Yes, consistent with the issue is the fuel supply, that is, my idle is very smooth and my intake vacuum is good with a very steady needle.

http://s785.photobucket.com/user/67dart ... sort=3&o=5

My AFR at idle is 12.5, so fairly rich. And yes, many folks with high performance engines tune their AFR to 12-13 over the entire rpm range to avoid detonation issues with advanced timing and higher compression engines.

For me, at highway speeds and under load, the AFR is 14.5-14.7, which is pretty normal, but perhaps not enough for this engine, at least those higher compression cylinders. I could go to richer mix in the main circuit, but I'd like to solve and keep 14.7 for highway driving (my mpg was 26 highway, and 18 city (I could up that city number by leaning out the idle circuit but its so smooth and performs well I didn't want to bother with it)). Of course if lowering the AFR in the main circuit is what it takes to solve this, then I'll do it. I don't have any flat spots or anything, the carbs perform well "as is." The thing is, this issue happened only sporadically in the first 5000 miles; but recently has been chronic. That makes me believe it's the fuel changing, not the engine or carbs (again, AFR readings are the same as before). Though I do have a compression difference that I was unaware of until now.

brian

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http://www.slantsix.org/forum/viewtopic ... 623525sted Dan has posted on this

http://www.slantsix.org/forum/viewtopic ... b768200eb2 In this post Dan mentions Small caps and high Zap ignition system, cross fire under cap.

http://www.counterpersontraining.com/in ... topicId=17 The after market is aware of this for some models. The ozone makes for ionization of the air. This ionized air is more conductive than normal air,making cross fire possible.Venting lets the ozone out. Never needed extra vent holes in my slants. My gm 4.3 needs extra vent holes or it will cross fire under load.

If a cylinder got spark too early on compression stroke, It would ping even with base timing backed off. What if a cylinder got spark while the intake valve was still open. Fire inside intake? I do not know just a wild guess

Thanks all for advice, tips and learning. I sure learned a lot these past few weeks.

So, I was using an NGK ZFR5N plug, which has a heat range of 5 (NGK scale) and an extended tip. You saw what they looked like, #1 was normal tan, #2-6 were white speckled, burning too hot.

http://s785.photobucket.com/user/67dart ... sort=3&o=7

Last weekend I tried an NGK BCPR5EGP, which is the same heat range, 5, but much shorter electrode, a platinum plug. I tried them simply because I had them, and I wanted to see if they would make any difference. Detonation was still there, but the plugs all looked normal tan brown when I pulled them today.

http://s785.photobucket.com/user/67dart ... sort=3&o=0

Dr. Dodge and I were corresponding, and we sent the same idea to each other simultaneously - try a colder plug. Today I tried NGK BR10ES, heat range of 10, coldest one I could find. The electrode is pretty much down inside the thread body and the thread body is much thicker metal (heat sink to head). It made a HUGE difference. I only noticed a very slight hint of pinging up steep hill under load. None to speak of otherwise. My temp gauge needle is running about 2-3mm to the left of where it was running, so a bit cooler.

I'll play with timing a bit more, but if this holds out, it may well be the answer.

brian

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Interesting about ozone causing arcing in the cap; hadn't occurred to me why they vented those caps....thanks Dan.

brian

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Drove it to work today, seems OK.

brian

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OK, so if you're a slant six owner and have a pinging or detonation issue, try a colder plug. So far with three days on her, she's running smooth and no issues.

I'll play with the timing this week, see if I can bump it back where it was without getting the issue back. It ran best at 10 degrees advanced. It is very smooth now, but doesn't have quite the umphf that it had at 10 degrees.

brian

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So where did you find a decent list of the plugs available for our drool tube heads. Seems like a daunting amount of numbers and types of plugs. I would probably just be thinking of the NGK line. Thanks

Rick

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Hi Rick,

Yes, NGK has a full range of heat/cold values that fit our drool tube heads, with the 0.75" reach and gasket seat (rather than tapered; I do remove the crush washer).

See NGK chart here: http://kcsbikes.com/pop_printer_friendl ... PIC_ID=100

You can see from the chart that the NFR5N is a "5" and pretty hot on their scale. They have plugs up to "12" on the cold range, I used a "10".

Summit has many of them, in resistor and non-resistor (racing) types. I used resistor type for street car. I couldn't find anything past 10 on Summit's site, but if you search their site with filters of NGK brand, 14 mm threads, 0.750 reach, gasket seat, you get about 358 plug types, with heat ranges from 4 to 10.5. If you further narrow to resistor type, there are 294. If you further narrow by heat range, there are:

35 in 7 heat range
16 in 8 heat range
22 in 9 heat range
9 in 10 heat range
2 in 10.5 heat range

You can narrow quickly by price, there are at least 100 plugs from $2-$4/plug range. Just for kicks I sorted from high to low price, the most expensive plug was $68/plug.

Brian

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Corrections:

1. There were some 10.5's on Summit's site, but they were $6/plug or more.

2. I meant to type "ZFR5N" above.

Also note: I had been getting an occasional run on condition or "dieseling" on shut down after long highway drives or after climbing a steep hill...that is also gone. I would also attribute that to the hotter plug I was using.

Brian

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Thanks that is good info!

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12.70 @ 104.6

So, with the colder plugs the detonation was drastically reduced, only a hint of it under load, problem solved, right? Not so much....no.

I was driving it home from work, about 50 miles, on Thursday and it started running rough during highway cruise. I thought I had lost a tire weight, because I could feel a vibration in the steering wheel, but there was a power fall off as well. I took it to the alignment shop just to be sure, the wheels are perfectly balanced.

The issue was gone by the time I got home, idles fine, no detonation coming up the hill to my house. Misfire? Fouled plug?

Anyway, today I played with the timing. I put it back to 10 degrees advanced, where it was before all this started, and the detonation came back. Dialed it back to about 5 degrees advanced, runs OK, not as strong but no detonation. Tried TDC, but it got didn't run any better and less power.

I put in another set of plugs, these were "9's" on the NGK scale, didn't have any more "10's". The "10's" that I pulled looked OK, brown, perhaps a little dark (that octane boost leaves a reddish residue and has higher chain hydrocarbons, so suity residue is not unexpected). See: http://s785.photobucket.com/user/67dart ... i.jpg.html Anyway, it ran exactly the same with the "9's". I took it out for a highway test and it did that rough running thing, no detonation that I could hear, but I could feel a vibration in the steering wheel, then it went away again. These are new plugs, so I doubt it's fouled plugs, and it idles really smoothly....hmmm..

I'm tempted to pull the head and try another one. I have one prepped. The idea is to try a head that results in lower compression see if this is really about the fuel supply and my higher compression head. Trouble is this new head I bought has been milled about the same amount as my current head, I measured from valve cover mounting lip to milled surface and it's 3.56". I suppose that's not all bad, if the new head does the same thing, then the problem isn't the old head, it's the compression and the current fuel supply (I was told that "summer blend" was recently released in the SF Bay area). If, on the other hand, the problem dissappears, then it was definitely the old head.

Brian

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Forgot to mention, when I got it home after work, right after the vibration/power fall off issue, there was fresh oil on the valve cover, same as when it had the detonation issue....hmmm.

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