In my mind there are three chief advantages to the Denso Iridium Power spark plug, if the engine is subjected to regular high loads or a street high-performance or racing duty cycle: Cut-back, U-Grooved side electrode, the use of an iridium/rhodium alloy in the center electrode and the laser welding process used to assemble the plug.


What I think are the two chief advantages of the Denso

Forty years ago, Nippondenso (now just "Denso" and part of Toyota) patented the "U-Groove" ground electrode design. initially, the U-Groove was made famous with car enthusiasts in the U.S. by Accel, which has had Denso make its spark plugs for many years. In recent years, Denso sort of "re-launched" the U-Groove as an in-house design feature using the "Iridium Power" spark plug which incorporates the U-Groove as well as other performance-oriented features. The theory behind the U-Groove is that, just after the spark gets the air-fuel charge burning, it allows the flame kernel to fill the gap formed by the U-Groove. This develops a larger and hotter flash front which enhances combustion. The flat ground electrode design of ordinary spark plugs tends to "crush" the spark, inhibiting its growth. Engineers call that a “quenching effect” and it can degrade ignition effectiveness and, in extreme cases, cause misfire. The U-Groove feature results in enhanced combustion in the early stages of the process. While the U-Groove side electrode has its performance advantages, its original development was for improved ignitability, which can improve exhaust emissions and fuel consumption. The U-Groove giving the flame a "groove to grow in", enhances the ignition process, especial with a lean air-fuel ratio. With a plug having a conventional side electrode, the overall volume of the flash front can be smaller. One conventional method to improve firing of a plug with a flat side electrode was to increase the spark gap. This can detrimentally affect the spark if the ignition system is not powerful enough. With a U-Groove, ignition can be improved without increasing the spark gap.

To help improve ignitability, besides incorporating the U-Groove, the Iridium Power plug minimized the portion of the ground electrode which that comes in contact with the flame nucleus by tapering it and cutting it back slightly. Because the area of the ground electrode where the tapered cut comes into contact with the flame nucleus is small, the heat lost to the electrode is lessened and firing performance improves. The surface area and mass of the ground electrode tip is reduced along with the load in terms of heat and vibrations. This means the spark plug can cope with heavier driving conditions.

In the late-'90, Denso developed and patented an Iridium alloy which is superior to platinum in hardness, strength, melting point and electrical conductance. The new Iridium alloy is composed primarily of Iridium with rhodium mixed in for oxidation wear. Pure Iridium has a melting point of almost 1300°F higher than platinum, and had the greatest potential for resistances against wear. Iridium by itself is susceptible to oxidation wear at temperatures over 1650º and testing showed because of that, iridium alone would not meet Denso's durability standards. After testing several metals alloyed with Iridium, rhodium proved to be the best choice.

Additionally, the Denso Iridium alloy's better durability allows Iridium Power plugs to have what may be the smallest center electrode (.4mm) in the industry. Denso was able to shrink the surface area of the center electrode without it being destroyed by heat. A smaller center electrode reduces the plug's voltage requirement, ie: less energy is required from the ignition system to produce a given spark. Also, its smaller size, combined with the tapered U-Groove ground electrode, allows more space for the flame kernel to develop and produce more efficient combustion. Lastly, the .4-mm center electrode and the cutback, U-Groove side electrode exposes more of the air-fuel charge in the combustion chamber to the spark which further enhances the ignition process.


The tip of the IVT-20 which works well in a 2.4

Iridium is, perhaps, the most significant technological advance in automotive spark plugs since platinum was introduced as an electrode material in the early 1980's. As vehicle manufacturers made increasingly powerful and efficient engines, there was a greater need for long-life plugs which can tolerate higher combustion pressures. Denso responded with its Iridium alloy, originally for OE applications. Initially, Lexus and Toyota models used ultra long-life Iridium plug with a .7mm iridium/rhodium tip and a platinum-tipped ground electrode which was capable of up to 120,000 miles of service. Later, GM began using Denso indium-tipped plugs, private-labeled for the ACDelco brand, in performance V8 applications, such as the 405-hp LS6 in the 2002 Corvette Z06. Iridium Power plugs, with their .4mm center electrode, have been developed for aftermarket high-performance applications.


Stock AC Delco plug for the 2.4L in an HHR

Denso's Iridium alloy had the properties to make an outstanding spark plug, but conventional tooling prevented mass production. Due to the high melting point, there wasn't a welding process which could be used in high-volume production and was hot enough to join the Iridium tip to the nickel base. This problem spurred Denso to develop, a 360º laser welding process to provide a weld so reliable; the electrode resists wear even in the most severe combustion conditions. The 360° laser welding earned Denso it's third spark plug related patent.

A spark plug must radiate combustion heat from its tips, down the insulator, to the plug metal body, into the cylinder head material and finally to the cooling system. A spark plug’s "heat range" is a measure of the speed at which it dissipates heat and how hot the tips remain. The “colder” the plug, the more rapidly heat channels out of the plug and the "cooler" the tips remain. In a high-performance application, a colder heat range may be necessary because of the higher combustion temperatures caused by higher compression ratios, higher RPM, supercharging or nitrous oxide injection. On the other hand, while colder plugs may seem to be the way to go, remember that the spark plug must achieve its “self-cleaning” temperature where it can burn off carbon deposits. Otherwise, the plug could foul causing misfire. In fact one big reason OE's put fairly warm plugs in stock engines is the problem with fouling in engines which are subjected to duty cycles which include frequent starts without warm-up, such has what occurs during a car's pre-delivery period if when on a dealer's lot, getting backed on and off a transport truck or a roll-back or in the hands of an owner who only makes short trips or subjects the engine to extended idle periods. If you trailer your HHR to shows, only drive it a mile to work and back or have a panel used in pizza delivery or courier service with long idle times, stick to the factory heat range.

For best performance, you want the coldest heat range which will not foul during light load use. HHRs which are driven hard, or see primarily city or highway driving with no starts-without-warm-ups or short trips, may run better with a colder plug. A plug that is too hot can make the engine more prone to detonation or, worse, pre-ignition, and, in the worst case, engine damage. A good, rule of thumb is, for engines which are subjected to a high-performance duty cycle, start with one heat range colder than the factory recommendation and, in the case of a Denso for the 2.2 or 2.4L engines, that's the IVT-20. For street use, regardless of the engine, I use Densos one range colder than stock and the main reason is it lessens the engine's tendency to detonate under high load. If you're engine is highly-modified and is raced regularly, I'd go two ranges colder. Denso heat ranges are numbered higher as they get colder. A 16 is the hottest Iridium Power plug available and 34 is the coldest. Available are 16, 20, 22, 24, 27, 31, and 34.

As for highway mileage using the IVT-20, the best my HHR could do is 31.6-31.8 mpg. That's on a 10-mile stretch of coastal highway which is a mix of gentle uphill, gentle downhill and level ground. I ran the section of highway in both directions. Speed was 70-mph. Tires were at 28 psi front and 26 psi rear. Fuel was E10 89-oct pump gas. Altitude was 80-100 ft (essentially sea level) and the OAT was 65°F. The ECM calibration is my own with the mileage-enhancing, delayed power enrichment feature disabled.

I run Densos in all the cars I own because they perform well. At one time, I used exclusively NGK spark plugs but, in the early-'00's I was looking for a plug in a specific heat range and and with a specific tip configuration for the modified LT5 V8 in a Corvettes. I couldn't get it with NGK, so I tried the Denso IT-22. They worked so well, I put them in the rest of my vehicles, either the -20 or -22 heat range and I have been happy with the results, whether it's street driving, testing on the chassis dyno, running the car hard on back roads or racing. Denso suggests 30,000 miles for best performance but I typically run them 40,000-50,000 miles before changes.

The last set of IVT-20s I put in the 2.4 in my '07 went 55,634 miles which, is darn good for a high-performance-oriented spark plug and quite a bit past Denso's projection of 30,000 miles.

 

Déjà vu

 

Can you get these plugs at your average car parts store?

 

Many, but not all, parts stores carry Denso spark plugs and other products.

I'm going to bet you can order them from Amazon.com, too.

 

I still can't find these plugs!

 

My understanding is that Advance Auto Parts is a Denso retailer. There are four Advance locations in Providence RI. I'd try one of them. If they don't stock the IVT-20, maybe they can order it for you.

 

I now have 126,000 miles on our HHR. I've been though two sets of Denso IVT-20s. The first set went 53634 miles and the second set went 51225 miles. In both cases, these plugs exceeded the manufacturer's advertised durability of 30000 miles. The factor determining replacement was the plug gaps opening up from .040-in to about .060-in. The vast majority of electrode erosion occurred with the ground electrode.

50K miles is outstanding durability for a high-performance spark plug such as the Denso Iridium Power design.

 

Mine went 7+ years and 90+K miles on the originals. When the ignition control module went south, I figured a good time to replace the plugs. My mileage never went down before changing.

 

I have not really closely inspected my AC Delco 41-103 plugs when I change them at 100,000 miles but I did check for gap erosion and they only opened up to .050, were a nice tan colour and not sooted or carbined , but I have a stock little 2.2 litre auto

 

Maybe I missed something in the reasoning here.

Go with the Denso's which will last somewhere between 30-50k miles: or keep the AC Delco original type which should go for 100k miles.

On a stock or mostly stock, daily driver, I don't see the need for change if the AC Delco plugs are working.