120-140 PSI compression reading at 5700' = Ratio?

[Richard/SIA]

120-140 PSI compression reading at 5700' on the 350 engine in my '57 pick up.
Most cylinders were very even at 130 PSI

Anyone able to give me an educated guess as to my compression ratio?

The engine codes as a 1969 255HP 4V.

It also appears to have been rebuilt before I got it.
If it's running high compression I need to know, so that I can avoid putting in too low an octane fuel.

 

[oldskydog]

Just a WAG here, but if the engine is original, I would expect 9.5-10 CR just based on the year.
The shop manual static cranking pressure for a high perf (10-10.5 SCR) is about 160 at sea level, as I recall. A 1" column of air is 14.7 psi at sea level and would be about 11.9 psi at 5700 ft .
At sea level 160 divided by 14.7 = 10.8 so that is ballpark.
At 5700 ft 130 divided by 11.9 = 10.9 which seems high so my numbers may be off and don't take into account any cam overlap.
I think I would assume about 10:1, at least.