yes, i do believe the gap is the problem. they did work well mid 500 for a while...

 

Just putting it out there, I was the one who had the NGK 6601 plugs machined down the 2 mm on the collar to make them fit perfect. If there is enough interest, the machine shop owner is a friend of mine and I could take orders for plugs from everyone like a group buy, order a whole bunch of stock plugs at a discount, pay him to machine them, and make some profit for my trouble (a lot of work and hassle). They still would be quite expensive since the plugs are expensive to start with, around $22, and the machining would likely be about $20 per plug or even slightly more. Once you add shipping, tax in some states, and the profit, I'm thinking likely around $75 per plug. Not cheap at all but you only need two and they should last for quite a while, unlike the leading plugs most of us use.

Just a thought.
Mike

 

Pretty interesting video with relevant info related to this topic!

 

Since this thread bumped I wanted to share that I had my car tuned in mid December with Denso IRE01-31 in the primary location and R6601-11 with trimmed shoulders in the trailing location. The car made 498hp/435tq on an E35 blend at 16# and 466hp/395tq on pump at 14#. There were zero complaints from the tuner on the plug combination and he was happy with them.

 

Not a fan of anything that comes out of that dudes mouth.

Yup, I've been a big advocate for the Denso Iridium plugs for many years now. With a proper tune, the 10 heat range work well a low 20s boost levels and are very resistant to fouling. Far superior to NGK anything in my experience

 

@mikejokich just wanted to mention I am definitely interested in purchasing machined r6601 plugs. If anyone else is interested they should post so that it is known. Otherwise they will not get made.

 

And the SA31s are also out of stock as well it seems. Did find 4 on eBay though.

 

reading on the M&W manual they specify max plug gap of .65mm. That doesnt really seem to comply with the 6601. Did you experience any issues by running this large of a gap? Do you run the M&W coils? Cop or wires?

 

The 6601 has a gap of .4mm. I did not experience any issues at all but the reach did bother me. I have been running the IRE01-34 11 in leading and trailing recently. I had been switching back and forth between RaceGrade IGN1A coils and the M&W Pro-14 with M&W coils since my engine harness has two separate ignition harnesses. One for CDI and another for the IGN1A's. I use custom made Magnecor R-100 wires on both setups. The last time I was at the dyno, my tuner wanted to try both. There was no doubt what so ever that the CDI was better. Cleaner and EGT's were noticeably cooler with everything being the same. This was using the same plugs on both setups, IRE01-34 11 gapped to .023 leading and trailing.

 

Very nice. I think the gap on the 6601 is more like 1.2-1.4mm range by the looks. The 0.4 must be a typo in the spec. Im also building a car and have a M&W CDI for it. For the leading it seems by reading this very good thread the IRE01-10 is among the best option. As some say on trailing the strap could become overheated therefore the S31A or 6601-10 could be a solution. The only doubt I had was that its wide gap could damage the CDI or Coils.

 

I also agree here that colder plugs will more then likely not (or if in the wrong way) affect the crowning of the rotor housing surface around the leading plug. Plug heat range gives a unit-less measure for how much heat transfer there is from either electrode towards the threads. Spark plug manufactures always select the hottest possible plug that will not exceed a given temperature limit that is previously set to guarantee the safety and durability of the electrodes. this way the plug has the greatest resistance to fouling. To do this they run a instrumented plug with a thermocouple fitted to one or both electrodes.

Here is an example of a M10 plug with an instrumented ground electrode.




Unlike intuition would tell, the peak temperature of the ground strap is not reached at peak torque or at peak power, at least not in the engine we ran this one in. it is a 500c/cylinder turbo DI engine, central injector and plug between exh valves. produces around 240hp/l on pump fuel. Y axis is bmep in bar. As one can see the hottest area is the point with the highest power while being stochiometric, when going richer the ground strap seems to benefit quite a bit from the cooling effect. the white islolines are lambda btw.




Apart from the visible design changes that could affect the heat transfer, the inside of the ground electrode can also be made in different ways giving more or less heat transfer. On this example of x ray of a failed electrode the center of the electrode is copper and the outer sheat is inconel and topped with a Platina tip. changing the cladding thickness of the inconel or the copper length or any other combination of materials has a big effect on how hot the tip gets. the same logic applied for the center electrode.