Spark plugs R46N or R45XLS ???

[Bill Balkie 24172]

Hello ,
     After two year of owning my 1970 Cadillac I thought it would be a good idea  to change the spark plugs .  The plugs Cadillac suggest are R46N  these plugs are still  available I can buy a set of 8 for about  20 Dollars .  I removed one plug out of my car today to find that the plugs in the car now are R45XLS . The car still runs with the standard points .  I  am Not an expert on Spark Plug  technology by any means.  I would think if I can get the original  equipment spark plug that would be a good move . But then again technology has changed over the past 50 plus years .  My question is should I go with the R46N or should I be using a more advanced spark plug ?
  Thank you very much,
                 Bill

[Scot Minesinger]

Bill,

Wish I knew the answer.  Fortunately, if one set is better than another you are not looking at a very steep loss.  The other nice thing, is that the spark plug change work can almost be done in a white dinner jacket, there is so much room and it is easy.  Changing plugs in a 1960's Thunderbird is difficult.  I have one 1970 Cadillac that runs on points the other HEI, and to me there is no difference.  If the car runs well now, i would replace with like.

[Bill Balkie 24172]

Scot ,
   Thank you for your opinion . No
Problem with the way the car runs . Just thought after 2 years and 4,000 miles it might be a good idea . Sometimes I think to much . 
      Bill

[76eldo]

Use whatever  the manual calls for.
I have about 20'sets of the green ring 46n's if you want an NOS set.

[Jay Friedman]

The late John Washburn, who wrote tech articles for the Self Starter, once did some tests on NOS plugs original to his 1949 Cadillac.  He found that a large percentage of his collection of NOS spark plugs deteriorated on the shelf over time, although they were never used.  The conclusion was that it's best to use brand new plugs which are the modern equivalent of the original plugs unless you need original plugs for show purposes.

For example, '49 Cadillacs originally used AC 48, then late in the model year AC 46-5 plugs, which can be found NOS on ebay.  However, I use brand new AC R45S plugs which are the equivalent.  Autolite 86 and Champion 63 plugs also work well in '49s. 

[James Landi]

In my opinion, respectfully submitted, the rule of thumb was 15-20,000 miles on a tune up--- plugs points, check rotor and cap...oh and wires (at night). 4,000 miles with the car running well seems unnecessary, especially if you mess up something in the process-- i.e breaking an ignition wire, or installing a brand new condenser that is defective.   (my father used to tell me, "James, if you're looking for trouble, you're sure to find it.)   ALas, his commentary turned out to be true--- when I'd screw something up in the process of being excessively concerned about preventative maintanence.   James

[fishnjim]

There's no difference.  The number usually indicates the "heat" or spark energy.   The XLS appears to be a convential replacement, look up AC Delco spark plugs.   I think the N is no longer made that's why it's in NOS.
You can opt for the platinum equivalent for longer life, but not getting much wear.
There was a gradual change of plug technology with the switch to HEI and decompression to improve ignition as gas mileage/emissions were an issue in this era.
You can look at the plugs and tell if they need changing.   If all carboned up and deposits, spark conductor edges burnt/rounded need change.   If they are like new and firing, no need to.   
They used to have spark plug machines to test and clean, but these met the fate of the dyno-soars when lead gas went out.   I saw one going to auction last week, but my history dust bin is full.
Take an induction timing light and plug it in and check each wire to see if the plug is firing.   Should flash uniformly.

[Daryl Chesterman]

This is a concise chart for decoding AC Delco spark plugs.

     https://www.jalopyjournal.com/forum/proxy.php?image=http%3A%2F%2Fwww.301garage.com%2Fforum_pics%2FSpark_P.jpg&hash=150a7d8cf4018ef70bcccb7ddf3a1155

I agree with James in that 4,000 miles on the plugs is no reason to change them.  Every time the spark plug wire is pulled off of the plug, there is a chance that the wire will be damaged.  If the spark plug that you removed looked to be in good condition, I would not worry about replacing them.  Also, the R45XLS plugs are the currently recommended spark plug for your car.  That recommendation is for an engine that is driven at highway speeds and occasional short trips.  If you only drive your car in town, in parades, or other slow speed driving for short distances, then a hotter spark plug would be recommended, such as an R46XLS, which is harder to find, but they are out there.  A car that does a lot of short trip drives should periodically be taken out on the highway and driven at higher RPMs (speed limit, not racing speed) for a sustained period of time to burn out the excess carbon in the combustion chambers and burn the moisture out of the exhaust system.  Short-trip driving is very detrimental to the exhaust system, unless it is stainless steel.

Daryl Chesterman

[76eldo]

I read John Washburn's article too but I can't figure out what can deteriorate on an unused plug no matter how old it is.
Does anyone know ?

[James Landi]

My guess--- moisture intrusion--- If my memory is correct, John was measuring the isolation characteristics of the porcelian (in resistance/ohms) that, over time, was providing a potential pathway to "ground" and therefore, potentially weakening or worse negating the spark at the spark gap.  Moisture, viz: water,  permeates--and high tension ignitiion voltage will follow a pathway of least resistance--- through the porcelian and to ground .  Please do correct me if I messed up.   James

[TJ Hopland]

Its been a long time since I smashed up a spark plug so I don't remember how the internal connections are made.  I don't think it was one solid hunk of metal,  seems like it was multiple pieces so maybe the joints could degrade? 

[fishnjim]

Going to need a medium to contact John now...
He also said same about condensers, which are a little more fragile.   Some didn't agree but presented no data.
I see the making of a myth here.   A better consequence would be how to store them adequately.*   I doubt the OEM packaging was very good for a hihg turn over item like plugs.   I get NGK plugs for the boat and they usually come in a cardboard package with a cardboard tube around the threads and bottom to protect them as they're factory gap pre-set.   They seem to be superior to anything.   I recall in the day, seeing plugs with the ground rod welded on off center, smashed down, etc.   I didn't like AC plugs, I used Champions back then.   Had better luck with them.   But it's a personal choice mostly.
You can take some of the older ones apart, you unscrew the top connector and that exposes the conductors in there.   But some break during torqueing, so any loss of the porcelain insulator integrity is a path to ground.   Micro crack, etc.   If it's complete saturated it won't fire.
The main issue I see would be the firing "pin" area would corrode and not spark good.   
They went to platinum and iridium coatings down there to protect that area at the high heat load and the HEI spark.   And every design combination they could think of.
Now we get multiple sparks.   
*- if you have a TIG, squirt a quantity of the argon gas in a steel paint can or heavy gauge plastic bag put them in there and seal it up.   Argon is heavier than air and keeps the oxygen and moisture out.

[Scot Minesinger]

2 years and 4k miles - don't bother changing the plugs.  Give it another decade.  Seeing that servo linkage connector you made, I get how you are (that is a compliment), but let the plugs have a chance to break themselves in first.

Waiting at the light to turn left with engine idling, the engine in my 1970 Cadillac DVC is so smooth, and at 80 mph it is a joy on the highway.  Cannot remember the last time I changed the plugs, guessing it was 2013 and 24k miles ago.  I am a nut about changing the oil and filter too much, 3,000 miles or 6 months which ever comes first.

[Cape Cod Fleetwood]

Hyannis Vintage Auto uses NGK plugs exclusively.
Reid's Automotive put ATL64's in my new engine, what he uses in dragsters.