Gap iridium plugs?

[NickCLotus]

Any thoughts on whether to gap Iridium tipped plugs or not. From what I have read they should not be gapped but default gap is 0.7mm whereas manufacturer recommends a gap of 0.9mm, is this likely to make any difference?

[E-bmw]

If it were me & the plugs are the correct type as recommended by the manufacturer, I would gap them.

If they were a "compatible" plug from a different manufacturer, then maybe not.

Having said that I don't really have a reason for the second line.

[GreenV8S]

If it was a big difference I'd regap them, but use a wire loop style gauge not the wedge style to avoid damaging the tip. For the small difference you quoted I'd only bother if the new plugs introduced running problems.

[Panamax]

Well, maybe I'm out of line but my recollection is "correct gap" on spark plugs is/was all about getting a decent spark to ignite the mixture. Back in the day ignition systems could be pretty weedy in terms of HT voltage so if the gap was too big you risked not getting a clean spark. These days with electronic ignition, individual coil packs etc there's probably enough voltage to jump just about anything.

Buy plug, stick in hole, job done.

[Panamax]

Following that last post I've just had a click around the internet. It looks like,

• Old fashioned ignitions systems - 15,000 to 20,000 volts. (see below)
• Modern ignitions systems - 30,000 to 45,000 volts.

It appears that in normal open air conditions a spark from 30,000 volts can jump as far as 3 centimetres. Things are obviously a bit different in a combustion chamber.

Some old coils would have ballast resistor installed. In summary, the input to the coil would be 12v during cranking for a big fat spark (although voltage tends to drop while cranking) and drop to around 8v during normal running. If the input voltage stayed at 12v the coil might burn out or the spark plug tip could get burnt away. Iridium is very tough and highly conductive.

[paintman]

Panamax said:

Well, maybe I'm out of line but my recollection is "correct gap" on spark plugs is/was all about getting a decent spark to ignite the mixture. Back in the day ignition systems could be pretty weedy in terms of HT voltage so if the gap was too big you risked not getting a clean spark. These days with electronic ignition, individual coil packs etc there's probably enough voltage to jump just about anything.

Buy plug, stick in hole, job done.

Always best to check there is a gap!

Competition on TV some years ago, trainee mechanics competing.
Included changing plugs.
Plugs BNIB but the judges had closed the gap completely on one of them for each competitor & repackaged it.
Sort of thing that could happen if someone has unpacked a plug, dropped it & put it back in the packet - never had on like that personally but I always look!
All the lads bar one checked the plugs.
The one that didn't spent the rest of the allotted time trying to figure out why the car had suddenly developed a misfire. Never occurred to him to pull the plugs.

[Bainbridge]

paintman said:

Always best to check there is a gap!

Competition on TV some years ago, trainee mechanics competing.
Included changing plugs.
Plugs BNIB but the judges had closed the gap completely on one of them for each competitor & repackaged it.
Sort of thing that could happen if someone has unpacked a plug, dropped it & put it back in the packet - never had on like that personally but I always look!
All the lads bar one checked the plugs.
The one that didn't spent the rest of the allotted time trying to figure out why the car had suddenly developed a misfire. Never occurred to him to pull the plugs.

I too check the gap when changing plugs, having seen large variances on new plugs in the past.

[NickCLotus]

GreenV8S said:

If it was a big difference I'd regap them, but use a wire loop style gauge not the wedge style to avoid damaging the tip. For the small difference you quoted I'd only bother if the new plugs introduced running problems.

Well they do seem to have caused running problems, rough idle and hesitation/fluttering at low throttle openings. Reverting to the old plugs seems to solve the problems so it seems in this case perhaps the 0.7mm gap is too small and needs to go to the manufacturers recomendation of 0.9mm.

Edited by NickCLotus on Wednesday 8th May 17:26

Edited by NickCLotus on Wednesday 8th May 18:02

[Panamax]

So long as you've got enough HT voltage to jump the gap a bigger gap should mean a bigger spark igniting the mixture.

Some may recall the Alfa Romeo "twin spark" engines. Two plugs in each cylinder for better spark ignition. (Not to be confused with aircraft practice where two completely separate ignition systems operate simultaneously for safety reasons.)

https://en.wikipedia.org/wiki/Alfa_Romeo_Twin_Spar...

https://en.wikipedia.org/wiki/Dual_ignition

[NickCLotus]

Panamax said:

So long as you've got enough HT voltage to jump the gap a bigger gap should mean a bigger spark igniting the mixture.

Some may recall the Alfa Romeo "twin spark" engines. Two plugs in each cylinder for better spark ignition. (Not to be confused with aircraft practice where two completely separate ignition systems operate simultaneously for safety reasons.)
...

Bearing in mind that the smaller gap did not work well, might be worth opening the gap out to say 1mm or even more as these are Iridium tipped plugs and so not likely to widen out any during their lifetime.

[Scarletpimpofnel]

Panamax said:

Well, maybe I'm out of line but my recollection is "correct gap" on spark plugs is/was all about getting a decent spark to ignite the mixture. Back in the day ignition systems could be pretty weedy in terms of HT voltage so if the gap was too big you risked not getting a clean spark. These days with electronic ignition, individual coil packs etc there's probably enough voltage to jump just about anything.

Buy plug, stick in hole, job done.

I'd agree with this. Partner has a Fiesta, the original spec was to have a 1mm gap but it seems over the years Ford have changed this to 0.8mm. The new Iridium plugs came gapped at 0.8mm and starting etc is noticeably crisper. Apparently the old 1mm gap put too much strain on the HT side of things. So I'd buy the correct plugs for the car and slap them in.

[richhead]

NickCLotus said:

Bearing in mind that the smaller gap did not work well, might be worth opening the gap out to say 1mm or even more as these are Iridium tipped plugs and so not likely to widen out any during their lifetime.

just be very careful of the iridium tip, very easy to damage, and you will find on a wasted spark system as most are half the plugs will widen the gap over time and half will decrease, not by much tho.

[NickCLotus]

New iridium tipped plugs now refitted but gapped to 0.925mm, running much better. Looks like this was the cause of the problem all along.