Spark Plug of Choice

[bd]

Aluminum heads transfer heat more rapidly than iron so better tolerate high compression w/o detonation.  But, installing a plug that is too hot can introduce preignition.  Choosing a performance plug is, in part, a matter of selecting heat range and electrode style so that the plug cools effectively w/o fouling while being hot enough to not introduce preignition over a broad temperature range.  An extended tip plug is a good choice for street use, because the electrodes are cooled by the influx of fuel and air while locating the spark kernel deeper into the combustion chamber.  However, selecting heat range is partly a matter of experimentation.

The Delco 41-106 is used predominantly in four-bangers.

Delco 19308030 (#5 RapidFire Platinum) is used in late 80's, early 90's Vettes and should work in your engine as a reasonable starting point.  You can always go up or down in heat range as you dial it in.

[LTZ C20]

Ok the delco 19308030 you provided sounds good. I put a set of rapid fires in my old engine and they didn't seem to work very well. BUT, that was an old iron 350 that was carb and I eventually found it was eating oil pretty bad. I possibly also got a cold plug, maybe 3 or 4. So, has anyone else had problems with rapidfires? One of the guys at work hates them lol. I got no problem using them as long as I know it was just my situation and not the plugs that was bad. I would prob get a 5 or 6 if I use those rapidfires.

[bd]

Kevin, don't misconstrue the information I've posted.  I don't "recommend" Rapidfire; it's just that you insist on Delco.  You could just as easily use a conventional 41-602 (5614285).  In my dealership experience Rapidfire is just another platinum spark plug.  They work no better and no worse than any other Delco plugs.  The biggest problem with Rapidfire is the number of offerings.  It's sort of a one size fits all, meaning that Delco attempts to cover too many applications with too few part numbers.  They're certainly no miracle - the mystique of Rapidfire amounts to advertising hype and sales!  Clever way to move a product.  But, then, that same approach is used by all spark plug manufacturers.  There's only so much to be accomplished by a spark plug - its actual purpose is limited and very specific.  Conversely, referring to claims that all "choose the brand" spark plugs are junk is ridiculous (although one exception does come to mind - ).  Ignore the hype!

The "secret" to choosing the right spark plug is to settle on the one that works "best" in the engine.  If the plug is selected accordingly, it will yield respectable service.  If not, it may well tank - and in some cases result in engine damage.  It's a matter of finding the right fit.  So, pick a starting point (brand, type, heat range).  Regarding Delco, I would stay within the heat range of 3 - 5.  With any manufacturer, a cooler plug will be less prone to preignition.  Then experiment if you're not satisfied.  Try various brands, gap styles and heat ranges, until you find "the plug" that functions the best in "your" engine, while providing suitable service life at an acceptable cost.  You will likely find several that work, a few that simply won't, and one or two that shine.  I used Champion, NGK and Bosch, in various gap styles, electrode material and heat ranges before settling on the Champion RC9YC5.  In another engine, even of similar build, the optimum plug might be hotter, or a different electrode material, or a different gap.  Brand loyalty should be the very lowest of your concerns.  Choose the plug that works the best!

One other caution I should mention:  The extended tip style plug I previously recommended may interfere with some domed pistons, because of the electrodes' protrusion into the combustion chamber.  As long as you are running flat top or dished pistons this will be of no concern.  But, if you have installed domed pistons, then spark plug-to-piston clearance should be checked along with valve-to-piston clearance.

[LTZ C20]

That's all I want! Lol I've been unsure where to start with. Should I be looking in the low or hi heat range. If what I've heard about rapidfires is true or not. Now you gave me a starting point, 3-5. You also said Rapidfires do there job in the right application, that reliefs my worry. So now I know if they don't seem up to par, it may not be the plug, it's that plug in my engine,  no big deal. What's better or not for aluminum and TBI.

I have dished pistons, I checked when I was assembling the engine and I have good valve clearances,  also I've had the plugs out before and they weren't damaged so I'm pretty sure that won't be an issue.

Thank you very much for your help and patience,  there is a science behind spark plugs and the more I've been working on this little plug search the more I learn and get confused also haha. I really appreciate all your help.

[bd]

You are welcome!

BTW - Be sure to chase the spark plug hole threads before installing the longer reach plugs if you have run the engine, since carbon builds up in the exposed threads of the holes.  Additionally, I recommend installing the plugs with oiled threads or a sparing amount of "copper" anti-seize.

[LTZ C20]

What should I use to chase the threads? Just a 14mm bolt? Specific size and thread pitch?

I always use copper anti sieze.

[LTZ C20]

Also, what's the heat range on the Champions you use.

[bd]

What should I use to chase the threads?

Use a tool designed to chase 14mm x 0.750" reach spark plug threads, such as a Powerhouse Products POW351690 or Lisle 20020, etc.  It's a good tool to have in your toolbox.  The last choice would be an M14 x 1.25 tap, because you do not want to remove any metal from the head.  I would not recommend using anything else to chase spark plug threads in aluminum heads!

Also, what's the heat range on the Champions you use?

Champion 9, which roughly corresponds to an Autolite 3 and an AC Delco 1.

[LTZ C20]

Ok. I will locate a tool or tap at the local auto parts store or hardware store.

Ok cool. I did I search on summit racing and selected the physical specs for the plugs, I didn't not specify brand so I got choices from alot of brands. I will sit down and make a chart so I can easily identify the best candidates and there availabilty.

On a side note, don't you live here in california?

[bd]

Purchase the rethreading tool.  Use a tap only as a last resort.

I'm about an hour and a half south of you.

[LTZ C20]

Oh ok. I'll do my best to find one. In fact one of the guys in the shop at work might have one.

Yea that's what I thought. Not to far at all.

[LTZ C20]

Hey BD, so I made a chart with a selection of plugs from AC Delco, Autolite, Champion and NGK. All plugs meet the physical specs for the heads. The chart has their part number's, heat range, price and material, including copper, nickel, platinum and iridium. Here is the chart.