Spark plug gap - Jeep Commander Forums: Jeep Commander Forum
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post #1 of 21 (permalink) Old 03-18-2016, 05:37 PM Thread Starter
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Question Spark plug gap

I am looking to change out the spark plugs on the XK this W/E and have bought 16 Denso 4703#4's. When I got the receipt it said to gap at .044, though the ngk's that would be stock in a 2010 5.7 should be gaped at .040.
The web info on the 4703#4's said to gap at .040, my thought is that the plugs no matter the brand should be gaped the same so as to fire at the correct time as is needed?The receipt is from the parts store.


Swanny
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post #2 of 21 (permalink) Old 03-18-2016, 07:03 PM
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The different internal resistance of various spark plugs will actually fire at different times depending on the electrode design and materials used in the plug. It takes a buildup of of energy before the spark cam jump the gap of the plug. Some plugs have a lower resistance so the energy requirement even with a larger gap will allow the spark to form with less energy. I would go with the plug manufacturers recommendation. The 5.7 liter engine's ignition is very sensitive to the plugs resistance value. Though apparently the same part number as the stock Champion brand some plugs from a parts house do not match this value and some have had problems with stalling and misfiring from aftermarket plugs. there is a post on this sight about this somewhere - it depends if the plug top cap is black or brass colored as to which plug works correctly. I use Denso Iridium plugs in my Hemi 5.7 and have had them for 35K miles without issue. The Denso with the fine wire electrode has less resistance than the stock plugs and works fine. it is interesting that Iridium is actually a worse conductor of electricity than copper or steel but it erodes less and lasts a long time. The way Iridium plugs can fire with less resistance is to make the electrode very fine so that is why Iridium plugs have such a small electrode as compared to a standard plug. The finer small electrode is also exposed to the combustion chamber a little better with less shadowing of the plug so there is less tendency for misfiring and a slightly better flame kernel produced from a fine wire plug though probably not noticeable in a stock engine.

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post #3 of 21 (permalink) Old 03-18-2016, 07:06 PM
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It sounds as if you have the Hemi and from what I have read on this site folks who have not put in the dealer OEM plugs have had problems, perhaps some 5.7 folks will chime in.
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post #4 of 21 (permalink) Old 03-18-2016, 07:19 PM
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Other plugs can be used in the Hemi as long as the resistance of the plug is the same or less than the stock plug. The problem was occurring because for some reason Champion plugs distributed through aftermarket parts houses had much higher resistance than the same part number plugs sold through Chrysler dealerships that were supplied directly from Chrysler. I can't figure out how that would happen but it did. Denso iridium plugs work fine in the Hemi.

Last edited by bdalameda; 03-18-2016 at 07:21 PM.
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post #5 of 21 (permalink) Old 03-19-2016, 08:44 AM
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Good to know
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post #6 of 21 (permalink) Old 03-19-2016, 02:43 PM Thread Starter
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GearWrench 41740D

Well I finished installing the plugs in my 5.7, all went well but not in record time but no problems, thanks to the socket set I bought for this job it has 3 different size extensions that are fixed to the socket and the socket has a magnet to hold the plug on removal and installation, worked great.

Swanny
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post #7 of 21 (permalink) Old 03-21-2016, 08:41 AM
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New ignition systems are very precise and some use wasted spark DIS which has a reverse current flow on half the plugs. If two plugs share a coil, in a distributor-less ignition system, the coil is usually wired in series in between the two plugs, pulling up current from ground from one plug (flowing current in reverse) and pushing down current to ground from the other plug (flowing current in the conventional way) and ground completes the circuit between the plugs.

Plugs from different manufacturer will vary and they have several different models of plugs with different features. Like a single platinum plug designed only to work with current moving in the intended direction, so that the plug lasts much longer, but use it with reverse current and it will erode away much faster than a standard plug. That is why there is also double platinum (platinum on both sides of the arc for these ignitions).

So people use the wrong plug, for one of many reasons, despite it being one of the many different "standard sizes" that should fit your engine, one of its features or how it varies from other manufacturer's resistance, quench area, etc, make for poor performance. So some plugs work well in certain engines and others work poorly.

We say use the Original Plug, by manufacturer and part number, the engine was designed to use. I "suspect" more than one brand/model plug will work fine, but there are more than a few that won't work well at all, to avoid a problem, its recommended to only go with what has proven to work, and we all know the original plug by manufacturer and model has proven to work since it comes with it off the assembly line.

As mentioned, Gap affects the resistance of the overall circuit and gap, shape of the electrodes, open/closed area around the spark, also effects the effective ignition of the air/fuel, which can be very complicated. I'd also wonder when a manufacturer recommends a different gap than the engine manufacturer, is it done from their own personal testing? OR are they just recommending a generic gap regardless of the actual engine. Keep in mind, the whole ignition creates a circuit, spark plug cables often degrade and their resistance goes up, depending on the type of the cable, resistive or suppressive (inductive), they eventually need to be replaced with new to get back to the original overall resistance.

Finally, over the years, Chrysler has switched back and forth from a coil for every plug to one coil for two plugs, the wasted spark with reverse current flow. Commanders have had both systems over the years. So what might work for one Commander owner, may NOT work at all for another Commander owner because their ignition systems are different.

Personally, I just stick with the OEM brand/model plugs and use the engine manufacturer gap recommendations, and avoid the ambiguity. That's NOT to say some other brands/models of plugs may work just as well, but you may pick a plug that doesn't work as well.

Last edited by Mongo; 03-21-2016 at 02:55 PM.
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post #8 of 21 (permalink) Old 03-21-2016, 05:49 PM Thread Starter
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Thanks Mongo, however there is a 3rd force working here that sometimes wrongly interjects, that force being my wife of 25 yrs. and that will be on the 13th of April when we ARE GOING on our 2nd Honeymoon and that will be a trip of some 2,500 miles so I thought I would change plugs as I didn't know the last time they were changed (only owned the Jeep 1 1/2 Mo's.), working on a time crunch the auto parts store only had 1 brand in stock, bought it did it now I'm not afraid of it and if there is a difference in performance I will deal with that when we return, as of now the gas mileage went up 2 mpg.


Film at eleven

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post #9 of 21 (permalink) Old 03-21-2016, 06:31 PM
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You all just GO ENJOY yourselves Swanny.
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post #10 of 21 (permalink) Old 03-22-2016, 10:31 AM
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Bdalameda, testified that he has used the plugs you bought in the same engine you have without problem, so good bet you won't have any problem either.

The same is going around about the O2 sensors, you have to use the OEM O2 sensors or you will have problems. I just replaced my O2 sensors with NGK OE sensors, they are OE supplier and claim its the OE model, they appear identical, but its NOT from the Dealer, which means its an aftermarket part and it may differ from the actual OEM because it doesn't have to meet the same specs and quality checks.

I tend to think the O2 sensor complaints may come from the BOSCH universal O2 sensors, I've heard complaints about them when used in all sorts of vehicles, which sorta makes sense, if they are what I think they are, one O2 sensor being marketed to replace several or more different O2 sensors.
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