain called cycle gear here in California with numerous locations.
Online retailers like bike bandit or dennis kirk probably have them in stock as well.

[mak]

Originally Posted by GGR

I drove a few days with both low resistance wires and non resistor plugs and the radio was not doing any funny noises. Is no resistance anywhere supposed to give a better spark and improve performance?

the correct combination would be low resistance wires and resistor plugs ,the standard norm now .
vintage was stranded ignition wires and resistance end caps to keep ignition chatter down.
Resistance wires and resistance plugs would pull down performance and starting issues may arise.
mak