I need help with an engine problem

[Tommy]

I am trying to diagnose a problem with the engine in my Cobra. My goal is to find the root problem without disassembling more than I need to.

Background

The engine in my Cobra includes the following parts:

Block: 1972 351 Windsor bored to 4.040"; deck height 9.503"

Crank: Speedomotive 408 stroker; Hi nodular cast with 4.000 stroke; balanced assembly; Feb 2004. Main studs ARP 154-5403

Pistons: Probe forged dish top; (10.48 comp ratio zero deck height); Total seal Max Moly rings; coated tops and sides

Calculated Compression Ratio: 9.19 (Gasket: 8.41cc. Piston dish + deck height: 34.2 cc. Head: 60 cc.)

Rods: 5140 forged steel; 6.200 inches long; rod ratio 1.55

Crank & rod bearings : Clevite Tri-metal main and rod bearings

Cam shaft : Comp Cams # 35-000-8RF; Grind # FW 3316F/3317F HR110+4

Lifters and retainers: Comp Cams 851-16

Rocker arms: Crane Gold # 86757-16

Heads, valves, guide plates: Edelbrock Victor Jr. #77189 (60 cc). Head bolts ARP 154-3603. Head gaskets Felpro 1011-2.

Intake manifold: Edelbrock Performer

Current situation

While driving a few nights ago, I heard a sound over the roar of the side exhaust that reminded me of an exhaust leak hiss. Simultaneously, the warning light on my Knock Detector illuminated and stayed lit. The Knock Detector is calibrated to flicker on briefly at its most sensitive engine speed (2400 RPM). I slowed to 2,000 RPM but the noise continued and the Knock Detector light continued to flicker on and off. As I had filled up with gas only a few miles before, I suspected the station had sold me a tank full of 87 octane gas from the 93 octane pump. So I dialed back the timing advance using the device I installed for that purpose on my dash. It didn’t help.

The next day I drained most of the tank of gas and refilled with 93 octane gas from a different station. Even after allowing time for the new gas to get to the engine, there was no change. The noise and warning light were still there.

Today I pulled the spark plugs and did a compression test. All the spark plugs were black and coated in carbon. It’s been several years since I drove the car hard at a track. All it has seen is occasional cruising.

I did the compression test with a fully charged battery and the throttle wide open. Cylinders 1-7 all showed between 200 and 210 PSI. Cylinder 8 showed 125 PSI.

I removed the valve cover for cylinders 5-8 and did a cursory inspection. There is nothing obviously broken.

What I need

I need advice in two areas. First, I need to compile a list of likely problems (e.g., blown head gasket, cracked head, cracked cylinder wall, cracked piston, etc.). Second, I need to know what to do next in search of the root cause of the problem without taking more apart than I need to.