After having done many of the standard many of the upgrades/tuning for my /6 (HEI, removal of miniox, 2 1/4" exhaust to aftermarket muffler, valve lash adj), I had the engine running pretty efficiently for a while... at about 20 +/- 1 MPG (which is good from the 16 it was running at before). I have known of a bit of a compression leak for a while as per compression tests of about 70-90 psi. Between kids, school, and work, I have not been able to put the time in on this issue yet, so I was hoping to talk it through here before going forward for efficiency sake.

I know about squirting oil in the cylinder to help diagnose ring or valve leak, but have not done so yet. My gut tells me that this problem is getting slowly worse since for no known reason the mileage is dropping again.

The symptom: at high RPM, such as shift points, I hear a tinny metal sound from the engine. My suspicion is that this is the sound of rings on the cylinder walls at high RPM due to lack of vacuum. Is this even a possibility? If so, would this be a result of a valve leak or a ring leak?

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That sounds like spark knock or "detonation". (Pinging)
What is your total spark advance at high RPM?
DD

Could be a tight valve. If your motor is mechanical valvetrain, then I would lash the valves, or loosen both valves just on that cylinder w/low comp and measure cyl press again.

If hydraulic motor, then you could have a stuck lifter or another problem causing the valve to be tight.

Lou

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