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Autolite AP3923 w/ 0,035" Gap


ednowicki

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Based on advice from this forum..

 

I installed autolite AP3923 plugs with a 0.035" gap. I did this after a valve adj and TBS.

 

My 2003 1150RT now runs BETTER THAN EVER. No pinging, no surging, no backfire, no vibration, and SMOOTH responsive acceleration.

 

Without hesitation, I recommend this plug and gap. I've done valve adj and tbs before (with improved performance), but nothing like now.

 

 

Ed

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The gap is what did it, not the plug. A wider gap results in a greater voltage needed to fire the plug (any plug), and this results in greater energy in the gap per firing, thus it better ignites lean-ish mixtures that cause surging.

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The gap is what did it, not the plug. A wider gap results in a greater voltage needed to fire the plug (any plug), and this results in greater energy in the gap per firing, thus it better ignites lean-ish mixtures that cause surging.

 

OK, why not go to .040" gap, which is the widest recommended in my '94 RSL owner's manual?

 

This afternoon I regapped my AL 3923 plugs to 0.40" and it ran better than before, although I'm not sure what the before gap was, other than being whatever came out of the box. Anyhow, very little surging before, but on this test run at 80 mph I was looking at other issues than surging.

 

Again, why not go to 0.40"?

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Again, why not go to 0.40"?

If the electricals (the coils and electronic ignition) is the same as that on the older bike, then no problem at all.

 

The only possible danger is the coil insulation or the electronics that drives is may not be designed for the increased voltage.

 

Basically, the voltage in the system rises only as high as is needed to break down the air gap in the plug, then it drops to a much lower value as current flows across the spark. The greater the gap, the higher the voltage rises before the the air in the gap ionizes and the spark occurs. With no plug at all (the spark lead is not connected) the secondary voltage can get high enough damage the coil, which is why you must never pull a lead off the plug when the motor is operating.

 

Anyway, I suspect that a 0.040" gap will cause no problem, so ...try it! a greater gap can allow more air'gas between the plug electrodes and may help the ignition process. Just remember that an increased voltage means less current, so increasing the gap too much is counterproductive.

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You da Man, Bob!

 

And, another few questions:

 

1. Suppose I were to replace the old leads on my R1100RSL with Magnecor wire, which can be bought by the foot. Any benefit or risk with that?

 

2. BTW, what plugs are you running in your K100RS? Is there any plug that helps the K-bike like the Autolite 3923 does the R-bike?

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OK, why not go to .040" gap, which is the widest recommended in my '94 RSL owner's manual?

 

This afternoon I regapped my AL 3923 plugs to 0.40" and it ran better than before, although I'm not sure what the before gap was, other than being whatever came out of the box. Anyhow, very little surging before, but on this test run at 80 mph I was looking at other issues than surging.

 

Again, why not go to 0.40"?

 

 

 

RSL, that is a good question.. There is a good possibility you could run .040” long term.. There are some positive reasons why it would work & other compelling reasons why it shouldn’t be done..

 

Let’s start with the spark plug itself.. Spark plugs come factory gapped to a specific gap range.. Not sure on the AL3923 but probably around .030” or so.. Opening the gap slightly from designed is OK but opening it too far places the ground electrode at a funny angle to the center electrode.. Basically when you bend the ground electrode to open the electrode gap up much past designed the ground electrode case attachment stays the same so the electrode will bend around that point.. You really don’t went to have the electrodes at an angle to each other as that causes angle war on the center electrode as it always fires off the same side.. (before I get flamed here I talk of ground electrode as the side electrode as everyone understands that,, on the BMW lost spark system we all know that one side sparks positive in relation to frame ground the other side sparks negative in relation to frame ground)..

 

Then you have the electrode wear factor.. That beginning gap WILL grow as the spark plug wears.. The factory recommended plug gap spec takes that into account.. If you start at .040” it could grow to .050” over the life of the spark plugs duty cycle..

 

Next comes the electrical side.. while it’s true the ign coil can produce more KV than the standard plug gap (even worn to the limits) calls for there are other factors..

 

_As the plug gap increases the KV to jump that gap also increases.. The coil has a basic limiting design that only allows it to supply a fixed current.. That current gets to the spark plug in both KV (high voltage) & spark duration length.. Seeing as the coil has a finite current offering as you increase the spark plug gap you also drive the KV firing voltage up that in turn lowers the spark duration length.. Usually not a problem if you get instant firing ignition in the cylinder but on a difficult to ignite mixtures you need both spark intensity & some fixed length duration to assure ignition.. It’s a trade off & a lot of testing probably went into the initial system here..

_Next you have the cold starting issue.. It takes a lot of spark KV to jump a .040” gap.. That might be just fine while cranking a hot engine in warm weather with a perfect battery.. Now how about cranking a cold engine with a low battery.. Under these conditions there might only be 8-10 volts available at the ignition coil.. What if the .040” plug gap requires 11 volts at the coil to achieve a decent starting spark (think it will start?).. Again I’m sure a lot of testing went into the original BMW design..

_Next you have the modern spark plug issue.. An awful lot of modern spark plugs don’t have the center porcelains glazed anymore due to modern fuel injection & electronic ignitions not requiring it.. It is a cost saver to the plug manufacturers.. On those non glazed plugs it only take a misfire or two on a cold start to allow the center porcelain to wet out.. Once that happens the spark that is building to jump the spark plug gap simply dissipates down that wet porcelain path.. Opening the spark plug gap increases the chance that will happen..

_Then you have the ignition system secondary insulation issue.. Such as ign coil insulation,, spark plug wire insulation,, spark plug upper porcelain length & plug boot sealing.. As you force the spark plug gap wider the chance that the spark will jump someplace else increases.. Remember that as the cylinder internal pressures increase that nice benign .040” gap could require much more spark to jump than a .040” gap on a plug laying out in the open.. Who knows if the ign coil itself can handle the constant output heat of sparking a .040” gap (that has grown to .045”) in hot stop & go traffic with the engine heat already topping out he RID.. Again I’m sure there was some initial testing on BMW’s part here..

_Next you increase the chance of a misfire at high RPM’s if you increase the spark plug electrode gap enough to push the coil to near it’s saturation point.. A different spark plug gap can change the ringing in the coil primary side..

_As a rule increasing the spark plug gap helps the idle & low RPM operation on a lean running engine.. So much so that early emission era automobiles tried to use a .060” plug gap.. That caused all kinds or problems from high RPM misfires,, to very difficult cold starting,, to damaged spark plug wires,, to back-arcing inside the ign coil,, to arc-tracing down the outer plug porcelains,, to forcing large diameter distributor caps to prevent arcing between plug towers,, to heat induced ign coil damage..

 

OK now to your original question> “OK, why not go to .040" gap”

 

You probably could.. How about .042” in a few thousand miles of wear?.. How about .042” with worn spark plugs, alcohol in the fuel, cold cranking with a slightly low battery??? I’m sure BMW would have liked to use a slightly larger plug gap to help with low RPM surging & idle emission compliance (why didn’t they?)..

 

Probably a real safe approach on plug gap would be to use the max sparkplug wear gap that BMW recommends then change your spark plugs often to keep the gap at close to that range..

 

Personally I have been running stock plugs lately as I found no long term gain from those AutoLite plugs but when I was testing the AutoLite 3922/3923 I ran the gap at .035” as I felt that was within the safety margin of the ignition system & that didn’t require bending the side electrode to an abnormal angle..

 

Twisty

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Great having you enganeeers here....Thanks Bob and Bill, enjoyed the learning..Very interesting......Now, if I can just remember it... tongue.gif

 

Phil............Redbrick

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Thanks, Twisty1, that's excellent information to have, and reflects much insight and expertise.

 

My '94 R1100RSL specifies max. gap of 0.040" and that's how I set it. I did not measure the gap when I first installed the AL 3923 plugs (used factory preset) or when I pulled them out yesterday, but guesstimate ~0.035". I used emory paper to clean the electrodes without touching the ceramic, set 0.040" so the wire gauge would clear with a bit of effort, then indexed the plugs when re-installed on the bike. By "indexed" I mean marked the plugs with a Sharpie marker to identify the open side of the electrodes, then oriented that side toward the intake valves in the cylinder. Reportedly, indexing is done by racers to get a better flame front, with supposedly a tiny bit better combustion and performance. Probably superstition, but I figure worth jiggling a bit.

 

Anyhow, per your and Bob's comments, I'll monitor the plugs often and keep them clean and adjusted to no more than 0.040" or maybe ~0.038" from here on.

 

FWIW, the bike ran noticeably better after the 0.040" adjustment, but that may be due to other extraneous factors, such as snugging up the intake clamps a bit, etc..

 

BTW, the rubber nipple caps on the TB nibbles had little cracks, so I painted them with red PlastiDip to seal any leaks, til I can get new rubber nipple caps...a cheap trick, but does work to seal small cracks in a pinch.

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Paul Mihalka

On my '99 R1100RT (170K miles) and on my '02 R1150R (76K miles) I regularly used the Autolite 3923 plugs, and gapped them a .040 from new. Never had a problem. I left them in the bikes for 20K miles each time, with very little gap increase.

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I also have been using the Autolite's for a few years, gapped at .032. The bike runs just a little bit better than with the original Bosch plugs. When the Bosch are $9 and the Autolite's are $1.20, and it runs better, it wasn't much of a decision.

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