compression too high:pinging?

[maxeuer]

Would anyone have a suggestion why the compression has increased on my 1098cc engine lately?
At the same time I have noticed 'pinging/knocking' when climbing so I had to retard the ignition timing.
It used to be -7° but now it is more like -4°.
Could those two things be related and how can I remedy this? Off with its head?
Compression used to be [Cyl 1-2-3-4] 12bar-12bar-10.5bar-12bar over the last couple of years but lately I measured 12.3-12.3-10.5-10.5
12.3bar = 178PSI, 10.5bar = 152 PSI, 12bar = 174PSI
Engine had been overhauled by KAR works with an unleaded cyl head 5 years ago
Thank you

[bmcecosse]

It's quite high compressions (good!) so 4 degrees before is to be expected. You could try top grade fuel and get back to the 7 degrees setting - not sure if the extra cost is worth it. Also check if the vacuum advance is working - suck the pipe and watch if the dizzy baseplate moves back and forth - it should !

[maxeuer]

Thank you BMCEcosse!
Everything seems to be working, the vacuum unit does move the baseplate, it just worried me that the pinging commenced only recently and that something might be brewing.
I probably have to wait and hear.

[maxeuer]

Eventually I decided to behead the engine, cleaned piston tops, combustion chambers, valves and reground the valves.
After refitting I could bring the ignition timing back to -7° without audible knocking and the >12 bar compression values are back to 11½.
So it seems this is settled then. Now back to the ever failing starter motor...

[bmcecosse]

170 psi is exceptionally high for a standard engine.... Has the head been skimmed? Otherwise maybe just a high reading gauge..

[maxeuer]

Bought this refurbished engine 5 years ago and the compression was always 11½bar on some of the cylinders.
Wether the head had ever been skimmed I can not say.
But judging from the valve stem oil seals, it seems to be a Mini head, maybe that explains it.
Seems to me that a value exceeding 12bar would indicate carbon build-up.
I might add car runs fine using Super 95 Octane E10 petrol after the overhaul.

[bmcecosse]

You should have measured the head while it was off... It would need a LOT of carbon to influence the CR - unlikely to ever happen,