Spark Plug recommendation needed

[Donald H.]

I need a recommendation for a 1966 427/390 L36. The owner's manual says AC-43N for city driving. The car currently has R43XLS plugs.

Thanks,

Don

[Owen L.]

How do the current plugs look as far as carbon deposits and the color of the porcelain?

For both my '67 427/390 and my '72 454 LS-5, I use NGK brand in the 4 heat range. NGK's heat range is opposite of ACD with a 4 being equivalent to a 5 or 6 Delco. The '67 with gasketed plug is a BPR4ES. My cars don't get a lot of sustained highway rpms and the hotter plug burns cleaner than the 3 or 4 heat GM recommended.

[Paul H.]

R45 XLS is the right plug for your 427/390. My L36 always ran great on those. It's the right heat range for a street driven 390 horse.

[Dave S.]

R45 XLS is the right plug for your 427/390. My L36 always ran great on those. It's the right heat range for a street driven 390 horse.

Paul’s suggestion is a good one. AC 45N will do the same job and might be easier to find.

[Duke W.]

All vintage engines used for normal road driving are best served by using AC heat range "5", which is the second number in the AC nomenclature system (The first number, "4", means a 14 mm thread) or equivalent in another brand. Any prefix or suffix letters define other features. For example, the prefix "R" means a resistor plug for better RFI suppression, but they cost some energy to the plug and did not become OE until 1969.

OE spark plugs were typically heat range 4 for small blocks and 3 for big blocks.

Heat range selection has nothing to do with rated engine maximum power. It has everything to do with average engine load. Since Corvettes are relatively light cars with powerful engines average load in normal road driving is low even at sustained freeway cruising speeds. The same engine in a pickup truck hauling a 6000 pound trailer would need a colder plug.

In my experience the OE heat ranges are too low, and they can rapidly foul in normal road driving because the tips often don't run hot enough to burn off deposits, especially short trips around town in traffic. One good thing about unleaded fuel is that there is no chance for lead oxide deposits to build up on the insulators, but carbon deposits will if the plug is too cold.

After fighting constant "plug fouling problems" on my SWC back in the sixties it finally switched to the AC 45, and prior to spending a day hot lapping the 2.25 mile Kent road course I swapped in a set of AC 43 plugs, which proved just right for sustained WOT operation and the 45 was just right for normal road use.

IIRC the "N" suffix means a "half-thread" long reach plug, which is okay for cast iron heads, and I don't think they've been in production for some time. The XL and XLS suffixes mean a fully threaded extended reach and S means extended tip.
The major plug brands have online catalogs with cross reference charts, so it's easy to find an alternative brand once you know the AC number you want.

Use AC plugs if your car is being judged, but I prefer NGK and Denso plugs because they have a more corrosion resistance coating on the shell than the black oxide on AC plugs. Also, GM has not manufacturered "AC" plugs for decades, and I have no idea who manufacturers them... probably the lowest bidder, and they no longer offer non-resistor plugs.

For small blocks with gasket plug seat heads I recommend the NGK B4 or Denso W14-U, preferably the latter due to it's "U-groove" ground electrode that provides more sharp edges that sparks like to propagate to than conventional ground electrodes. Both of these plugs are equivalent to the long-out-of-production, non-resistor AC 45.

Duke

[Mike T.]

When I got my 66 L72 Roadster, now running a sedate but strong running 396 engine, it had 43XLS Delco plugs in it. I made the change to 45XLS but while the pic below will show a dramatic difference between two, there were other changes that improved the look of the plugs such as rejetting (car had been in Florida now at the 5500 foot elevation or Prescott AZ), increase initial timing from 4 to 12 and moved the ported VAC to manifold vacuum and lastly, found the secondary float level to be a bit high so dropped that too. I've probably got less than a thousand miles in it since purchase but happy with the AC Delco 45XLS...whoever makes them. :-).
Mike T - Prescott AZExtg 43S on Right New 45S on Left After Rejet Conv to Man Vac Timing Reset from 4 to 12 and Lowered Sec Float Levels on Blue 66 L72.jpg

[David M.]

Another vote for NGK V power. Its the modern equivalent to the once high quality USA made ACs

[Donald H.]

Thanks for all the responses.

I may just order the NGK BPR4ES and give them a try. Per Duke's recommendation, I run the NGK B4 in my 67 L79 small block and have been happy with them.

Don

[Duke W.]

Another vote for NGK V power. Its the modern equivalent to the once high quality USA made ACs

The B4 for small blocks is NOT a V-power plug. It's a conventional center and ground electrode plug like the old AC 45. The Denso W14-U equivalent is a "U-groove" ground electrode which has the same benefit. I don't know if the NGK equivalent for the 45XL or 45XLS for big blocks is V-power or not.

Duke

[Duke W.]

Thanks for all the responses.

I may just order the NGK BPR4ES and give them a try. Per Duke's recommendation, I run the NGK B4 in my 67 L79 small block and have been happy with them.

Don

The BPR4ES is a resistor plug. do a little more research in the NGK and Denso cross reference charts for an equivalent to the AC 45LS or 45XLS.

Duke

[Duke W.]

When I got my 66 L72 Roadster, now running a sedate but strong running 396 engine, it had 43XLS Delco plugs in it. I made the change to 45XLS but while the pic below will show a dramatic difference between two, there were other changes that improved the look of the plugs such as rejetting (car had been in Florida now at the 5500 foot elevation or Prescott AZ), increase initial timing from 4 to 12 and moved the ported VAC to manifold vacuum and lastly, found the secondary float level to be a bit high so dropped that too. I've probably got less than a thousand miles in it since purchase but happy with the AC Delco 45XLS...whoever makes them. :-).
Mike T - Prescott AZExtg 43S on Right New 45S on Left After Rejet Conv to Man Vac Timing Reset from 4 to 12 and Lowered Sec Float Levels on Blue 66 L72.jpg

It's clear that the 43XLS is carbon fouled. This could be caused by too cold plugs, too rich mixture, or some combination of both, but the 45 XLS looks good... nice clean insulator, which is the way it should be if ignition system components, including plugs and carburetor calibration is dialed in.

Dujke

[Donald H.]

I have not found an AC 45XLS. All I can find is the R45XLS.

Don

[Paul H.]

I have not found an AC 45XLS. All I can find is the R45XLS.

Don

That's what I recommended in this thread.

[Jeffrey S.]

Don,
The 45XLS and the R45XLS are essentially the same plug with the exception that the "R" plug has a resistor in it to help eliminate radio noise. Both are a 5 heat range, fully threaded, 3/4" reach with and extended tip. That is the same as the 45N except that it is half threaded and does not have an extended tip. The XL plugs, being fully threaded, were used primarily in aluminum heads where the half threaded "N" plugs were used in cast iron heads. I tried to PM you with some more info but couldn't get through. Email is easier anyway (jeffstuarts@gmail.com).
Jeff

[Duke W.]

I have not found an AC 45XLS. All I can find is the R45XLS.

Don

Non-resistor "AC" plugs have not been offered for decades. If you need "AC" plugs for judging, okay. If your car is not in the judging process I recommend Denso or NGK.

I'm not sure it the old "N" suffix AC plugs are extended tip or not. I don't think they are so a suffix "XL" would be a better geometry match. the "S" suffix indicates an extended tip. Extended tip plugs may have some anti-fouling advantage, but they reduce chamber volume by a cc or two, so if your engine is on the ragged edge of detonation, I don't recommend them.

Here's cross reference info I can offer for non-resistor, gasket seat plugs from old paper catalogs.

45XL = Denso W16ES-U or NGK B6ES

45XLS = Denso W14EP-U or Denso BP5ES

The Densos are their patented "U-groove" type. The NGKs are plain electrode, not their patented "V-power" type.

Note that in the AC heat range system the higher the heat range number (second digit) the hotter the plug.

The Denso and NGK heat range systems are "opposite polarity". The higher the number (two digits for Denso and one for NGK) the COLDER the plug. Given that it's not clear to me why they recommend one heat range hotter for extended tip plugs. I think it should be just the opposite since extended tip plugs tend to burn off deposits better so one heat range colder may be tolerated.

Another thing to note is that when you go to a parts store and ask for a specific plug number the guy may not know what you're talking about. That's because the sales part number is a "stock number" not the actual plug number and the stock number is usually for four spark plugs, but sometimes six. He should be able to cross the plug number to a stock number on his computer, but don't count on it. Know the stock number for your selected plug number If the part store doesn't "stock" what you want, ask them if you can special order them.

Spark plugs can be confusing, so owners need to do some homework. Don't expect to get the "best fit" plug when you go to a parts store and say you need plugs for your '66 427 Corvette. You'll get whatever they cross cross reference from the old 43N, which is too cold. If you insist on AC your choices are the R45XL and R45XLS, which are both resistor type. If you want another brand, research their online catalogs that have the most recent cross cross references for the 45XL or 45XLS and the appropriate stock numbers.

Once you locate what you want, make a note in your maintenance records, and if they prove satisfactory in service buy another set or two. The correct plugs for our vintage Corvettes are not used by modern vehicles, so who knows how long correct plugs for our cars will be available.

Duke