Spark knock under mild acceleration

PursuitSpecial

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So after I gave up on the thermoquad and put it on my 400, I switched to a Holley Economaster for the 318. It really doesn't run any different, though it has much better cold manners. Anyway, now I'm noticing that the car is spark knocking very badly under moderate loads like would be encountered in everyday driving getting up to speed, but it only does it in certain weather conditions. It's an audible tap-tap-tap sound that manifests only under acceleration. Last weekend it was 25F with a barometric pressure of 30.54 inHg and humidity of 75% when it did it, the engine was as warm as it would get. Once the pressure dropped and the rain moved in this weekend it stopped. I have already experimented with timing, it is as conservative as it will get without hurting the power too much. I currently have it at 10 degrees initial and 30 total with a stiff advance spring so it won't see total until about 3000 rpm. There is 6 degrees of back and forth slack in the timing chain. I fed some water down the intake with the engine revved up and it didn't blow any carbon out of the exhaust. As it is a bone stock 318 HP engine it should have 8.2:1 compression due to the larger 360 heads. The engine has a history of doing this. When I first got it out of the woods and started driving it, it would knock on regular gas, the only way to stop it was to lock the timing at less than 20 degrees and make it a dog to drive or burn premium in it. I experimented by running a tube to the carb and feeding it pure oxygen out of a tank at different concentrations and it would knock very, very badly. For some reason it seems this engine is incredibly sensitive to atmospheric conditions and fuel octane, which it definitely shouldn't be ast such a low compression. The only other thing I can think of is spark plugs, I'm running a 0.750 reach plug, which should be correct for 360 heads, unless they were different or something in the early 80s. Any ideas what's going on?
 
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Mikes5thAve

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Don't know on plugs, I always run based on year of the engine.
With an MP distributor my base timing is around 6 or it pings. I was able to run 12 or so with a stock distributor.
 

PursuitSpecial

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It has to be in the gas, I took the Aspen out today and once the engine warmed up it spark knocks very badly if you slowly accelerate in high gear just enough to not let the transmission kick down. And it's warmer and wetter today, 60F and rainy. There's no way a slant six should knock like that in normal driving. The fuel I'm burning is ethanol free mid grade 89 octane from Circle K, I've been burning it for over a year with no issues but now I'm suddenly having issues. I haven't noticed it yet in the big block Gran Fury yet and it runs the same gas, but the secondaries and my cherry bomb knockoff muffler are so loud under acceleration I probably wouldn't notice. Once I burn a tank out I'll replace it with top tier premium and see if it stops
 

Camtron

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Plug reach is the same across LA engines, gap should be around .035. Do you know what plugs are in there? Wonder if they’re just a little too hot.
 

kkritsilas

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There is also the possibility that there is a lot of carbon build up in the combustion chamber (surefaces of the cylinder head and top of the pistons). This raises the effective combusion ratio, and the carbon build up also tends to retain heat a lot more than metal surfaces do. Both together can make an engine more octane sensitive. Hard to know how the engine was used in the past, but a lot of idle time, as one example, can increase carbon build up. I think getting a borescope in a cylinder or two might put this theory to rest, one way or another.
 

LSM360

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Sounds like you've narrowed it to bad gas. I wonder if the octane mix didn't get done properly when gas was delivered to station, or there was water in that tank (station tank). Hopefully you can run it all out soon and fill up somewhere else and see.
 

Oldiron440

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The mix can be a problem, depending upon how close your station is to the distribution point, it’s mixed in the tanker just by adding this and that by volume. Weather it gets mixed well before it is delivered to the station can very. That’s why some stations have
e-5 and the next has e-30 and very few if any get the e-10 or 15 they were supposed to get.

When I had my shop I bought gas at the station across the street from me and he would verify alcohol level in every tank delivered to him. At the time it was supposed to be 10 percent but it would vary up to around 30 percent. The explanation he would get was that the fuel hadn’t mixed in the tanker.
 

PursuitSpecial

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I put half a tank of shell 93 in it, and it still knocks. It could be the plugs, I'm running a Champion RN11YC4 which is not a hot plug, but the 81 factory service manual recommends a "65PR4Y" which I think is an Autolite 65 plug but I can't find info online or in my 90s Autolite catalog about it. I put the same gas in the Aspen and it stopped knocking so the gas is definitely an issue, but for some reason the 318 engine is very sensitive. I'm wondering if the car was doing this when my grandpa parked it 20 years ago since it had avgas in the tank, I thought he did it to preserve the tank but he could've been burning it to stop the spark knock.
 

Camtron

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Hmmm, failing fuel pump/gunked up fuel lines? Maybe throw an inline pressure gauge/regulator on to see if you’re getting full pressure to the carb.
 

Camtron

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Anecdotally, when I bought my car I was it’s 6th or 7th owner and it came with a 14mm Snap-on socket stuck on the inner fuel pump bolt from someone messing around with it in the past, lol
 

PursuitSpecial

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I pulled plug #3 and took some pictures of the piston with a bore scope. It's pretty dirty in there and the piston looks like the surface of the moon, and there looks to be some loose carbon. Not sure if that is causing the detonation or if the detonation is what's knocking the carbon loose and marring the piston.

20250105221659773.jpg


20250105221747835.jpg


20250105221908956.jpg
 
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