Spark plugs are a critical element of any internal combustion engine. They are the proverbial wizard’s wand that ignites the air/fuel mixture to create the glorious and mysterious magic we know as horsepower.

If we look around, there is an endless array of different plug offerings, different electrode styles, different heat ranges and even different material. To the list of different plug varieties, we can add changes in plug orientation, or more commonly known as indexing.

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Being performance enthusiasts, the question we want to know is, do these changes in spark plugs really make a difference in power?

After all, the goal of the spark plug is to ignite mixture and initiate the flame front, but can one of these changes do a better job than another? Can multiple electrodes provide multiple sparks to supercharge the ignition process? Can a wider plug gap provide greater spark energy to achieve the same result? What about plug indexing, does the direction of the spark change said result?

Well, after endless plug changes and dyno pulls, here is what we found out!

man holding a pair of spark plugs
How much power can trick electrode designs, heat ranges, or even plug indexing really add? (Image/Richard Holdener)

Test 1: Comparing Plug Types

In this test we ran a comparison of four different plug types on our junkyard 5.3L test motor.

The test motor was a tried and true 5.3L, but not your usual (for us) iron-block, run-of-the-mill base LM7. This test motor was of the all-aluminum variety, meaning in additional to the usual array of aluminum LS heads, this 5.3L was also sporting the very desirable aluminum block. The all-aluminum L33 5.3L was a big score from a local wrecking yard, and featured flat-top pistons (a la 4.8L), high-flow 799 heads and a slightly more aggressive cam than the base LM7 5.3L.

Over time, we had changed out the stock cam for a Red Hot cam and matching valve springs from Brian Tooley Racing, along with a FAST LSXR intake manifold and 102mm throttle body. Additional components included a set of 1-7/8 inch long-tube Hooker swap headers with collector tensions, 80-pound injectors and the Holley HP engine management system.

For this test, we ran the following spark plugs:

The plugs offered changes in both material and electrode design, but all three were the same heat range. Run with these three (actually four) different plugs, there was very little difference in power (one to two hp variations). The Copper Core plugs showed a maximum difference of four horsepower in two spots, but this test showed that there is not big power to be had from changes in different styles of spark plugs.

We also ran the combination of used plugs (various heat ranges and manufacturers) that came from the wrecking yard—they produced the same power as the fancy new plugs.

Test 2: Plug Indexing & Heart Ranges

Starting with heat ranges, it should be noted that changes in the heat range of the plug are usually considered when running boost. We typically go to a colder plug when running a turbo, blower, or even nitrous, to help ward off detonation—but we did nonetheless, test the effect of heat ranges had on power on our naturally aspirated 5.3L.

In this test, we compared two sets of Denso Iridium plugs (hot heat range) Q16s to a set of (much colder) IQ31s. The IQ16s would be similar to stock heat range plugs for our NA 5.3L, while the colder IQ31s are plugs we normally run on high-boost turbo applications (like the 1,543 hp Big Bang 6.0L).

There was no significant difference in power between the two heat range plugs on our cammed 5.3L test motor.

While the motor was still on the dyno, we also tested the effect of indexing (electrode orientation) the plugs. The idea behind indexing was to aim the opening of the plug gap (and initiation point of ignition) toward a particular position in the chamber (i.e. intake valve, exhaust valve, dead space etc.). By marking the plugs externally to indicate the electrode opening and using washers, we oriented the plugs first toward the intake valve, then toward the exhaust valve, then randomly however they happen to tighten into the head with no washer. We also tried randomly moving the plugs to different cylinders.

In the end, regardless of the plug position of there was no significant (meaning measurable and repeatable) change in power.


Having also tested different plug gaps (reducing it down to just a 0.010 inch gap finally showed a slight change in power), we can say that on this test motor, there was not big power to be had from any of the plug changes. The winner, if any, were the cheap copper core plugs, but even that power gain would diminish over time as the copper core wore out.

The great thing about the premium plugs might not be power, but longevity, as the precious metal plugs will run for years. Some OEM recommend plug changes (with platinum) at ridiculous mileage intervals (like 100K). Copper plugs will definitely not last that long.

It should also be noted that our test motor had something many test motors do not, and that was an excellent (multi coil) ignition system. We even (as we always do) had the dwell cranked up to 4.0 ms, all but ensuring a hot (misfire free) spark. With a single coil/distributor, where misfire might be more common, curing or eliminating the misfire with one or more of these changes, could certainly yield more power, but the culprit would be the lack of power from misfire, and not the component or change itself.

The takeaway here is don’t go looking for big power gains with spark plugs unless you are curing misfire!

engine dyno chart
This graph shows the minimal power gains offered by the change in spark plug design and material. We ran the standard copper Autolites vs the NGK Laser Platinums and E3 Diamond Fire plugs with very minimal changes in power. In fact, the cheap Autolites might have been a touch better in power than the exotic plugs, but in terms of longevity, the cheaper copper plugs would certainly not last as long as precious metal plugs. The same results came from the testing on plug indexing, heat ranges, and even mixing and matching used plugs from the junkyard. On this motor (with an excellent individual-coil ignition system), there was no significant change in power from any of the plug tests. Simply put, if the plug did not misfire, the power was always there. There is a discussion to be had about other (non-WOT) gains or changes with plugs (emissions, mpg, or idle quality) but just don’t go looking for big power gains. (Dyno Chart/Richard Holdener)
ls engine on a dyno test run
Perfect for our many plug tests, our test motor started out life as an all aluminum, high mileage junkyard L33 pulled from a local wrecking yard. (Image/Richard Holdener)
5.3L casting mark on an ls engine cylinder head
Always a desirable find in a junkyard, the L33 5.3L featured an aluminum block along with higher-compression, flat-top pistons and a slightly more aggressive factory cam profile—compared to the de rigueur iron-block LM7. (Image/Richard Holdener)
btr camshaft in a box
From previous testing, the L33 test mule was equipped with a BTR Red Hot cam that offered 0.617/0.619 in. lift split, a 221/22X degree duration split and 113 LSA. (Image/Richard Holdener)
close up of 799 casting number on an ls engine cylinder head
Finding the L33 aluminum block 5.3L also meant the motor was factory equipped with high-flow (compared to 706/862) 799 heads. These, along with their 243 counterparts, were the highest flowing cathedral port heads offered by GM. (Image/Richard Holdener)
close up of valvetrain, rockers, and springs on an ls engine
To prep for the BTR cam upgrade, the 799 heads were treated to valve spring upgrade, though the rockers remained factory stock (no trunnion upgrade). (Image/Richard Holdener)
top end of an ls engine on a dyno
Working with the high flow 799 factory heads and BTR cam upgrade was a FAST LSXR intake manifold. In the sub-7,000 rpm range on an LS (like our 5.3L), the FAST would be tough to beat for average power production. (Image/Richard Holdener)
102mm throttle body on an engine
The FAST LSXR intake featured a 102mm throttle opening so we saw no reason not to utilize the maximum airflow potential of the intake with a matching 102mm throttle body. (Image/Richard Holdener)
headers on an engine on dyno
The exhaust system consisted of a set of 1-7/8 inch, long-tube Hooker (swap) headers feeding simple collector extensions. (Image/Richard Holdener)
close up of fuel rail on an engine
To ensure plenty of fuel flow to the modified 5.3L motor, we replaced the stock L33 injectors with these 80 pounders from Accel. (Image/Richard Holdener)
man at dyno desk doing engine tuning on a computer
Rather than rely on the factory ECU, we dialed in the AF and timing curves on the 5.3L using a Holley HP management system. (Image/Richard Holdener)
man holding packages of spark plugs
The first test was to compare four types of spark plugs, the used and mismatched plugs that came from the wrecking yard with the motor to three new sets: basic Autolite copper-core plugs, high-zoot NGK Laser Platinum plugs, and E3 plugs with Diamond-Fire Technology. (Image/Richard Holdener)
the tip of a spark plug with 4 electrodes
Would the trick, multi-prone electrodes (designed no doubt to provide multiple access points for the voltage to jump the gap) show any gains? As it turned out, the answer was no, as the cheapest Copper Core Autolite plugs actually made a few horsepower more in a narrow rpm range. Obviously the longevity of the copper plugs would not compare to the precious metal plugs. (Image/Richard Holdener)
a pair of spark plugs with indexing marks
Next up we tried indexing the plugs. We marked the orientation of the electrode and positioned it first facing the intake valve, then the exhaust valve, then away from both. (Image/Richard Holdener)
a spark plug with indexing washers and spacers
These spacers were used to properly locate the electrode orientation, and yes, we tried spacers with no change in plug orientation (minute compression change and recessed plug). There was just no power from these changes on this motor with its (it must be pointed out) high-power, individual coil (near) plug factory ignition system that has successfully powered 1,500+ hp turbo motors. (Image/Richard Holdener)
long spark plug tip in a cylinder head combustion chamber
This photo not only shows the positioning of the electrode, but also a hotter plug (which we also tested). How much of a change you might see (we saw none) would come just from displacing some of the chamber volume? (Image/Richard Holdener)
spark plug tip in a cylinder head combustion chamber
The colder plug is shown with an altered electrode orientation (open toward intake valve). Again, we saw no change in power from either orientation or heat ranges. We do, however, run colder plugs on boosted/nitrous applications—though this stock compression truck motor was hardly in a position to need colder plugs (Image/Richard Holdener)
spark plugs and feeler gauge on a table
We took a big swing at different plug gaps as well, testing a very narrow 0.010″ gap against a very wide (but no still misfire) 0.100″ gap! (Image/Richard Holdener)
ls engine on a dyno test run
The bad news is that our 5.3L test motor didn’t really respond to changes in plug design, orientation (indexing), or material—but the good news is that it seems hard to lose power as well. If the plug goes sparky sparky, you are probably in good shape. (Image/Richard Holdener)

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Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.