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I have a 425hp 427ci and I am running stock plugs in it. I've seen it posted that a hotter plug is recommended. I don't see why. Is it the way we drive these cars now? Is it for better performance? Is it because of the gas that's available now? I find very few opportunities to use the available power from this engine, so I'm considering changing plugs. What is the currently recommended plug? And will I need to reset the timing if I change the the plug temp? Thanks.
Most of the C1/C2 cars like plugs in the 45 heat range..............vs 43 or 44 heat range specified by the factory. This fact was known back in the 1960's by many of us.......but just not communicated to everyone.
Not for a 427 engine. Service Manual shows AC 43N as factory plug........so I am recommending OP go to AC 44N or 45N. I believe the 45N will ultimately be his best choice.
Since these plugs are not readily available, OP should see what is available from others with 44 or 45 heat range equivalent.
If you follow EBay (fleabay) for awhile you’ll find nice NOS plugs in the range your looking for. After a couple of months I was able to find OEM plugs for my ‘63 that were new in box for $40….now I just need to install plugs and wires (date coded). 😁
L72 with lowered compression, and putt putt street driving? a 45, like a R45XLS would work fine. Go colder for hi compression/ hi performance setups. The higher performance the engine, the higher the cylinder temperature, the colder plug is then desirable (plug just hot enough to avoid fowling). The GM engineers specified 43’s ? they probably assumed the big port, four bolt, solid lifter 427’s would be used as they were designed to be used :-)
Time to pull the plugs and have a look. It does sound like a switch to a 44n equivalent would be more suitable. Engine appears to be unmolested with TI. What sent me down this rabbit hole was a bit of inconsistent idle after in town driving. A quick rip through the gears seems to sort it out. Unfortunately that isn't always an option. Plus I'm out of safe driving speeds b4 I'm out of 2nd. Steering is needs some love. Thanks for all the help.
I have used AC 43N in my L71, L72 and two L36 cars with no trouble, perhaps I am just lucky in this respect. New AC 43N are running around 150.00 a set.
If the car is running well ,I would not try and fix it but if miss firing or fouling then listen to the fellas on the forum. If not pinging heavily or detonating leave the
timing alone.
Mike
Unless you need a set of 43Ns with the old AC logo on em for judging, the XLS series plugs will work fine and they're much cheaper. I'm with 435er. 43 heat range plugs (43XLS) work just fine, even in extended idling in traffic. They may get a little dark after a lot of idling, but I haven't fouled one yet after 12 years and about 8000 miles on the current set. And that's with 10.25 compression, not 11+ and about the same cam specs as factory L72. Yes, as powershift points out, most C1/C2 cars like 45 heat range plugs. Then again, most of em aren't big blocks.
2020 Corvette of the Year Finalist (appearance mods)
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I was AC all the way until I tried to remove one from my truck's 350, 1992. The darn thing broke off and left only the threaded part in the HEAD!!! I've never seen one do that in my life but I had to wonder where those metal casings are made. Seems like more of a Chinese metallurgy type thing. I had to track down an oversized ez-out and just barely got it to bite into the thin casing left in the head.
I switched to Bosch Platinum and have never looked back.
He said its a L-72, so it needs a long reach plug. R45 is a 3/8" reach small blocks. The current "correct" plug for a big block (resistor type only) for normal road driving if you insist on AC is R45XLS. If you don't need AC plugs for judging my recommendation is a non-resistor plug, which was OE. Non resistor equivalents to the out of production non-resistor 45XLS include the Denso W14EX-U and NGK BP5ES
The OE plug was 43N. These were replaced with the XLS series but heat range 3 is too cold for normal road operation. AC heat range 5 or equivalent in another brand is best for normal road use on both small blocks and big blocsk. All major spark plug brands have cross reference charts online.
Most of the C1/C2 cars like plugs in the 45 heat range..............vs 43 or 44 heat range specified by the factory. This fact was known back in the 1960's by many of us.......but just not communicated to everyone.
Larry
In the AC numbering system the first digit, in this case "4", indicates the thread size, which is 14 mm. The second digit is the heat range running from 1, coldest, to 6, hottest.
L72 with lowered compression, and putt putt street driving? a 45, like a R45XLS would work fine. Go colder for hi compression/ hi performance setups. The higher performance the engine, the higher the cylinder temperature, the colder plug is then desirable (plug just hot enough to avoid fowling). The GM engineers specified 43’s ? they probably assumed the big port, four bolt, solid lifter 427’s would be used as they were designed to be used :-)
For road use the compression ratio, cam specs, whatever is not an issue. The issue is the average load the engine sees. Whether big block or small block Corvette engines in road use operate under low average load, so you need a hot plug to keep the insulator from fouling due to excess deposits that can short energy back to the steel shell and cause misfires. At the same time you can't get AC heat range 5 or equivalent to overheat in normal road use unless you use your Corvette to haul 10,000 pound trailers up over Loveland Pass.
The current "correct" plug for a big block (resistor type only) for normal road driving if you insist on AC is R45XLS. If you don't need AC plugs for judging my recommendation is a non-resistor plug, which was OE. Non resistor equivalents to the out of production non-resistor 45XLS include the Denso W14EX-U and NGK BP5ES
Duke
Trent - Perfect timing with this thread, I was just about to look up the spark plug recommendations for my Nassau Blue 66 L72, now running a replacement Bigblock with a solid lifter cam. Just got it a few days ago and I'm already going through the service replacement #3247 due to it sitting for long periods and old gas.
The plug I just pulled out of it is an AC Delco R43XLS and very sooty looking, which no doubt was caused by the old gas (color of 'tea'), incorrect carb adjustments (mixture screws 1/2 and 3/4 turns out), etc but while I'm freshening up the carb, I'll be looking for a set of replacement plugs and the R45XLS seems to be the one I'll need.
Mike T - Prescott AZ
For road use the compression ratio, cam specs, whatever is not an issue. The issue is the average load the engine sees. Whether big block or small block Corvette engines in road use operate under low average load, so you need a hot plug to keep the insulator from fouling due to excess deposits that can short energy back to the steel shell and cause misfires……..
Duke
Well said. HD use, colder, mild/street hotter…… like i said the GM guys must have assumed more HD use than not when they selected/specified the 43’s.
Interesting on the impact of compression?? I do understand the “load” being the primary consideration / dominant factor.
The below from “Champion” is what we were always lead to believe? However, i’m not aware of their technical basis for this recommendation?
Champion Spark Plugs, for normally aspirated, gasoline-fueled engines, a good rule of thumb is to go about one heat range colder for each full point in compression ratio increase from 9:1 through about 12.5:1, and two heat ranges colder for each point increase between 12.5:1 and 14.5:1. Beyond 14.5:1, 3-4 heat range reductions per point may be needed.
Another old hot rod adage was when you modified a stock engine, for every 75HP or so added drop the plug one step cooler from factory, the bases? Who knows, lol….. probably shade tree engineering.
Resistor plugs have a (smaller) secondary air gap vs non-resistor plugs which have none. If I remember correctly, one advantage was to reduce misfiring and cross firing from induced voltage from spark plug wire routing. But obviously, jumping the secondary gap results in less spark energy at the primary ignition gap inside the combustion chamber.
Duke or others will likely have additional information to share.
Sparkplug recommendations?
