This last weekend I decided to do a tune up on my '58 which has a '71 LT-1 350 engine. Bought new wires and AC Delco R44TS plugs as per GM spec. Right out of the box 2 of the 8 plugs failed. They all had good electrical signal to the plugs. Replaced the 2 failed plugs with some old ones that still worked to verify it was the plugs, which it was. Returned all 8 AC Delco plugs and have switched to another brand. The AC Delco plugs are made in China. Thought you all should be alerted.

 

They don't make anything that I'm aware of anymore. My Delco remain starter lasted less than 300 miles. You really have to watch oiI filters also. No. Blue or orange for me.

 

NGK made in Japan, high quality. I use NGK 7355 Iridium spark plugs with a 0.040 in gap. Can't go wrong with these spark plugs.

 

I just rebuilt an engine and it had a vibration, checked every possible source to cause a vibration all the way to disassembling engine and rebalancing it. Still had vibration. Finally decided the spark plugs were at fault, put in old plugs and no more vibration. Three of the new plugs were only firing about 70% of the time. They were NGKs. Exchanged for a new set, it runs smooth now. New spark plugs can be a problem to consider.

 

Bosch Iridium for me. My last Delco experience with my GM truck had one plug actually BREAK OFF! leaving the threaded part in the head with no visible part sticking out. NEVER seen that happen before. I saved the broken plug to send pics to Delco. I had to do some fancy dancing to get it out with an oversize easy out, and of course it had to be No.7, the hardest to reach on a high 4x4.. Very lucky to get it out without part going into the combustion chamber.

 

Couple spark plug notes, Champion, NGK, Autolite still make spark plugs. I am sure there are a couple more manufacturers from overseas also. AC Delco no longer make spark plugs and they have not for maybe 30 years +.

Old stock engines run the old stock distributors with a single coil. These are the lowest voltage spark systems out there.

Modern engines and even some in between run high voltage systems but not our old Corvette engines, at least before 1974 or so.

High voltage systems run so much voltage they needed better spark plugs to get more life out of them. Platinum, then Double Platinum and later Iridium plugs were introduced to allow the plugs to last longer in a high voltage engine.

Every time a plug sparks it blows a small amount of material off the electrodes on the spark plug. The Platinum extends the life by resisting the amount of wear. A double platinum became a 100,000 mile spark plug.
The only negative is the platinum actually needs a stronger spark to make it work. They don't work well in a low voltage system and will cause misfires.

Iridium plugs were introduced later, Iridium resists wear better than platinum, these are advertised to be a 200,000 mile spark plug, the Iridium works so well they could make a fine tip and a more focused spark with that fine tip. These are upgrades over the Platinum they replaced, a bonus was Iridium will work well with lower voltage than the Platinum plugs. Iridiums are more expensive than Platinum so people would swap Iridium plugs for Platinum to save money at a tune up and suddenly they would start getting misfires. Most likely they should have left the Iridium plugs in that engine since they last so long. Even with this ability to spark at voltage lower than a Platinum plug the Iridium still takes more voltage than a Copper plug to spark but they are better in all ways than the Platinum plugs. Also, Iridium plugs are not yet available for all applications.

Now, for our C1, C2 engines that are stock, stock Coil and stock voltage system the plug that works the best because it needs the least amount of voltage to produce a quality spark is the Copper Plugs. These are the least expensive option but they were designed to work well in the low voltage system. Copper plugs also wear the fastest because they don't use the precious metals on the electrodes to resist wear. Wear in a spark plug is seen as they age. The gap becomes larger over time. Time in this case might be 10,000 miles. At that point you can re gap the plugs and put them back in or replace them. They are only $2 each. In old carbureted engines plugs will foul out, the fuel mix is not ideal like it is with a fuel injected engine so checking plugs is part of the tune up procedure, you need to see them and read them to know how well your fuel is burning so I see no negative in running Copper plugs in these old engines. If you run an HEI, MSD system or some other high voltage coil your old Corvette engine you can certainly run a Platinum or Iridium plug but you still have a carburetor and the plugs will need to be inspected from time to time to see how your engine is burning. Even with super high voltage systems the old Copper plugs will still last over 5,000 miles, in a classic old Corvette that might be 3 - 10 years of driving. Food for thought.
Mark

 

Thank you.

 

Thank you. I will give them a try.

 

Thanks. Sure caused you a lot of work. Glad you solved the problem.

 

Thank you for the suggestion.

 

Thanks for the information. Great analysis! Appreciate you providing that history.

 

My set up in my 1959 fuel injection Corvette is as follows:
NGK 7355 Iridium Spark Plugs with 0.040 in gap
Breakerless SE electronic ignition
MSD Street Fire 5520
Starts and runs great.
Oh I also have an electric fuel pump.

 

Mark,
Now that is how you contribute to a thread! Thank you.
And it gives me great pleasure to learn that I do have the best plug for my engine installed.
Phillip

 

Phillip, glad you appreciate the note but we only discussed plug type, heat ranges play into this also and other members have the hands on experience to share on that subject.

I should have noted for consideration that quality upgrades with plugs are done on both the center electrode and the ground electrode. This enhances life of the plug. Single platinum plugs only use platinum on the center electrode while double platinum plugs are on both sides of the spark. Same for iridium plugs some companies only use iridium on one side to save money. If they are using it on both sides making it double iridium it will be in print.

 

Thanks for explaining spark plugs and when to use each plug type.