After 2 days have realized that my gas mileage has dropped prettyt bad, getting 19-22mpg (my average for the last two days was 19.0 on the street) when I was getting 27-28mpg and now I can't get over 28 at 65mph when I used to get 34mpg, higher when crusing on a slight downgrade.   Returning these plugs for the same plugs I just pulled out - NGK ILZKBR7A-8G with a .028 gap (same that was in there before).   The car runs better but gas mileage has suffered, not cool. 

Thanks, 

- Murf

(pssst... he's writing about a new MINI - I don't think they used Lucas ignition.)

In any case, I'm old school - I don't trust "pre-gapped" plugs and always verify gap. You never know how they've been (mis)handled between factory and you. 

Plug gaps-
1. I had Iridium plugs in my s/h 2001 4.0L Ford Falcon when bought. It ran fine but fuel economy was bad. When I checked, they were worn out to .080" gap.
I regapped them to .040", and economy came good. It was due for some new plugs, as I'm not rich I bought some NGK conventional ones. They are still in and running fine.

2. Watch plug gaps when buying for a Mini. Most sets in a blister pack now are gapped to .041". Too wide for stock Lucas ignition, so close em up before fitting.

Wow, thanks for the feedback.   I agree with finding the correct manufacture gap, just having trouble finding it online.   The car still does have a little pop, but probably down about 50%, even with the resonator removed. 

Cheers, 

Mike

I don't know about mew MINIs or what they should sound like, but to my thinking, the sound that comes out of the tailpipe is a result of engine peformance, or lack of it. Having "pop" come out of a tailpipe and then not, would suggest to me that the "pop" was a sign of below optimum engine tuning, especially if the engine seems to run better.

Spark plug basics:

• "Laser" is a marketing gimmic word
• "Iridium" is a mineral coating on or mineral content in spark plug electrodes that prolongs its useful life by reducing wear.
• The gap between spark plug electrodes is critical. Engine designers select a gap and spark plug that works best with the engine they are building (to their thinking at least). They also design the spark timing to optimize combustion.
• Spark plug heat range is also critical - also selected by the manufacturer.
• as an engine ages (wear, dirt etc.) its tuning characteristics change - a change in spark plug heat range might be needed, but spark plug gap shouldn't.
• adding "special" different plugs to the mix ususally just adds new parameteres (issues or problems) to the tuning equation - unless the tuner is very experienced in engine tuning and plug selection.
• the difference between 0.028" and 0.032" is only 0.004" - not very significant, unless the gao you choose is at the end (or beyond) of the optimim range. In the old days, spark plug gap spec was given as a range, typically 0.032" to 0.036" (still only 0.004" difference). The engine tuner would use a gap to suit his engine's particualr condition.
• new spark plugs are "supposed" to be pre-gapped. Don't trust 'em. Check and adjust.
• a spark plug gap that is too small does not make a big enough spark to ignite effectively.
• a spark plug gap that is too big produces a weaker spark that does not ignite effectively. 

My guess is your "pop/burble" was a result of incomplete combustion, making a sound you liked, but not optimum performance. Find out what gap is specified for your engine and use it. If you care about performance and fuel economy, have it tested with an analyzer or instruments. Your car does exhaust amaylsis continuously. If it determines your combustion is not optimal, it adjusts timing and air/fuel mixture to try to compensate. THis means it is "de-tuning" your engine, resulting in a drop in performance and possibly the added tailpipe noise. By changing spark plugs, you may have corrected a problem and the computer has restored some of the tuning.

Just swapped my spark plugs on my 2009 mini cooper s manual and seems I have lost a little "pop/burple" in my muffler.   However car does seem to run a little smoother.

Had - 

NGK (5992) R5 ILZKBR7A-8G Laser Iridium Spark Plug

Installed - 

Bosch ZR7SI332S Original Equipment Fine Wire Iridium Spark Plug @ .032 gap

Is the difference in gap affecting my "pop" or was I getting too much pop and now better gas mileage, cleaner ride?

Thoughts?   Should I adjust my new gap to .028 to match what was there before?

- Mike