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#3197047 - 11/24/13 08:23 PM Spark plug gap
tayto Offline


Registered: 01/22/13
Posts: 5
Loc: Vancouver, BC
Hey guys, about to do a tune up on my new-to-me '97 Subaru Legacy 2.2L. I have pretty much always owned North American, RWD, V8 cars/trucks and have recently made the switch to imports. On all my vehicles in the past, as well as other family/friends north american cars, I have usually found I got more power and even better economy in gapping the plugs .005-.010" over what factory recommends. When I use to work in the automotive world on the bench you would get some call backs after a tune up. Usually the customer reported of worse economy than before the tune up or not as much power. The mechanic I worked with would increase the gap .005-.010" over factory and everyone would be happy. It was explained to me that the factory undergaps, to get longer service intervals out of spark plugs. I know I would usually regap my own plugs halfway through the service interval. I did a full tune up to a friends '94 Jeep Cherokee 4.0L last summer and gapped plugs @ factory gap. Friend was happy enough better milage, power, etc. A month or so later, I had to repair a broken MAP plug that was being held on with a ziptie. While doing that I also regapped everything .010" over. He never knew I regapped the plugs and about a week later got a call about how much power it had picked up.

Can anyone else back this up? Is it something I just need to play with and record my milage? I know with a higher compression ratio you usually want a smaller gap? Is there any good rule of thumb for determining gap based on compression ratio, coil output, wire size, etc. If given a factory "range" of spark plug gap (ie: .039-.044") should I go with the lower or higher?

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#3197064 - 11/24/13 08:40 PM Re: Spark plug gap [Re: tayto]
eljefino Offline


Registered: 06/15/03
Posts: 24664
Loc: ME
Sounds good on paper. I wonder about losing the "knife edge" on the electrode and/or tip as it wears, though and its effect.

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#3197073 - 11/24/13 08:53 PM Re: Spark plug gap [Re: tayto]
Highline9 Offline


Registered: 08/24/11
Posts: 498
Loc: ca.
I will give it a shot on the elantra. Plugs are easy to get to. Have a set of ngk v powers in there.
_________________________
2010 challenger R\T
2006 jeep commander
2003 hyundai elantra
1992 dodge w250 cummins

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#3197123 - 11/24/13 09:48 PM Re: Spark plug gap [Re: tayto]
Blue_Angel Offline


Registered: 07/24/13
Posts: 111
Loc: Ottawa, Ontario, Canada
This has become a BIG deal on the Cruzetalk forum (Chevy Cruze).

Running the gaps a little wider than spec makes a BIG difference in the low RPM drivability of the turbo 1.4L engine on the Cruze. Factory spec is up to 0.028", but we are seeing great results pushing the gaps as high as .035". Low RPM torque is improved and the engine is noticeably smoother at idle, especially with the AC running.

Another large improvement with this motor comes from switching out the factory iridium plugs to old school "copper" plugs (actually just ordinary nickel steel tips). Running a standard NGK V-Power BKR7RE plug at the wider gaps makes another large improvement over re-gaping the factory plugs. I can lug my little 1.4 down to 900 RPM under load going up gradual inclines in 6th gear without protest, where the same car with smaller gaps on the stock plugs would shudder in protest and choke, forcing a downshift.

As with anything, YMMV. This seems to be the case for this engine but may not for others. Apparently turbo motors don't like iridium plugs... I find this "logic" hard to subscribe to and there seems to be a lot of non-factual "evidence" floating around. As much as my head told me there would be no advantage to changing out my plugs, after seeing so many positive reviews of the mod and it's low cost of about $10 I decided to try it out and I'm a believer, though I can't understand why there's a difference.

I don't expect everyone to read this and accept it... I sure didn't before trying it for myself. All I can say is in this case it works, and YMMV. I don't work for NGK, and some have shown similar results with standard Autolite plugs as well. The difference seems to be in the tip material.

Anyway, back to plug gaps, yes they do affect how the engine runs. I would absolutely believe that the OEM specs err on the smaller side as to get extra mileage out of the plugs as the gaps widen with use, however with iridium plugs as factory equipment and their slow wear rates I'm not sure why they would still be doing so...

For the record I've made every reasonable attempt to quantify a mileage difference between plugs and gaps. I couldn't find ANY repeatable measurable difference in my real world testing, though I was hopeful I would. At best I can say the wider gaps and V-Power plugs allow driving in a more fuel efficient manner since keeping revs lower is now easier than before. On level road two-way testing at various speeds I observed no difference. Obviously, those with automatic transmissions are likely to see no gains as the car decides when to shift and doesn't allow lugging the engine to low RPMs to begin with.
_________________________
'02 Chev Corvette ZO6: TGMO 0W-20/M1 0W-40, 6:1, HTHSV 2.72, VI 212
'12 Chev Cruze Eco 1.4T MT: M1 0W-30 AFE

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#3197242 - 11/25/13 06:07 AM Re: Spark plug gap [Re: Blue_Angel]
jorton Offline


Registered: 07/04/03
Posts: 2784
Loc: San Antonio, TX
Originally Posted By: Blue_Angel
and some have shown similar results with standard Autolite plugs as well


Folks on Ford Focus forums are running Autolite standard plugs gapped to .004 - .009 over spec, so I guess the wider gap doesn't damage ignition systems. I think most have cars with 5 speed manual transmission. Tune up intervals vary from 7000 miles to 20,000 miles, which is a little short for me.
_________________________
"If everything seems under control, you're just not going fast enough."
- Mario Andretti



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#3199285 - 11/27/13 12:47 AM Re: Spark plug gap [Re: Highline9]
Highline9 Offline


Registered: 08/24/11
Posts: 498
Loc: ca.
Originally Posted By: Highline9
I will give it a shot on the elantra. Plugs are easy to get to. Have a set of ngk v powers in there.


gapped to .055 seems to run responsive.
_________________________
2010 challenger R\T
2006 jeep commander
2003 hyundai elantra
1992 dodge w250 cummins

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#3199294 - 11/27/13 01:41 AM Re: Spark plug gap [Re: Blue_Angel]
Clevy Offline


Registered: 11/11/10
Posts: 7912
Loc: Saskatoon canada
Originally Posted By: eljefino
Sounds good on paper. I wonder about losing the "knife edge" on the electrode and/or tip as it wears, though and its effect.
Originally Posted By: Blue_Angel
This has become a BIG deal on the Cruzetalk forum (Chevy Cruze).

Running the gaps a little wider than spec makes a BIG difference in the low RPM drivability of the turbo 1.4L engine on the Cruze. Factory spec is up to 0.028", but we are seeing great results pushing the gaps as high as .035". Low RPM torque is improved and the engine is noticeably smoother at idle, especially with the AC running.

Another large improvement with this motor comes from switching out the factory iridium plugs to old school "copper" plugs (actually just ordinary nickel steel tips). Running a standard NGK V-Power BKR7RE plug at the wider gaps makes another large improvement over re-gaping the factory plugs. I can lug my little 1.4 down to 900 RPM under load going up gradual inclines in 6th gear without protest, where the same car with smaller gaps on the stock plugs would shudder in protest and choke, forcing a downshift.

As with anything, YMMV. This seems to be the case for this engine but may not for others. Apparently turbo motors don't like iridium plugs... I find this "logic" hard to subscribe to and there seems to be a lot of non-factual "evidence" floating around. As much as my head told me there would be no advantage to changing out my plugs, after seeing so many positive reviews of the mod and it's low cost of about $10 I decided to try it out and I'm a believer, though I can't understand why there's a difference.

I don't expect everyone to read this and accept it... I sure didn't before trying it for myself. All I can say is in this case it works, and YMMV. I don't work for NGK, and some have shown similar results with standard Autolite plugs as well. The difference seems to be in the tip material.

Anyway, back to plug gaps, yes they do affect how the engine runs. I would absolutely believe that the OEM specs err on the smaller side as to get extra mileage out of the plugs as the gaps widen with use, however with iridium plugs as factory equipment and their slow wear rates I'm not sure why they would still be doing so...

For the record I've made every reasonable attempt to quantify a mileage difference between plugs and gaps. I couldn't find ANY repeatable measurable difference in my real world testing, though I was hopeful I would. At best I can say the wider gaps and V-Power plugs allow driving in a more fuel efficient manner since keeping revs lower is now easier than before. On level road two-way testing at various speeds I observed no difference. Obviously, those with automatic transmissions are likely to see no gains as the car decides when to shift and doesn't allow lugging the engine to low RPMs to begin with.



Wider gap creates a larger sparks and less shrouded. So yes what your feeling in the buttdyno isn't placebo.
Here's the thing. You want the largest gap possible while not being so wide that the spark doesn't bridge the gap.
If an engine tends to be driven at higher rpm the velocity of airflow may impede the ability to bridge the gap,boosted engines also can potentially have an issue so with engines exposed to these conditions smaller gaps increase the reliability of the spark event.
Wider gaps may also shorten service life. Over time plug gaps do shrink while in service too.
So as time passes long life plugs gaps shrink which may cause a slight reduction in mileage and we either don't notice or just accept the condition assuming that "the ole girls gittin tired"
Up until last summer I bought the cheapest spark plugs on the shelf. I wasn't buying the longer lasting feature because to be honest I wanted to see the condition of my plugs. The plugs are a fantastic piece of hardware because of everything they can tell me about each cylinder as well as expose problems with fuel,ignition,ring seal as well as a/f ratio and other possible problems.
So I didn't want long life plugs,nor did I want to pay more for the feature. I also believed a spark was a spark and that wall at the parts house with every possible variation/gimmick was nothing more than a well planned method of separating fools from their money.
However I've had a few experiences lately that has made me re-think my long held belief that plugs were plugs however I'm not interested in opening that can of worms. Those conversations end up similar to the "which is the best oil".
If anyone is interested just pm me.
Back to plug gaps and plugs in general today's high energy ignitions have enough juice to bridge larger than spec gaps and there is a trade off. Wider gaps become less likely to fire consistently and reliably as the engine rpm increases and may be noticeable if the engine operates around the rev limit and high load.
Again this possibility is a non issue for most drivers since most people only use around 60% of the powerband. Those types of running conditions may benefit from a wider gap since the consequences are felt at rpm the engine will never operate at.
Cool topic guys.
_________________________
2006 Charger RT
Miles x 2 per oil filter

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#3203966 - 12/02/13 01:44 PM Re: Spark plug gap [Re: tayto]
PandaBear Offline


Registered: 08/05/02
Posts: 12555
Loc: Silicon Valley
The sharpness of the plug edge makes a huge difference in how much voltage does it cause to spark, as well as the gap, as well as the electrode diameter.

So for a car that's sensitive, you can increase the gap and buy fresh plugs that have sharp edges or narrow electrode, and replace them often before they are worn out to round edges or wider gaps.

Just make sure your car is not calibrated to a specific plug model for any misfire measurement or computer control, or you make see codes left and right by playing around.
_________________________
"You keep asking questions PandaBear and you'll end up a vegetarian like my wife" - Camu Mahubah

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