Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

[Bob B.]

Hi all,

I am running a '70 LT-1 350 in my '60. I was going to change the plugs and got a set of AC Delco R43S plugs at the parts store, but i see they have and eighth of an inch more threads than the non-S. The pistons are stock, so they are domed, and I don't want the piston hitting the plug. Should I use the R43S's?

I have three NOS boxes of Holley 25-54 performance spark plugs. In smaller letters, it says 43W. I assume 43 is the heat range, but no idea what W means. Anyone know what W means? Can I use these in my engine? What was the original application?

Lastly, I have a couple of sets of AC Marine M44C. Again, I assume the 44 is the heat range. But what does the C mean? Any reason to not use these?

Thanks,

Bob

[Joseph L.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

You can use an R43 with regular thread reach. That is a very cold plug unless you are racing or never idling in traffic. In 1957, they were AC44 at first and 46 later from the factory. 58-62 were AC44, but these were always too cold. I have successfully used AC R45 or R46.
Joe

[Jeffrey S.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

Bob,
The "W" suffix on AC plugs refers to "Recessed Termination" (whatever that is). The reach or length of threads of the 43S and 43 should be identical- 3/8". The "3" in the 43 is the heat range which, as Joe correctly stated, is too cold. The original plug specified for the '70 LT1 is an R43 which was good for the intended purpose of the LT1 which was higher RPM racing. A "5" heat range is perfect for that engine for just driving around.
The C suffix refers to a colder version of the older M44 plug. M=marine, 4= 40 mm threads, 4= heat range.
Jeff

[Duke W.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

GM consistently over the years installed plugs that were too cold for normal road driving including extended freeway trips at 80+. In sixties vintage small blocks the standard plug was the non-resistor AC 44. In the AC numbering the first "4" means 14 mm thread, 3/8" reach, 13/16" hex. gashet seat. The second number is the heat range, and the higher the number, the hotter the plug. Due to constant fouling I replaced the OE type AC 44 with AC 45 in my SWC's 340 HP engine, and replaced them with AC 43 when I spent a day hot lapping the Kent 2.25 mile road course.

GM never used extended tip plugs like the AC 44S in small blocks. Theoretically the extended insulator "broadens" the heat range and will burn off deposits faster than the standard type. Also they subtract one to two cc from the combustion chamber, so there is a slight increase in compression ratio, and high compression pistons can actually hit the ground electrode, though usually only if the heads have been excessively milled.

GM began using resistor type plugs in 1969, supposedly for better RFI suppression, but the resistors dissipate energy, so less arrives at the plug and breaker point systems are marginal offering only about half the per spark energy as the TI and HEI.

I have no idea who currently manufactures "AC" spark plugs. Also, they no longer offer non-resistor plugs that were OE thorough 1968. GM hasn't made them in house for 30 years or so. AC/Delco is just a marketing outfit that contracts with manufacturers and likely goes with the lowest bidder. What does that tell you?

I don't recommend AC plugs unless you are campaigning your car in NCRS judging. For small blocks used in normal road driving I recommend the Denso W14-U as a first choice and the NGK B4 as a second choice. These are equivalent to the long-out-of-production non-resistor AC 45. As long as the carburetor/FI system is delivering a reasonable cruise and power A/F ratio these plugs should last at least 15K miles with breaker point ignition systems and at least double that with TI or HEI.

Chevrolet also specified heat range 3 for big blocks, and I have no idea why, but its seems like the OE heat range recommendation for big blocks and the LT-1 was as if they thought owners were racing them all the time.

Heat range choice is based on average service load, not peak engine output, and one can't operate even base engine Corvettes at high load for any significant amount of time unless you are hauling a heavy trailer in mountainous terrain, so Corvette engines need a relatively "hot" spark plug to keep the insulator temperature above the minimum value to burn off deposits in normal driving including long distance freeway trips at not-to-high above the legal limit.

I spend the summer of '67 In Southern California and prior to heading back to Seattle for my senior year at Udub, I drove to Las Vegas for a weekend. Nevada had no highway speed limits back then and the brand new I-15 was dead flat and straight for the 40 miles from the California-Nevada border to the Strip, so I dialed up 5000 revs from the engine room (130 MPH with the 3.08:1 axle).

I made it to the first Strip exit in 18 minutes, and the "hot" plugs didn't seem to mind.

Duke

[Owen L.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

GM consistently over the years installed plugs that were too cold for normal road driving including extended freeway trips at 80+....

Chevrolet also specified heat range 3 for big blocks, and I have no idea why, but its seems like the OE heat range recommendation for big blocks and the LT-1 was as if they thought owners were racing them all the time.

Duke

Duke,
What, if any, role might the gasoline additives (like lead) play in GM's spark plug decisions? It appears that when unleaded was mandated for '72 and onward, the spark plug for the LT-1 and the big blocks went to R44T. Of course the hotter plug likely helped with meeting emissions, but could the hotter plug be necessary for unleaded to perform better?

[Jeffrey S.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

Duke,
There is one exception (that I know of) to your comments concerning resistor plugs starting in production is 1969. That is 1955 Corvette V8 used 43-5R. This was a resistor plug. I guess GM used colder plugs based on the intended use of the car. BB, LT1 , etc were generally purchased with high performance use in mind. Just a guess but on my part but you know far more than me about this stuff.
Jeff

[Duke W.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

Duke,
What, if any, role might the gasoline additives (like lead) play in GM's spark plug decisions? It appears that when unleaded was mandated for '72 and onward, the spark plug for the LT-1 and the big blocks went to R44T. Of course the hotter plug likely helped with meeting emissions, but could the hotter plug be necessary for unleaded to perform better?

The primary reason for "fouled plugs" back in the day of leaded fuel was lead oxide deposit build-up on the insulator, which bleeds energy away as voltage builds up on the center electrode, eventually leading to misfires. Unleaded fuel about doubles spark plug life assuming proper carburetion/FI calibration.

GM 1971 models and up were designed to operate on 91 RON unleaded fuel (equivalent to today's 87 PON), and Ford and Chrysler followed suit the following year.

Thanks for reminding me of the T suffix... tapered seat rather than a gasket seat. I think they became standard on most Chevrolet engines for the '71 model year, but other than that geometry change they had no functional effect on spark plug performance. If Chevrolet changed the big block heat range to 4 in the early C3 era, it was probably due to them realizing that heat range 3 was too cold for normal street and highway driving.

They were likely conservative on spark plug heat range out of concern that a few owners would run their cars hard for extended periods. Overheated insulators can cause pre-ignition, which leads to detonation and possible severe engine damage.

Spark plugs have no effect on emissions unless they are too cold and foul out causing misfires that will drive HC through the roof.

Duke

[Duke W.]

Re: Sparkplugs: Holley 25-34, AC Marine M44C, AC R43S. Applications?

Duke,
There is one exception (that I know of) to your comments concerning resistor plugs starting in production is 1969. That is 1955 Corvette V8 used 43-5R. This was a resistor plug. I guess GM used colder plugs based on the intended use of the car. BB, LT1 , etc were generally purchased with high performance use in mind. Just a guess but on my part but you know far more than me about this stuff.
Jeff

Thanks for the correction. I'm not as familiar with early C1 models as I am with later models. IMO their choice of heat range 3 was way too cold, and I recommend the same plugs for early small blocks as I do for later models up to the end of gasket seat heads.

If non-resistor plugs increase radio noise, use a resistor plug of equivalent geometry and heat range as the AC 45.

Duke