11/24/2023 0 Comments Ngk plug heat range chart![]() At the same specific output as the WRX, the B5234 could produce about 260 hp for that NGK '6' plug, and about 310 hp to be at EVO NGK '7' levels. This is closer to the WRX (a '6') than the EVO (a '7'). Take a B5234 at 240 hp (for the 850R), wave your hands a little, apply the same ratios, and you can calculate 104 hp/cyl*liter. The earlier USA market EVO's were 271 hp for the same displacement and number of cylinders or about 136 hp/cyl*liter. The old WRXs were rated at 227 hp with a 2.0 liter four, or about 114 hp/cyl*liter. The WRXs seem to consistantly use NGK '6' while the Mitsu EVOs use NGK '7' heat ranges. I looked at some older turbo cars for inspiration of what the manufacturers felt comfortable selling with a warranty. I don't think it's enough to divide by cylinders to try to correct the ROT's - I think you need some kind of specific output value like hp per cylinder * liter. There are all kinds of rules-of-thumb about when one needs to go colder, but most of them are very general and based on carbureted V8s. This is what I used for reference for heat ranges - it's from Denso. The NGK's didn't look bad, so in went the new set. The Champion '8' comes up on cross-reference charts as a hair cooler than the NGK '6', and it's close to the now unobtainable Champion RC7GYC. I bought another set of NGK 6418's, plus a set of Champion 9801 Iridiums (RC8WYPB3) as back-ups in case the NGKs looked bad when I pulled them. So after 25k miles, I thought it was time to check. When I originally installed them, I wasn't sure what heat range I wanted to use because the NGK '6' is perceived by many to be a little warm. The car was running fine when I replaced the NGKs, so the 25k is more of a reference point. They are definitely more expensive than standard plugs, but hell, a Grande Latte probably costs the same as one plug, and it only lasts 15 minutes! (ADDENDUM: In all fairness, I also did replace the compressor bypass valve at about the same time, but I would be surprised if that was as much of a contributor as the exhaust.) I did not expect this, but it was a nice surprise. The Simons exhaust gave me almost a 1.5 mpg increase. As setup, this engine has a consistently smooth idle and is good for about 24 mpg in 'mixed' driving. I mention the heat range aspect because cold plugs invariably will run a bit rougher under light loads, which is where we drive most of the time. On this car, major modifications as of now are a 628 (850R) ECU and a Simons Sport exhaust. Keep that in mind when someone tries to convince you that you need colder plugs after any modification. Given the fact that they do not appear to be cooked I suspect going to a cooler 7 heat range is unnecessary in anything remotely close to a stock engine. ![]() These are a NGK heat range 6, which they recommend for nearly all Volvo turbo 5 cylinders, and almost all Subaru turbos for that matter. Because the deposits are consistently on one side of each plug I suspect it is the side pointing at the intake valves as that part would run a bit cooler from fuel impingement. The deposits are heavier than I expected them to be, and notice how the corners of the ground strap (as best seen on #4) have eroded away a bit, but otherwise the plugs look good. I only run 93 octane of no brand in particular. I replaced them with an identical new set. This is what they look like as of today, 25k miles later, in decending order of cylinder 1 to 5. In Oct 2012 I installed NGK 6418 Iridium plugs (BKR6EIX) in my 1994 850 Turbo.
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