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Spark Plug Gap Questions....How much gap can I run?


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So I was at Mesa talking to the guys...I love going in there...you can pick up alot of valuable info from guys who have been in the BMW community for probably more than I have been alive. So Spence is telling me about plug gap..then says, "Yeah the bigger the gap the hotter the spark"...Uh oh...I thought the opposite and was probably gapping my plugs at .24 thinking it was a tad hotter, when it was colder...Duh. I had a bit of a lumpy idle, and the smell of unburned fuel in the exhaust. Tried to lean out the carb, but it would stall about the time it was lean enough...So I got a hotter Bosch plug, WR9DC and gapped them to .34. WOW....the car runs much smoother...idle was smooth as butter. Just gave it a quick drive and wondered if I should try a bit more gap? I have a Tii dizzy with Pertronix, Ireland Step Headers, 284 cam and Weber 38/38 carb.

1976 BMW 2002 Fjord Blue Ireland Stage II • Bilstein Sports • Ireland Headers • Weber 38 • 292 Cam • 9.5:1 Pistons • 123Tune Bluetooth 15" BBS

2018 BMW M550i X-Drive

1964 Volvo Amazon Wagon

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I've run .45 with my MSD set-up.

Wow, intersting. I'll give this a try.


post note: Just gapped @ .35, took a 1/2 hour drive. Ran very nicely. I have a streetfire/msd box w/ weber 40's. My gauge tool's thickest blade is .35, where'd you guys get ones w/ .40+?

1976 2002 Custom Dk Blue w/ Pearl

1975 2002A Sahara (sold Feb 2008)


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If you have a stock coil, anything beyond .030 to .035 or so may be pushing it as far as reliability is concerned...

If you have MSD or something similar, just get rid of the spark plug and stick a nail in the cylinder; the spark will make the jump to the head. I have seen MSD throw a 6 inch spark.

So the rule isn't quite 'the wider the gap, the hotter the spark', its more like 'the hotter the spark, the wider the gap'. If you have a 'hotter' spark, then your spark will be able to jump a larger gap. That larger the gap, the larger the "seed" for the explosion that round. The larger the "seed", the more uniform the explosion and the car can be tuned more precisely which creates more power and more efficiency. Stock coil's produce 20,000 V or so, and a Flamethrower or high performance coil can make up to 40,000 V. A capacitive discharge system can make 500,000 V or more depending on the system. With that many volts, you can make the gap much larger.

I think that MSD says to start at .040 with their 6AL. And I also believe that they say you can go all the way to .060 if you feel like it. I have always seen MSD cars gaped at around .045 or .050 (Twice the gap of stock, remember).

My Opel is gaped to 0.35 with a Flamethrower coil and it never misses a beat.




I lost control of my balance and had to sit down for a couple of minutes (When I say had to sit down, I mean that I fell over and could not get up). Thank goodness it just a buttload of volts without any amps, or else I wouldn't be laughing.

J Swift
Global Formula Racing (Oregon State University)

1972 Opel GT "Mae"

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I'm running a Crane XR3000 and I've set my gap to .036. Car runs just fine. I get a little less smell of unburnt gas, so I think I'm getting a much better spark.

More importantly, what coil are you running? The points has nothing to do with the size of the spark produced other than the dwell...but points replacements should have similar dwell times as points.

J Swift
Global Formula Racing (Oregon State University)

1972 Opel GT "Mae"

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Hotter spark is mostly a bunch of bologney - myth.

The spark plug sparks, period.

Once the AF starts to combust, the process starts and any spark exceeds the energy necessary to begin the process.

Spark duration plays a part (which is why multi-spark CD systems have some merit).

Points and to a lesser extent Pertronix have a more gradual release of the energy stored in the coil then either a Crane or ECU spark - these are instant all-in.

Probably more important to a mostly stock M10 motor is spark indexing - that is, the open end of the electrode is properly facing the designed Flame Front. This is one reason to stick with the OEM spec plugs. The head was tapped with these in mind so the electrode faces the correct way.

But, in discussing a street car, this is all pretty much moot. The subtle differences offer no noticeable advantage.

Don't worry about it.


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1976 BMW 2002

1990 BMW 325is (newest addition)

1990 Porsche 964 C4 Cabriolet

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Actually, Jay is right-

bigger gap= longer spark= bigger flame kernal, or better chance for one.

And Mr Yunick also said 'longer is better', in reference to TIME.

So the LONGER the sparks stays struck, the better.

I seem to remember he said that the biggest gap you could

RELIABLY run seemed best, but that it wasn't a big deal, and

that you should run the gap conservatively to avoid misfire from

a spark that can't jump due to too BIG a gap...

...but I read that a long while back...


"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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