spark plug questions for 427

[Bill S.]

i bought plugs for my L72 they are R43XLS. what does XLS stand for and are they correct? the manual shows a cold and hot plug. what would be best for todays octaine. i suspect this to be the root of a problem i have.

[Clem Z.]

Re: spark plug questions for 427

XLS stands for 3/4" reach with extended tip. i would install R45XLS unless you are racing the car. since i never understood this L-XX stuff so what year is a L-72 because after 1970 BBC used short reach plugs

[Bill S.]

Re: spark plug questions for 427

L72 is the 66 425 hp with 11:1 and solid cam also used in the COPO's ,thanks bill

[Dick W.]

Re: spark plug questions for 427

Follow Clem's advice. The 43's are too cold a plug to drive on the street. We never put anything colder in the 427's than a 45. Even L/88's.

[Mark K.]

Re: spark plug questions for 427

Where is the best place to buy these R-45 spark plugs? I also have a 427 currently running R-43s which, when hot, causes stalling. I assume R-45s will alleviate that problem.

[Terry F.]

Re: spark plug questions for 427

Can you explain a little about hot and cold plugs. Does it have anything to do with the spark or is it simply a heat range that describes how hot the plug will run while in use? Does a hotter plug improve the quality of the fuel mixtur burn? If you use too hot a plug does it potentially lead to pre-detonation? Also, what was the purpose of transitioning to resistor type plugs? I always thought it had to do with radio interference, electrical interference?? Sorry for all the question but it was always stuff I have wondered about. Terry

[Duke W.]

Re: spark plug questions for 427

A hot plug does not transfer heat to the cooling jacket as fast as a "cold" plug, so you should use a "hot" plug for normal road driving and a "cold" plug for sustained high output like road racing and top speed running on the autobahn. The relative heat transfer rate depends on the detail design of the insulator geometry. If you look at some of the major plug manufacturers
Web sites, you will likely find more detailed information including cutaway drawings and the proper insulator temperature range.

Too cold a plug will foul out quickly in normal road use because it doesn't get hot enough to burn off deposits. Too hot a plug for conditions can run too hot and damage the insulator or cause pre-ignition (There is not such thing as "pre-detonation".), and since pre-ignition is effectively the same as advancing the timing, it can get the engine into detonation, which can cause damage if it's sustained.

For the vast majority of road use AC heat range "5" or equivalent in other brands is best for ALL vintage Corvette engines, even though the OE plugs were colder, like heat range 4 or 3. Heat range five is the recommendation from most experienced vintage Corvette owners. The second number in the AC system is heat range. The first number "4" means a 14mm tread. The major plug manufacturers explain their numbering systems on the web and have online catalogs and cross reference charts.

The resistor type plugs came into use for additional RFI control.

Duke

[Terry F.]

Re: spark plug questions for 427

I see. Interesting because there are so many little variables. I forget the math involved but each variable that you add to the equation to try improve an engines performance increases the possible combination of variables by some factor. Wonder which is more important, having a hotter plug vs being able to advance the ignition timing. I suspect that the timing map has an ideal configuation based on the cam profile regardless of what else is going on. I also suspect there is a limit to the point at which you can improve the burn of the air fuel mixture based on spark plug design and you should stop there if it requires you to monkey with your timing map too much. There are a lot of variables to an engines performance. Thanks for the information. Terry

[Duke W.]

Re: spark plug questions for 427

You are correct that there are a lot of variables, and most are interrelated, but spark plug heat range choice is primarily a matter of driving conditions. It doens't matter is you have a '58 Biscayne with a 283 2-bbl. or a L-72. For road use, AC heat range 5 or equivalent is usually the best choice.

You will read that compression ratio and engine power affect heat range choice, but this is not true unless the car is being raced, driven at top speed, or towing a heavy load. A high power engine in a light car is rarely under significant load for more than a few seconds.

For a given combustion chamber design, the total WOT timing required for peak power is pretty much the same regardess of engine configuration. The vacuum advance must be matched to the engine's idle vacuum characteristics, which is why a 30-30 cam engine needs a different vacuum advance control than a base cam engine. Then you endeaver to bring the centrifugal advance in as quickly as possible to the detonation limit. A SHP engine will tolerate a much quicker centrifugal curve than a base engine due to the late closing inlet valve and lower dynamic CR with SHP cams.

So. though both base and SHP engines will likely work best with the same spark plug and require the same total WOT timing, their ideal spark advance maps throughout the operating range will be very different.

Duke

[Terry F.]

Thanks, Terry *NM*