I’m confused. I see all kinds of opinions about iridium and platinum spark plugs versus the normal copper plugs. Some say they’re good while others say not to use them because they’re only for late model cars that are concerned with emissions. I like the idea of spark plugs that could last longer. I have a 355ci small-block Chevy in my ’67 C-10 pickup. It has a mild cam, aluminum Trick Flow cylinder heads, an Edelbrock Performer RPM dual plane, 750cfm Holley and an HEI distributor. What plugs will work the best? – E.P.
Jeff Smith: This is a highly volatile subject on which everybody has an opinion. The problem is that engines don’t run on opinions—they operate based on physics and facts.
Reeves isn’t just an engineer. He’s also an enthusiast with a small block Chevy-powered Nova.
To get started, we need to establish a few foundational facts. Before we can get into deciding if the fine-wire plugs like platinum and iridium are better, let’s make sure that the design and heat range of the plug is correct because tip material is irrelevant otherwise.
In this case, we’re dealing with a non-stock application. Most, if not all of the Trick Flow Specialties street aluminum heads call for a 3924-style Autolite spark plug. This is a 14mm thread that’s 0.750-inch long, with a copper core center electrode and a gasket seal to the head.
This plug is also an extended-reach design—Autolite calls it a Power Tip.
With this extended-reach design, the center electrode is extended beyond the end of the threaded portion of the shell. This projects the spark closer to the center of the combustion chamber and is also the hottest spark plug in this range of plugs. This makes it a good choice for the street. The heat range of the plug establishes how efficiently the ceramic core will burn off carbon deposits. A hotter plug will burn these deposits more quickly than a colder plug.
Hotter plugs typically work better for the street while colder plugs are better suited for competition applications where the engine will run at full power for extended periods—like in drag racing or circle track racing. This extended nose plug is, by design, a hotter plug because the extended nose makes the insulator path longer back to the body of the plug. The longer the insulator path, the hotter the heat range.
So now that we know the basic spark plug design and its heat range we can move on to some of the other details. Your question is about the fine-wire spark plugs like the iridium or platinum spark plugs.
There are plenty of opinions floating around the internet, but few facts. The truth is that these fine-wire spark plugs are an outstanding choice for a performance street car.
The fine wire spark plugs require less voltage to fire compared to the larger copper electrode, which makes things easier for the ignition system and allows for an increased plug gap to fatten up the spark a little. Some believe these plugs will hurt a turbocharged or supercharged engine and yet the 2018 Dodge Hellcat sports a 707-hp supercharged Hemi and calls for an iridium spark plug as original equipment. The OEs choose these plugs because they are durable and will last a long time.
For the street, the heat range is among the most important decisions you need to make.
If the engine is only mildly modified, then a stock heat range spark plug is a great plan. Even a high-powered supercharged engine may only need a one- or two-step colder plug and even then, only when the engine is pushed very hard.
In a drag racing application, you’d want a colder plug for a boosted or nitrous application. In those situations, you want a cold plug with a very short ground strap so that it doesn’t get super-hot and cause pre-ignition. I’ve had that happen on the dyno and the engine destruction isn’t pretty, plus it’s very expensive.
It’s beyond the scope of this column to get into tremendous detail on spark plug selection, but the point is that upgrading to a set of fine-wire plugs like an iridium or platinum spark plug with the same heat range, is a good choice.
These plugs will last a long time and do the job with no ill effects. Spark plugs rarely help make power unless the previous plug has shorted out, but choosing the right plug and the best heat range will ensure that it doesn’t cost power. It’s all about the details.