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Spark Plug Gap


jschunke

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Here's one for the folks that are familiar with the Oldsmobile

307. For the first time I decided to gap my plugs to the

correct specification (0.08). Usually I just install the plugs

right out of the box (0.05). Now I'm getting less torque

with the 0.08 gap and it just don't feel right. Would ya

go back to 0.05?

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I would not. Factory gap for a 307 with 5A heads is .080". If your 307 has the 6A or 7A heads, the gap is a more conventional .035" or .045" (can't remember exactly). I would think that something else is wrong, if you can "feel" the difference just by changing the gap by .030".

The heads were changed drastically around 1985 (to the 6A or 7A heads -- a swirl port design), and that's also when the plug gap changed considerably. There was no change in the rest of the ignition system, so I'm sure the change in gap was driven purely by the change in head design, so I would stick with what the OE recommends. I don't know if you could do "damage" with a much smaller gap, but I always found mine ran the best with AC plugs at the stock gap. I tried AC Rapidfire and Bosch non-platinum plug and neither ran as smooth or steady as the regular AC plug.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Thanks Jadcock. Went against my better judgement and got Autolight spark

plugs (that's all NAPA had). Usually it's AC Delco all the way. So I'll get

AC plugs and take it from there. Been experimenting with gap on my Camaro

350, and beleive it or not there was a huge loss in torque going from

0.035 to 0.05. Put it back to 0.035 and all was well. That's why I'm wondering

if I'll get my torque back by going with a smaller gap on the 307.

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If there's a huge loss in torque, then your ignition isn't capable of generating a concentrated enough spark with that gap. If you had a much higher power ignition then it might have increased the torque.

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That's why I'm wondering if I'll get my torque back by going with a smaller gap on the 307.

I bet you'll get your torque back with the AC Delcos. :)

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Thanks Jadcock. Went against my better judgement and got Autolight spark

plugs (that's all NAPA had). Usually it's AC Delco all the way. So I'll get

AC plugs and take it from there. Been experimenting with gap on my Camaro

350, and beleive it or not there was a huge loss in torque going from

0.035 to 0.05. Put it back to 0.035 and all was well. That's why I'm wondering

if I'll get my torque back by going with a smaller gap on the 307.

The larger gap requires a higher voltage value to "arc-over". And it takes just a little longer for the coil(s) secondary voltage to build to that higher value. The effect of increasing spark plug gap is the same as retarding the ignition timing (when all other values are left unchanged).

If the engine runs with the larger gap, then your ignition coil can obviously develop sufficient voltage to fire the plug. It is just firing later than it would with a smaller gap.

Jim

Drive your car.

Use your cell phone.

CHOOSE ONE !

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What year is the 307? I don't think I have ever seen a gap that big .080. Thats a huge gap from my experience, as the largest I have seen has been .060. There is no question that using a smaller gap than .080 will improve performance. I have a book on spark plug gaps if you let me know that year. Is the engine stock? Some of the electronic ignitions used a larger gap but if the distributor was changed to a conventional distributor a smaller gap may be correct. Mike

Early on sailors navigated by the stars at night and the North star became the symbol for finding ones way home. Once you know where the Northstar is you can point your ship in the right direction to get home. So the star became a symbol for finding ones way home or more symbolically even finding ones path in life.

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The larger the gap (as long as you can complete the arc over) the bigger and better the spark. UNless you have a wickedly powerful ignition, 0.80 is a HUGE gap. 0.60 maybe 0.65 at the most i'd use unless you have a badass ignition. Using a larger gap will strain the coils and cause them to fail unless they are designed for it. Using a smaller gap can't hurt anything although it might reduce performance if it's under the required gap. The arc-over is the most important part of the plug firing, not the spark itself. If the arc-over takes less voltage to occur the spark will be less (the arc-over takes more voltage on regular steel plugs than it does on platnium or iridium plugs). However if you have a platinum or iridium plug and you can increase the voltage or amperage to the plug during the spark and you set the gap higher than the steel plug you can get an even better spark. I love electricity and know lots about it :>

From what I know about cars, a 080 gap might even hit the top of the piston or a valve.. that's quite large. Luckily the caddy's have a very good ignition system (at least the DIS does) and even when mine were worn down to more than 070 gap it was still firing reliably..

The 'reason' you lose torque due to higher gap is the inability for the arc-over to complete which is most likely causing missfires which you may or may not even notice driving the car. If the gap is really supposed to be 080 then parts of your ignition are bad.

On another note the richer the air/fuel ratio is the lower the arc-over voltage that is required. the higher the compression, the more voltage is required.

Platnium/iridium plugs may actually increase horsepower/torque because of the fact that the arc-over voltage is less, but ONLY if your ignition system is incapable of handing the voltage. (if your ignition is weak and you install platnium plugs it will help, if your ignition is strong and you install platnium plugs your HP may actually decrease unless you adjust the gap in the platniums/iridiums to compensate)

I hope this helps :)

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Heh I just read what I posted.. I must be feeling very long-winded today.

If anyone actually read all of that, congratulations :>

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I'm starting to sound like Guru :)

If it's an electricity question, a computer question, or a networking or internet related question, or a mechanical design type question, i can probably go on forever about it :)

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The Olds 307 was produced from 1980 to 1990, and was carbureted by a computer-controlled Quadrajet the whole time. There are two basic flavors. 1980-1984...had a hydraulic cam, and 5A casting heads, which were "conventional" heads. Also had the .080" gap. 1985-1990...had a roller camshaft, and 6A or 7A casting heads, which were "swirl port" heads for better torque and mixture...and had a smaller plug gap...and also a totally different plug design.

The 5A 307 uses R46SX plugs and the 6A/7A 307 uses FR3LS6 plugs (both are Delco numbers). Again, the .080" gap is the OE gap for the older 307 engines...don't hesitate to gap the plugs at that gap -- that's what it was set at at the factory, and that's what they run hundreds of thousands of miles on. The ignition is plenty able to handle it -- it's the standard GM HEI distributor.

Like on the Northstar, plug wires and other ignition parts are a non-issue. My Cutlass still had the original wires on it when I bought it in 1995 (at 58k miles). I replaced them with a new set from Delco (Packard) just "cuz"...they were cheap.

Any Cadillac with a 307 engine will have one with the 7A heads and roller camshaft...because they were installed later than the 1985 model year. I've owned both flavors of 307 and there's a distinct difference in "character". The older one is a bit rougher, and a bit "rowdier". The newer 307 was a lot smoother...almost felt like a fuel-injected engine. Both are 100% reliable and almost impossible to kill.

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Hmm I thought I could edit that msg... I made a typo, the richer the mixture the more voltage not the less (more density the richer)

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Heh... You sure do a lot of research jad :)

Wish I had the time to play.. maybe when i'm retired.

My dream car ever since I saw it in 1992 was the 1992-97 generation Seville. That was very much out of my reach when I started driving (1995), and I bought an '84 Cutlass from someone who worked for my father. Unbeknownst to me, I would come to fall in love with that car, and in love with Oldsmobiles in general. It was all I did through college (go to car shows, fix up cars)...oh, and I got an education on the side. :) Anyway, I've owned a few of the 307 cars, so I know a lot about them. I bought an '87 Regal because it had a 307 engine in it, but it was real tired. There was some sort of a knock in it, but man that thing ran smooth. I think it was a piston wrist pin. Anyway, I bought a '79 Regency 98's engine from a local junkyard -- it was an Olds 403 engine. It dropped right into the Regal and made that car a real screamer. :)

Someday I hope to know as much about the Northstar as I do about Oldsmobile rocket V8s. I'd like to think I'm getting there, just not quite yet. The Oldsmobile engines were designed back in the 1960s, so it's impossible to find anyone who actually helped work on those. We're lucky to have Bbobynski here, who actually DID work on the Northstar, and the "inside" knowledge is just fantastic. What we'd give for someone like him on our Oldsmobile G-body mailing list. :)

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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Yes.. I agree info from Guru is very good, especially the 'don't worry about that, it's supposed to do that' info. I know a lot about engines but not really all the model numbers and ratings and stuff off the top of my head so I have to use the manual.. even when building my own engine i still followed instructions to make sure I never missed torquing something down right. I love my friends 85 delta 88 royale .. It's just as smooth as my 99 caddy (when going in a straight line of course hehehe). And the steats are so bouncy. (or rather, the SEAT since it one big front seat which Is cool like the devilles). I wish they still made cars like that with the big cooshy seats but it doesn't seem to happen any more. It's all about 'performance' and leather interiors. Now I'd be happy with a 1968 deville sedan with the 472(i think).. suicide doors.. now THOSE were cool :) but i'd still miss the style and handling and performance of the STS.. would be fun to have both, or hell even 5 or 6 cars someday :>

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would be fun to have both, or hell even 5 or 6 cars someday :>

I think that's what I'm going to have...one day a long time from now. :) My dream list:

1) A 92-97 seville (hopefully the one I drive now).

2) My old '84 Cutlass (I want to buy it back).

3) A mid 1970s Olds 88 Convertible, triple white, with the 455.

4) Some form of a pickup truck, to haul my kayak and other stuff.

and 5) Something for the wife to drive, of her choosing. :)

Jason(2001 STS, White Diamond)

"When you turn your car on...does it return the favor?"

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