For this test I used straight Platinum 9s for 9000 miles. Carbon build-up was really bad. The electrode which starts out flat on the top was rounded off. I should have changed these plugs out sooner. Performance was suffering and my idle wasn't perfect but still ok. It was a bit like a slight backfire sound coming from the exhaust. I didn't even notice it until I put the new plugs in and there was a sudden difference. Both the leading and trailing position plugs looked the same. Here is a new picture of the plugs. The left one was a leading and the right was a trailing. The corner that is whitish on the left one is where some of the carbon flaked off as I was taking them out. I wanted to know if it would have blown my engine had it come off while the engine was running so I took the flake and shoved it through the spark plug hole. Yea right! Like I said, I should have changed these sooner. I haven't heard of carbon from plugs blowing an engine but I have heard of carbon flaking off of the other parts like the rotors and then getting wedged between the apex seals. This is normally in cars that are driven short distances repeatedly like 2 miles to work every day. I usually go at least 10 and normally around 30 miles in a trip.

NGK also makes standard non-platinum plugs and racing plugs. I have also tested the standard plugs along side some Platinum plugs and could see no difference. Below is a listing of NGK plugs that can be used in rotary engines.

 

Plug Part #  Retail $ Normal 7 BUR7EQ 6.40 Normal 9 BUR9EQ 6.40 Platinum 7 BUR7EQP 7.60 Platinum 8 BUR8EQP 7.60 Platinum 9 BUR9EQP 7.60 Racing 9 R6725-9 36.04 Racing 10.5 R6725-105 25.74 Racing 11 R6725-11 25.74 Racing 11.5 R6725-115 25.74 Old 9 BR9EQ14 6.11 Iridium 7 Alt BR7EIX 6.95 Iridium 8 Alt BR8EIX 6.95 Iridium 9 Alt BR9EIX 6.95 Iridium 10 Alt BR10EIX 6.95 Rx-8 7 RE7A-L   Rx-8 8 RE8A-L   Rx-8 9 (Short) RE9B-T

BR9EQ14 used in earlier rotary engines. Another version had 3 ground electrodes.  Also available in other heat ranges. Some People are reporting good results using BR*EIX Iridium plugs, which are for motorcycles and snowmobiles. A special thin walled socked is required to install them though. Rx-8 trailing plugs are shorter. I have heard people are having good results in the Rx-7 with these plugs.

HKS Platinum: For this test I used HKS Platinum heat range 8 plugs in the leading position and NGK 9s in the trailing. I figured this way I would be able to tell if there were any abnormal conditions that caused the HKS plugs to perform differently by comparing the NGK plugs to the old ones. I drove my car 5000 miles with two track events. Carbon build-up was still bad however the HKS plugs electrode held up much better. It is much smaller than the NGK electrode as well. In theory a smaller electrode is better for a normal plug. After seeing the amount of carbon I decided to try to clean the plugs. The NGK plugs wouldn't come clean at all but the carbon on the HKS plugs was easy to scrape off. I decided to reuse these plugs to see how well they work after such a cleaning. The only reason I did this so soon is because I had a hell of a time getting my HKS Iridium plugs. Anyway I got a new camera so was able to take some pictures this time. The plugs were in there for 6000 miles with one autoX and my dyno time. Carbon build-up wasn't as bad this time for some reason. The plugs were also white in more areas, which indicates I'm running lean. Perhaps it's due to a change in driving habits, more highway miles, or a real problem. I'll know when I get my car dynoed later on. Overall they still look better than the new NGK plugs I put in there with them. I preferred these plugs over all the others I tested but unfortunately they are no longer produced.

HKS Iridium: These plugs are the racing style with an ultra small electrode. I used heat range 9 in the leading position and the normal NGK 9s in the trailing. Apparently these plugs are suppose to be the best out there but I really couldn't tell a difference. Maybe there was a horse power increase but until I have money to burn I can't afford to dyno every plug out there. Anyway these plugs were in there for a little over 6000 miles and didn't look good at all. Both plugs had large places where carbon flaked off. For a daily driven car these plugs aren't an ideal choice, thus "racing" plugs. Also after testing the Denso Iridium plugs I learned that these are re-badged Denso plugs.

I haven't even bought these plugs yet. Very few people use them. They are nothing more than NGK racing plugs.
 

These plugs are custom made and have a different spark pattern that has been proven to increase performance in piston engines. They are much smaller than the other plugs but have a much much bigger electrode. So here is how the story on these unique plugs goes. I tested them for 6 thousand miles and one autocross. Near the end of testing I was reading posts on a forum and found a guy that had the tip of one of these plugs break off in his engine. He had only 2000 miles and two drag races on them. He had them put in by Pettit, which was also testing them in several other cars. It was found that several of the plugs in the other cars were also cracking. Torque Master would not pay the bill for the new engine and turbo but Pettit did. The part that cracked off in these cases is called the Ground Electro-Ring and is the part around the electrode. Later, when I contacted Torque Master they said the cause of this is a lean running motor. I consulted Dave at KD rotary and he confirmed that he saw a car or two with broken Torque Master plugs and they were running lean. The person with the broken plugs reported they were "awfully black, like they had been burned in the engine". However this does not negate this problem. Update: I have discovered that "hispanic non certified welders" could also be the cause of this problem. The plugs should not be the weak link in the rotary engine, it should remain the apex seals. I figure that because of the fact that the Ground  Electro-Ring is raised and supported by four points it might not be able to dissipate enough heat or perhaps doing so unevenly. Dave said he thinks they are a good idea but that the materials used aren't standing up to the high heat levels of a turbocharged rotary engine. So what did I find in my engine? Well the Ground Electro-Rings were fine and the plugs held up much better than the others tested. Carbon buildup was minor and the electrode wasn't all worn down. However, one of the plugs had a cracked off part of ceramic that goes around the electrode. Thankfully it didn't enter the engine because it didn't fit through the gap between the Ground Electro-Ring and electrode. Torque Master said this is caused by improper timing and that "the ceramics are made by the same people". Well, I have never had this problem with other brands. So my conclusion is that these plugs are better but have the risk of breaking. If they were redesigned I'm sure they would be the best however until they are buy at your own risk. They were $20.00 each plus shipping. For the complete conversations with the owner of the broken plug and Torque Master click here. Update: Torque Master just sent me a free set of plugs! This set has a solid ground electrode ring (the four hole were not cut out), which should prevent cracking and might protect the ceramic a bit more from the combustion chamber. I might re-test these plugs but not until I finish testing all the other brands.

NGK seems to lead in the market but Denso is also a big name in spark plug technology marked by their extensive product offerings and technology such as using Iridium. Normally big business doesn't take much interest in the little guy like me but Denso surprised me when an engineer from their Europe division contacted me. This was the second company to contact me, the first being BRISK, below. In my book I give a lot of respect to a company when they are willing to take the time to directly interact with part of the community they serve. This can be anything from sponsoring a small time SCCA racer to posting some information on a public forum to answering a letter. Click here to view a very informative document Denso sent me on the history of their rotary spark plug development. In addition to this letter the engineer told me they have developed a new rotary plug designed for the renesis rotary engine found in the Rx-8. They should hit the market late in 2004. (This info is for general purposes only and no rights can be derived from it.) In the chart below you will see the part numbers for these new plugs.

After testing these I was informed by a vendor that they are the same plugs offered by HKS. I tested the HKS plugs earlier and they do look identical. So apparently HKS racing plugs are re-badged Denso plugs. I wasn't very happy with the results of either test. Carbon buildup had a tendency to flake off of these plugs and my idle was a bit rough from the start. I had these plugs in for over 12K miles and it shows. I also ran two autoXs on them. I won't be wasting my money on any more racing plugs unless I'm racing. Here is what the Denso engineer had to say regarding this test.

"The new HKS iridium RX7 spark plugs indeed share the Denso iridium technology. ... You were concerned about the carbon flaking off on the HKS iridium plugs, but this is not the important part of the plug. As long as your insulator is not fouled with carbon, the thin electrode iridium plugs will always outperform other style of plugs. The ground electrode of one of the plugs shows some deposits. This is strange and reminds me of 2-stroke racing engines that have burned too much oil."

While I was doing some research on NGK non-platinum plugs I stumbled across a cross reference to some other Denso plugs designed for Rotary engines. There is virtually zilch info on these plugs anywhere but from what I can gather they were an alternative to the early NGK SD10A and SD11A plugs that were later updated to the current BUR7EQ and BUR9EQ. One cross reference did however list these plugs as an alternative to the current NGK plugs. Obtaining info such as heat range and application was nearly impossible even when I called Denso. They had zero info but then again they also had zero info on their Iridium plugs. So after doing several cross references I deduced the first heat ranges listed below. The numbers in the part numbers however correspond to slightly colder plugs. See chart below. The picture above is a S31A plug I bought to see what they look like. It's very similar to the other surface gap plugs I have tested. The electrode is a bit higher and the ceramic comes further up. Naturally the ground electrode ring isn't divided in to four sections. I plan on testing these a bit down the road.

The plug to the left is an example of an earlier design for rotary engines similar to the ones offered by NGK. Again there were versions with 3 ground electrodes and naturally there are additional heat ranges from what is listed in the chart below. There is more information on the theory behind these plugs in the document from Denso.

After testing every plug I could get my hands on I thought I had pretty much concluded my spark plug testing when I was approached by the Prenco Development Coordinator Garrett Simpson regarding a new line of plugs. BRISK spark plugs are made in the Czech Republic and have been around since the 1930s. This company really impressed me and I'm sure we will be seeing more of them in America soon. I have seen testing that indicated that these plugs provide a solid boost in performance for piston engines but unfortunately they are not suitable for the rotary engine. Here are two interesting sites if you want to learn more about these plugs for your other cars. http://plugs.janmax.com/  BRISK Tábor

Autolite

I was able to find one example of a rotary plug produced by Autolite however this one design, part number 2626, was designer for early rotary engines and is apparently only offered in a heat range of 6.

I don't like NGK, that much is certain but when you consider cost it's hard to argue switching brands. Torque Master looked great and seemed like their design would out last the others, which would more than justify their price but they had their own problems. HKS platinums are my favorite to date especially after I was able to clean them. I bought a spark plug cleaner which did a pretty good job. However the seals are only meant to be used once and any unnoticed damage to the electrode during cleaning could have disastrous results.  

If the HKS platinums were still available I would run HKS platinums leading and NGK standards trailing. The reason for this is because the leading plug is the one that does most of the work and thus would benefit the most from a better plug. The trailing position fires half as often and from what I am told is more of an emissions plug. Mixing plugs like this should have no adverse effects because not only are the positions run off of different coils but Mazda also essentially recommended different plugs because a different heat range is a different plug. Since HKS no longer offers these plugs I'll be using NGK standard non-platinum plugs. They don't last long but either do the more expensive platinum NGKs.

There are also rumors that the plugs used in the Rx-8 are producing some awesome results so expect a review on them soon as well. They are NGK iridium plugs though I have not yet seen a part number.

I'll be testing the NGK iridium plugs next.

Note that all prices listed were found at http://www.sparkplugs.com/ with the exception of the HKS plugs, which are listed at retail. I currently shop at www.rockauto.com for the best prices and super fast shipping.