Hi guys, I pulled my fairly new plugs the other day and it looks like I have a tiny bit of aluminum on the center electrode and also some pepper like specks on the insulator. The plugs have maybe 500 miles on them, mostly highway but also with some full throttle blasts now and then (all street miles). I always see several degrees of knock retard when I change gears (T5) at full throttle but its not excessive. I only run 30-32 degrees WOT and it is not in until 3500 rpms, so I don't think I have too much total timing, just some mechanical noise (old crappy motor mounts are probably original to the 1977 car, new urethane trans mount ,mechanical clutch) when I bang shifts really hard. I also have forged pistons which may be slapping a tiny bit (inaudibly but the knock sensor might hear it) when I'm getting back into the gas hard. Anyhow I run around 42 deg max during highway cruise, commanded 15.5:1. The car runs very very smooth on the highway and I get 20 mpg highway, which I am very impressed with considering the 3.73 gears and the fact that I average 75 mph. Could the slight alum. on the plugs be from actual detonation on my few and far between WOT blasts, and the knock sensor is correct in picking up a little too much timing there, or could I be putting aluminum on the plugs during highway cruise (at such light throttle??? 42 deg with 9.7:1 and iron heads doesn't seem too excessive to me, what about to you??) because of too much timing, even though the car runs and sounds flawless at cruise? I am 6E MAF. BLM's on the highway are a tiny bit on the lean side but not bad, no higher than around 132.

 

As an experiment, how about pulling at least 4* out across the board and running another 500 miles with a few new plugs?

 

Take a good picture of a plug and post it here.

It depends on where in the map you're at for the timing. Cruise for some isn't cruise to another. I'd hesitate to go far away from stock timing (of similar combination) in search of economy, without a definitely noticable wtf in there.

 

I will try to post a pic. The digital camera I have is an old piece of crap that's only like 2 megapixels but it may be good enough on the highest resolution.

 

You're running 15.5 to 1 AFR so you might be melting your piston tops... I know you said you are running a BLM of 132 but that doesn't mean the cylinder temperature isn't excessively high. Try going back to 14.7 with the same timing settings and see what it does after 500 miles. If everything looks fine, try going to 15.0 then 15.2 etc.....

Also, your timing will depend in large part on the type of heads you have. Sixty's style combustion chambers will mean you need more timing, newer style combustion chambers on aluminum heads require less timing.

 

I have 1980s 081 305 tpi heads. Fully ported and polished. It's a fast-burn scenario especially because the small chambers on a big bore engine is giving a really large quench pad.

 

If in fact true, then you're running too much timing.
FWIW, in my TR, I run 31d timing in cruise mode, and ~16:1s AFR.

 

You should not be having a problem with detonation with that timing and that af and compresion ratio at cruise. Im running more timing at cruise (vette aluminium head table plus added timing on the upper RPMs).

I run 87 octane until the dead of summer then I have alot of retard in the mid rpm range at full throttle then. Go with 91 octane as the intake air temps go up. (64 cc head, flat top 350)

I have never seen aluminum on the plugs in the 50,000 + miles, and I have audable detonation.

You may be over reacting to what you are seeing and being overly protective to your new engine. The knock sensor does work well in protecting the engine. Remeber the old carb days and driving a car years with it pinging ever time you got on it hard with no damage (noticable).

I have seen 49 degrees 16 to 1 af 87 octane at low map with out a trace of knock counts. I would be looking at your mid rpm high map for the problem.

 

You might have high quench chambers but remember that the pistons at TDC relative to the combustion chamber is what make it fast burn. Keeping the pistons "in the hole" and running a thick head gasket can negate much of the gains to be made from a small quench chamber!
You said 9.7:1 but what is your dynamic compression? I would think 40 degrees would be plenty and I don't think your detonation is coming from cruising at that AFR. I also don't think you're melting anything cruising with your current timing/AFR. It would be wise to experiment with less timing and keep the lean AFR at cruise. I also believe every sbc should have an oil cooler or at the very least an oil temp gauge.
With a properly working OEM knock system I wasn't able to detect the detonation on my spark plugs. Infact as I was playing with the spark settings I found that 44 degrees with winter gas 87 octane in Columbus Ohio do NOT work with the 330hp crate 350! I tried closed loop and highway mode, only difference was that highway mode was choppier when the detonation happened. It was detonation from pre-ignition and that was only 44 degrees with 9.4:1 vortec fastburn. It was so bad that on the highway the car couldn't pull enough timing out and there was a noticable stumble herky jerky feeling! The pre-ignition had heated up the combustion chamber so much that it just snow-balled. The plugs showed nothing and were amazingly clean. My understanding is that the heat produced from the pre-ignition was keeping my "cold" plugs clean. From then on I went to 91 octane in the winter and ran 40 degrees with the same lean AFR of 15.6:1. Never had the problem again.

 

I always run 93 octane. I'd never try to run anything with more than 9:1 compression on 87. My pistons are 0.017 in the hole, so I have around 0.056 quench including the 0.039 head gasket. 0.040 would be better but it could be worse.

 

A friend's Imp 96ss with LT's and home-maid CAI has gotten his best ET and mpg with 87. He was running 93 but I told him to look into running the cheap stuff and he said he could feel a difference. Maybe it's just me but my engine ran it's best ET/mph/mpg with 87. I had to to take out a few degrees of timing but that means nothing... the best mph and mpg say everything.
Did you know that a 600cc Honda F4i can run on 97 (blend 50/50 of 94 and 100) up until 8500rpm with a smaller F3 cam (higher dynamic compression) AND 14.7:1 static compression. Obviously it isn't a comparable motor in almost all respects but I do believe people have been "had" by the "high octane only" disclaimer. With your 327, if you rev high and have a high dynamic compression (small cams do this) then you should stick with the higher stuff for peace of mind if nothing else.
Back to my friends Lt1 for a moment. His stock cam and heavy weight required we go back to near stock timing where as with premium we were able to run almost 4 degrees more at WOT without tickling the knock sensor! Moral of that story; More timing with higher octane doesn't always make you faster.