Sparkplug gap and fuel economy

[Al's Red Wagon]

When I have put plugs in, in the past I trusted that they were pre-gapped as they said on the box...

When I put my new plugs in The other night and put them on my feeler gauge I found that even my .88mm feeler couldn't get through. I opened it up to 1.1mm.

However my fuel economy was quite low for the trip I just took. Only 20mpgs - usually I get 24-26 with a luggage carrier on top and a full load. The car is running fine, no CEL's or anything...

Would a gap that is too big cause this?

[saabcog]

I'd be willing to adjust mine to .88mm and see what it does to my MPG. I get 25mpg with 1.1mm gap at the moment. I'll run upstairs and re-gap mine and then post again at the end of the weekend with any results.

[Al's Red Wagon]

Awesome - Thanks...


FYI Aero spec is .9mm-1.0mm

[saabcog]

I took a drive to dinner and back with the new gaps. Right away I see a major difference; the car's idle is much quieter and smoother. I'm also happy to report that my butt-dyno says the performance has not decreased.

Even if this experiment doesn't show any gas mileage improvement, I think I'm sticking with the smaller gap for the improved idling.

Have you experienced a change in the car's idle with these larger gaps?

1.1mm


.88mm

[rolly]

Since I have only had mine for while and going through all the service items to give it a clean bill of health. I checked the plugs and they were at 1.2mm a bit due to wear no doubt, so I gapped them down to .9mm and straight away noticed better idle - cannot comment on the fuel milage as I have only filled up with my second tank of 98 when the PO only used 95, and it is on the decrease anyways(l/100km)

[blinktude]

ohh!
this could possibly be the solution to rough idle on our 4 cylinder models!
high five

[Al's Red Wagon]

Originally Posted by Al's Red Wagon

FYI Aero spec is .9mm-1.0mm

Any gear heads that explain why the Aero gap spec is a bit smaller than the lpt gap spec?

[turbophille]

Originally Posted by Al's Red Wagon

Any gear heads that explain why the Aero gap spec is a bit smaller than the lpt gap spec?

talking from experience (not with Saabs), a larger gapped plug can have its spark blown out under higher boost

[Raven18940]

Originally Posted by turbophille

talking from experience (not with Saabs), a larger gapped plug can have its spark blown out under higher boost

Exactly, the denser the fuel/air mixture, the harder it is the jump the gap.

[stipud]

Might also have something to do with using platinum plugs instead of copper.

[turbophille]

Originally Posted by Raven18940

Exactly, the denser the fuel/air mixture, the harder it is the jump the gap.

spot on mate. the lpt pushes 8psi whereas the aero pushes 15psi, thats a big difference! on lesser cars, if u doubled the boost you could then have to look at race coils, bigger leads, uprated ignition amps ect

[Al's Red Wagon]

So what about the Nordic/BSR folks who are at at 20+pressures with the lpt? I haven't heard anything about those people gapping their plugs smaller...

[Chris 9-5]

Originally Posted by Al's Red Wagon

So what about the Nordic/BSR folks who are at at 20+pressures with the lpt? I haven't heard anything about those people gapping their plugs smaller...

Thats true - I was told to run the NGK BCPR7ES with a 1.0mm gap.

But I dont think its called a LPT anymore

[Al's Red Wagon]

Originally Posted by Chris 9-5

Thats true - I was told to run the 7's with a 1.0mm gap.

But I dont think its called a LPT anymore

that's confusing 'cause that's upper limits of aero and lower limit of lpt...is that smaller than you were running before?

[Chris 9-5]

Originally Posted by Al's Red Wagon

that's confusing 'cause that's upper limits of aero and lower limit of lpt...is that smaller than you were running before?

I didnt really have a "before"

However I tended to use NGK BCPR7ES-11 (pre gapped at 1.1mm)

[turbophille]

Originally Posted by Chris 9-5

I didnt really have a "before"

However I tended to use NGK BCPR7ES-11 (pre gapped at 1.1mm)

thats the plugs i was given by my local parts store when i did my first service. a month ago i re-gapped to .9mm and car runs much smoother now

[saabcog]

Would anyone else be willing to narrow the gaps on their plugs and report back? I'd like to see more data sets.

The idle is smoother for sure. No change (up or down) in MPG so far, but it has only been a day.

Where I really want other people's impression is with the car's behavior at WOT when running smaller spark plug gaps. I don't want to say anything more until others try this out because I have no way of really measuring what I may be feeling and I know how easily the mind can be tricked when it comes to cars. (How is that for an abstruse statement.)

[Chris 9-5]

Originally Posted by saabcog

(How is that for an abstruse statement.)

That was so abstruse I had to look it up!

http://en.wiktionary.org/wiki/abstruse

[saabcog]

Originally Posted by Chris 9-5

That was so abstruse I had to look it up!

http://en.wiktionary.org/wiki/abstruse

Yeah, I'm usually obtuse.

[Al's Red Wagon]

Originally Posted by saabcog

Would anyone else be willing to narrow the gaps on their plugs and report back? I'd like to see more data sets.

The idle is smoother for sure. No change (up or down) in MPG so far, but it has only been a day.

Where I really want other people's impression is with the car's behavior at WOT when running smaller spark plug gaps. I don't want to say anything more until others try this out because I have no way of really measuring what I may be feeling and I know how easily the mind can be tricked when it comes to cars. (How is that for an abstruse statement.)

I'll be going down to .9-1.0 for the ride back on Monday....but my only real concern in MPGs...My wagon has never let me down at WOT (except for that mustang once... )