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I wonder if the 6601's could be machined to 21 or 22 mm reach... 🤔
who can make this experiment?
Short answer, Yes. Longer answer >
Reading this brought me back a ways to when we were trying to find clever ways to add additional speed to the outboard race boats. These two cycle 30 hp engines had a long list of specs we could not alter. The classes were designed to keep the equipment consistent, and they were inspected to prove it. They even tested our fuel for additives. Consequently, these were Yamato y-80 Japanese engines that were used in Japan for their stadium racing. But I digress. After tons of time, energy and testing we found a few tricks that made a difference. We had a minimum cc that we had to follow. The cylinder head was removable and had some loose measurements to adhere to. We found that the compression ratio was around 7.6 / 1 on just about every one we tested. The cc ratio on most of them were well above the minimum. Since we only had to meet the minimum cc, we would cut the heads down and re machine squash band so that it would just be above the minimum. This effectively raising the compression ratio, made a difference. Not by its self, but a bunch of small things like this led to a noticeable increase. To take it a step further, we had two cylinder head options ( since the engines came oem with both > think class rules here) a 14mm spark plug and a 18mm. We only had two plug sizes with VERY few options to run. The surface area of the 18mm plug was much larger making the total cc of the combustion chamber larger than the 14mm, or so we thought. We started with a spare head, much like Howard did with the rotor housing and began removing the crush washer and measuring. We started cutting the shoulder down on the spark plugs till it was at the minimum allowable clearance we felt comfortable with. After a while, people started wondering why we were running the “junk” 18mm head instead of the “better” 14mm one.
You could possibly cut the shoulder down enough to run the crush washer and have the depth you’re looking for. It takes some time and energy to measure / adjust. You will have to destroy a couple plugs to see exactly how much material can be removed consistently plug to plug. This may or may not yield the results needed. It’s worth investigating though.
as usual, when considering a change there are a number of factors to consider and to make the best decision the weight of each factor negative and positive is important to get right.
we have heat range, gap and length as primary considerations. and of course it isn't as easy as it may seem.
take a look at the above dyno plot showing Torque. notice the wiggles as the torque increases. these wiggles are misfires and are not noticable from the driver's seat. they are not noticable on a typical dyno sheet unless it is set to zero smoothing.
you can get misfires from weak coils, leaking plug wires, mis gapped plugs... and you probably won't know it unless you are on a dyno and looking at a zero smoothed plot.
we were evaluating the Autolite AR3932X plugs.
we found that as we moved towards 400 hp we started to get the wiggles. notice how they directly correlate to the torque readings/combustion chamber pressure.
the problem was the .031 gap.
we switched to the more common AR3932 gapped at .023 and no wiggles.
6601 gap is .016. .4 mm.
Denso 3111 (S31A) gap .055 1.4 mm
Autolite AR3932X gap .031 .8 mm
as we juggle variables it is worthwhile to again consider someone (RX3Wagon) who has actually run the 6601s...
" I just happened across your spark plug article. I actually stumbled across these plugs in an old Mazdacomp catalog a few years ago. They recommended the 10 or 11 heat range version for road racing FD RX7s in both leading and trailing. I actually use them in the leading and the trailing on my rotary swapped 808 wagon. I can say I feel that they work extremely well in the leading position. Being a surface discharge plug, they never seem to foul. Objectively, they crisped up the throttle response off idle as well as at lower rpm. I am running 20psi with water injection on a S366SXE on a half studded, (street ported), S4 FC block. I have had zero issues even while tuning the car myself and occasionally making silly mistakes. I also noticed a power difference using these plugs. I have no way to quantify it but, the butt dyno says so. I believe this is due to the way the spark projects outward from the surface of the plug and exposes more of the spark surface area to the fuel/air charge with no ground strap on the way. I am actually shocked that my lowly LS2 coils are able to fire these plugs off with (700cc of) water at 20psi with no issue (running 4.9ms dwell). To make up for the fact that the plugs are 2mm shorter, I remove the washer on the leading plugs. I checked this on a junk rotor housing I have and there is still plenty of clearance. I find these plugs to be much superior to the race plugs most people recommend for rotaries; R7420 R6725. I personally found these to not idle as well and foul much more easily.
I researched spark plugs a lot while building my car. I found the BUR9s in all 4 holes worked quite well up to 15psi. I figured if I was going to run more than that, I needed colder plugs and water. I was unimpressed with how the R7420-10 plugs worked. I found they would foul fairly easily if idling for any length of time. I also felt the made less power than the BUR9s at the same boost..... Butt dyno feel anyway. So I tried these R6601s and haven't looked back. They are relatively cheap and Ihave been running the same set for 2 years. My car gets about 3000km per year and most of them are hard, abusive, kilometers.... With a few street legal drags thrown in for good measure. best to consider."
thanks for the info RX3Wagon.
my experience the AR3932X and your experience w the OE 9s underlines that gap matters as power rises... the 6601s are looking pretty good on this score. the other major point is heat range which relates to rotor housing/apex seal health. another score for the 6601s.
I have a set of the 6601-9s for my sub 350rwp with HKS twin power. Currently using the BUR9EQP. While I dont have a dyno, I can maybe datalog and see if there any any different results with Xaviers FC tweak across the RPM range. Will try them in trailing to start to see if Idle and knock are improved and move to leading as well.
All interesting and fun information to play around with.
So we're good with crush washer in the trailing, but ideally remove in the leading? I wonder if there's like a thin copper washer than would do the job with less thickness?
i am starting w 11s w crush washers in the trail... primarily because they are already on board, and 11s w no crush washer in the leads. if for some reason they need a washer i am going the thinnest copper washers i can find as they will do a much better job of heat transfer. currently the thinnest i find are .04
I ordered a few of the R6601 plugs and will see how much of the shoulder you can cut down. Hopefully enough that you can use the crush washer. Will report back with my findings.
if we are trying to remove as much heat as possible from the plug to lower spark plug mountain the steel crush washer is exactly the wrong material. it acts as a barrier to transferring the heat to the receptive aluminum rotor housing. if i end up running some sort of washer it will be copper. the best setup would be no washer.
i did a brief run with four 11 heat range 6601s yesterday and all seemed to be fine. no crush washers. i will pull the plugs today and check for leakage etc.
Last edited by Howard Coleman; Mar 17, 2023 at 10:03 AM.
A metal-metal seal w/o a crush washer or gasket is fine as long as the seating surfaces are true (essentially no runout) and particularly since it's steel against aluminum which will flex/yield slightly as the plug is tightened. The only time metal-metal sealing is questionable, IMO, is for sealing liquids that may eventually seep out. For a mainly dry seal that does not need to be absolutely impermeable, metal-metal is OK.
A steel sealing washer does little to enhance the seal if the surfaces are true, and in that case, all you're adding is another joint to possibly leak.
took a look at my 6601 11 heat range plugs running no crush washer and DaveW is on the money.
"A metal-metal seal w/o a crush washer or gasket is fine as long as the seating surfaces are true (essentially no runout) and particularly since it's steel against aluminum which will flex/yield slightly as the plug is tightened."
my primary concern was if there would be leakage without the washer and there is none. i also checked the crush washer and it is steel so it acts as an impediment to heat transfer so buh by crush washer.
Last edited by Howard Coleman; Mar 21, 2023 at 11:07 AM.
took a look at my 6601 11 heat range plugs running no crush washer.
my primary concern was if there would be leakage without the washer and there is none. i also checked the crush washer and it is steel so it acts like an impediment to heat transfer so buh by crush washer.
any specific torque when tightening? or just good all tight? I normally torque my plugs with a torque wrench
even missing AC and PS (actually not missing) there's too much stuff around my plugs (primarily the external oil line to the front bearing) to get a torque wrench easily connected. while i am a big fan of doing everything to proper torque i can't remember ever using a Tq wrench on plugs.
So we're good with crush washer in the trailing, but ideally remove in the leading? I wonder if there's like a thin copper washer than would do the job with less thickness?
I don't think I'd personally like to run without a washer long term. I'd much prefer a piece of sacrificial material that is closer to the same Rockwell as the housing itself.
Received my (4) 9 heat range plugs to test out. Need to clip the washers and put them in the motor to see what happens. My stock BUR9s are about 1 year old with about 1500 miles. Will have to review those to see how they are burning and if I need to go to colder plugs for my power level. Will update everyone as I make progress.
Received my (4) 9 heat range plugs to test out. Need to clip the washers and put them in the motor to see what happens. My stock BUR9s are about 1 year old with about 1500 miles. Will have to review those to see how they are burning and if I need to go to colder plugs for my power level. Will update everyone as I make progress.
Dumb question:
Depending what brand they are, is there any possibility that the ground strap might protrude into the combustion chamber?
Dumb question:
Depending what brand they are, is there any possibility that the ground strap might protrude into the combustion chamber?
Dave - I am using the same R6601 that others have. Judging by Howard's measurements I am assuming his .15 with the crush washer removed, although maybe he can clarify. That being said, he did mention that the actual electrode is protruding .01, meaning that without the crush washer, the R6601 plugs will be .14 as opposed to the stock plugs at .11. If my math and logic is correct, these new test plugs will be slightly farther out than a stock plug. If I am reading this wrong and the .15 includes the crush washer, he mentions .05 as the crushed washer depth. Without the crush washer on the new plugs including the extra .01, would leave.09 to be flush with the rotor housing.
I also noticed that Howard and the original R6601 user mentioned they are using these plugs without washers in their motors.
@Howard Coleman - can you please clarify the measurements for us?
Alex sent me these pics... note the ground strap is showing almost fully annealed. proper heat range is half annealed on the ground strap. too cold is less than half.
Since we are looking at existing plugs, figured I would post mine too. I just swapped these stock 9s for the new R6601-9s. Have about 1500 miles on these plugs. Looks like I might be able to put a colder plug in the trailing? Interesting how the trailing are half white/black. Would like to hear feedback from smarter people than I on the plugs.
Regarding the new plugs, I installed all 4 9s without the crush washer. Started the car and let it warm up. Unfortunately its cold and rain today, so I was not able to drive the car at all.
I did notice that idle with the accessories on seemed like it was actually rougher than with the previous plugs. I did notice that my AFRs do seem richer than before swapping the plugs. I will have to go back and look at my PFC logs. I try to shoot for 12.7-13 at idle and with the accessories on it dipped into the high 11's a few times. Maybe the extra fuel is causing the slightly rougher idle. Could be from the car coming off of fuel and enrichment for startup.
Update-- So I was able to actually drive the car today. Car runs fine with these plugs, maybe a touch smoother. I will have to plug in my laptop and pull a log. For some reason it looked as if my AFRs were slightly richer than before. Dont know if this is due to the weather being slightly warmer, fresh plugs or both. Currently running these plugs, without crush washer in leading and trailing spots.
) until the gasket just touches and then turning it down 1/2 - 2/3 of a turn from there.
