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Bosch Platinum vs. NGK Platinum Spark Plugs

by Jarrod K. Wright

SR20DE owners have engaged in an ongoing and at times very heated debate regarding the effectiveness of Bosch Platinum spark plugs in this engine. Because the stock NGK Platinums are fairly expensive, costing $8 to $15 apiece at dealerships and parts houses, some owners have elected to substitute the less expensive Bosch product come tune-up time. Bosch Platinums typically cost between $2 and $4 apiece at large retail parts outlets. Both plugs are, as their names imply, of the platinum variety and last upwards of 60,000 miles in normal applications.

Several owners have replaced their stock plugs with Bosch Platinums and immediately encountered problems such as poor idling, part-throttle hesitation, surging under deceleration and power loss. Other owners have reported that these plugs worked well for several thousand miles, at which time a gradual decrease in performance over time occurred, accompanied by some or all of the above symptoms.

Conversely, many owners have reported no change in engine behavior after installing Bosch Platinums and went on to experience many thousands of miles of trouble-free motoring. A few even claimed an improvement in overall driveability, but this can probably be discounted as the result of replacing worn-out stock units with new replacement parts.

So where exactly does the truth lie in all this?

In January 1999, in the midst of a flurry of debate on this subject, SE-R Mailing List member and NX2000 owner Chris Pinthong decided to find out. He performed a straightforward peak horsepower comparison between the two brands using a Dynojet 248C Chassis Dynamometer at Group 5 Motorsports in San Diego, CA. This type of dynamometer measures horsepower at the drive wheels.

Pinthong's engine was slightly modified and included a four-into-one header and 2.5-inch custom mandrel-bent exhaust. He removed the air filter during the test period. List member George Roffe provided a set of new NGK Platinum spark plugs, while Pinthong contributed a set of used Bosch Platinum plugs with approximately 15,000 miles on them. (Ideally, two sets of new plugs would have been used, but new Bosch units were unavailable at test time. Instead, Pinthong installed the new NGKs and ran them for a few days before the test, putting about 1500 miles on the plugs in an attempt to partially level the test conditions. This mileage and use discrepancy probably skewed the results toward the NGKs, but more on that later.)

Pinthong and the techs at Group 5 hooked his NX up to the dynamometer and performed several runs with the new NGK plugs installed and gapped properly. He then shut the engine off for 20 minutes and aimed a large fan across the engine to cool it down. With the NGKs still in the engine, he fired up the car once again and measured power at the front wheels. He recorded a peak value of 132.0 horsepower.

After completing the run, he shut down the engine and immediately replaced the NGK plugs with the used Bosch Platinums. This procedure took roughly five minutes. He started the engine without a cool-down period and once more performed the dyno test. The result was 133.5 peak wheel horsepower, a 1.5 horsepower increase over the NGK Platinums.

This brings up a very puzzling question. Why would a set of well-used Bosch Platinums produce 1.5 more wheel horsepower than a set of almost-new NGK Platinums? The results are even more confusing when the test methodology is considered. In an e-mail to the mailing list on January 30, Pinthong wrote, "Now, under those conditions, the second run should have been lower [in measured horsepower] if the plugs were equal, since there was not enough time for the engine to cool off like with the NGKs."

In other words, a hot engine typically drops a few peak horsepower to a cooler one, so the test should have revealed a lower measurement for the Bosch plugs if they were comparable in power production to the NGKs. And if they were inferior to the NGKs in this regard -- as some have claimed -- the test should have revealed an even wider peak power differential between the two.

The validity of this test can and should be questioned. The methodology was suspect and the controls were poor. And like most hot-rod exercises, this was never intended as a scientific endeavor. Group 5 Motorsports offered free use of their dynamometer, and Pinthong graciously volunteered both his vehicle and his expertise. Under the circumstances, the test conditions were about as good as could reasonably be expected.

Nevertheless, the results indicate the Bosch Platinums compare well to the NGK Platinums in peak horsepower production. This is despite the conditions being considerably more favorable to the NGKs (a fresh set run in a semi-cool engine) than to the Bosches (a used set tested in a hot engine).

Finally, in an effort to provide some balance to Mike Kojima's observations on this subject, I'll relate some of my own thoughts on Bosch Platinum plugs. I've installed well in excess of dozen sets over the years in various cars and trucks -- including my 1991 Sentra SE-R -- and I've always been very satisfied with their performance. But I am not prepared to take the results of Pinthong's test and claim that Bosch Platinums are necessarily better than NGK Platinums. (And neither is Pinthong. In fact, he still discourages their use in applications involving turbocharging or nitrous oxide. "I would never use them with these applications because of the poor experiences other list members have reported," he said.)

However, I will say that Bosch makes an excellent product that is comparable in performance to the factory NGKs in stock or near-stock applications. And depending on where you shop, you'll pay 50 to 85 percent less for them.

If you're debating about whether to install Bosch plugs in your SR20DE, rest assured that they'll probably work at least as well as the factory plugs in everyday driving.

Many thanks go to Chris, George and the fine folks at Group 5 Motorsports for making this comparison possible.

Group 5 Motorsports
8268 Miramar Road
San Diego, CA 92126
(619) 693-FAST