Bosch W7DTC Plugs Fouling

[Mark]

Been having issues with the plugs fouling on the Vitesse running 1.75 Sprint SU's carbs. Using the car as my daily instead of occasionally, to give the Gt6 a rest since mothballing my Mk1 Eunos, (ULEZ). Prior to the Bosch plugs I had been using NGK BP5E, never noticed a problem but I wasn't doing the mileage I am now. Changed leads, cap, condenser etc. Carbs are in great condition, new float needle valves, and set up using Gunson Colourtune and gas analyzer balanced etc. Car ran well, but would gradually develop a miss, and the plugs would begin to foul up. Reverted back to the NGK plugs for a few weeks and checked them the other day and they seem to be burning ok, going in the right direction colour wise.

Read good things on here about the Bosch plugs, and think I may need to revise my ignition system to get the benifit of the plugs.  Currently standard coil etc, so may not have enough oomph to work with these plugs. Any thoughts?

Edited April 27, 2024 by Mark

[2.5piman]

Hello Mark,

I have no experience using this type of multi electrode plugs, but it seems that you effectively triple the gap of the single lectrode and hence require a lot more voltage at the plug?

I am old enough to remember (1960s) 'Golden Lodge' plugs that were announced with huge fanfare for being so wonderful. Many fitted them and suffered misfiing etc and sunsequently binned. They had triple electrodes also.

Alec

[RedRooster]

Voltage will find the easiest path to earth, those plugs don't triple the gap. 

[JohnD]

Multi point plugs are intended to prolong their life.

Spark uses the least resistive route as said above, those points wear, the gap gets bigger and thenext one is 'chosen'   

No ignition advantage.

John

[Nick Jones]

As said before, the triple electrode plug design is primarily to extend life. Only one gap jumped at a time, takes easiest path each time, wear spread three ways. 
 

These come with a larger gap than would be normal for points ignition. They can be adjusted but not as easy as a conventional plug and really need a tubular gap gauge.

However, I suspect the actual issue here is heat range. BP5ES is a reasonably hot plug (and NGK are known for having a good range tolerance) I suspect the Bosch plug is colder (can’t find a decent comparison chart off the cuff) and this is why they are more susceptible to fouling.  Not had this problem myself even on non-injected cars), but I don’t do much town/city work.

Incidently, my theory on why the Triumph engines seem to like them is that they are side-electrode plugs and the spark is exposed to the combustion chamber, not shrouded by the earth electrode.

[PaulAA]

52 minutes ago, Nick Jones said:

However, I suspect the actual issue here is heat range. BP5ES is a reasonably hot plug (and NGK are known for having a good range tolerance) I suspect the Bosch plug is colder (can’t find a decent comparison chart off the cuff) and this is why they are more susceptible to fouling.

Purely anecdotally and without data support, I'd agree with this. There's no question the multi-electrode plug suits the Triumph engine, but short runs/town driving do increase the fouling considerably, to the extent that I have to pull the plugs and clean them if there hasn't been a recent 'burner' run.

[Mark]

32 minutes ago, PaulAA said:

but short runs/town driving do increase the fouling considerably

Well, that reflects my usage of late. Starting up on choke, and then maybe a few miles round trip dosen't give the plugs much chance. Once cleaned up the Bosch plugs run well.

 

[2.5piman]

Hello Red Rooster,

I find that hard to accept, the gap will, certainly from new, be equal and hence the easiest path is not just to one electrode. It makes no sense for a plug manufacturer to design a multi electrode plug so that only one fires at a time?

Alec

 

[DeTRacted]

It's electrically impossible for it to do anything else Alec. The electrical breakdown must occur at just one 'weakest' point. That may not be the same electrode for every subsequent firing though as it depends on a number of variables including local air/fuel mixture, electron space-cloud and the presence or absence of carbon deposits all of which will influence ionisation. 

Edited April 28, 2024 by DeTRacted

[Nick Jones]

^^^Exactly this.

It was the main way of extending plug life/re-gap intervals before the special electrode materials and pointy tips came along. The latter have somewhat taken over from the multiple electrode these days, but I don’t find they work as well.

Heat range comments apply regardless of electrode design.

[JohnD]

Interesting you mention needle tipped electrodes, Nick.  That's what SofS uses.

Haven't looked at them.since Saturday.  I'll report how they're doing.

John