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Discussion Starter #1
OK, I tried to search, and have gleaned that ideally, for a stock, "low voltage" ignition system, .035 is a good gap for original or slightly modified ignitions.
I just put a MSD 6a on my 68, with performer RPM heads. Edelbrock only lists the size and range, not the gap for the heads. The MSD instructions call for .050 for compression up to 10.5:1.

Pretty wide variation there.
Would it be a safe bet, to split the difference, and gap at ~.040?
 

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I have mine gapped at .045. I tried .035 and it didnt run as well. I have an MSD 6AL2 and just a little more compression than you. Generally the plug gap will be based on the strength of the ignition system and compression.
 

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.040 here MSD - platinum Bosch Seems to be about right
 

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Discussion Starter #4
I'm going to try it at .045, with the champion 12yrc's that Edelbrock recommended.
 

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The data I've seen for the 302 4V engine calls it 10.5:1, stock, and the shop manual calls for a gap of .032-.036 if I remember right. I just put in new plugs and gapped them all to .035 since that was toward the larger end of the spec range. With the stock head and coil setup, is it better to not go to 0.040? Is the key player there the coil?
 

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I've got my Autolights gap set at .045. It a 302 4v with reworked head and a Performaer intake. The ignition is a Mallory unilite distributor and the big coil with a 6AL box. The car starts easy and runs great.
 

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not the same but my old flat head 6 cyl with the mallory coil likes .045 gap on ngk plugs---can't believe the differance in starting and performance---the wider the gap the higher the voltage until the compresion starts to kill the spark . i don't think you can do much damage to the engine by playing with gaps (maybe burn an electrode off if way to wide)--i'm not going to experiment with gaps on our 67 big block as my bandaid supply is down to three boxes
doctordesoto
 

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go for the .050 gap. the mds box has the power to push the spark across the gap. which means bigger spark to fire the fuel, better burn.
 

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The larger the gap the shorter the spark lasts so its a balancing act. Trying to get as large a gap as possible without loosing too much spark time. Generally larger gaps will have better idle quality while smaller gaps work better at high RPM. I'd stay between .035 and .050 as anything smaller or larger will net you nothing and most likely cause either poor idle quality or high rpm misfire. Stock ignition stay between .035 and .040. Some good info here. http://forums.corvetteforum.com/c1-and-c2-corvettes/2723461-ignition-analyzer-oscilloscope-pattern.html

Bill
 

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I thought that the MDS A box fires the spark duration is specified as “20 deg crankshaft rotation.” I assume that this means that the unit will
fire multiple sparks over a 20 deg rotation.
 

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I thought that the MDS A box fires the spark duration is specified as “20 deg crankshaft rotation.” I assume that this means that the unit will
fire multiple sparks over a 20 deg rotation.
Yes MSD got its name from muti spark discharge. As far as I know MSD is only muti spark at lower RPM. Pertronix claims to have multi spark at all RPM' when using their Ignitor III system. Can't see how its possible as there is not much time to have muti spark at high RPM as there is less coil saturation time available as the RPM's increase. Even with multi spark discharge each spark will be shorter the larger the gap.

Bill
 

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Discussion Starter #12
I went with a .040 just to see how it does. I still haven't been able to get it out on the road, but I will report back when I do!
 

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I have been running .045 for over 15-20 years w/ no problems. This recommendation comes from Vic yarberry who has been vintage racing for many many years, with Pertronix/MSD setup on the Cougar. Must have a coil like pertronix flamethrower.....
 
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