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#1 Compression ratio..? How to know.?
Posted: Fri Jan 30, 2026 8:29 pm
by pafagt
Hello forum..! I need to know what the compression ratio of my engine would be..? It is a 4.2 67 car built in April. Is there a standard for these engines or a what to find out..?
Thanks you very much.!
Pablo
#2 Re: Compression ratio..? How to know.?
Posted: Fri Jan 30, 2026 8:50 pm
by pafagt
Stamping on the engine block is 7E13106-9 does this mean the compression ratio should be 9.6:1..?
Thank you..!
Pablo
#3 Re: Compression ratio..? How to know.?
Posted: Fri Jan 30, 2026 9:55 pm
by abowie
The 9 suffix indicates 9:1 compression from the factory.
Are you in the process of rebuilding the engine? If so give consideration to the fact that previous head and block skimming will push CR above 9:1 if you use 9:1 new pistons without CCing the head and calculation it accurately.
Too much compression on these engines can be bad and make it very hard to time the engine properly without pinging.
We routinely used 8:1 pistons on all rebuilds unless there was a good reason not too.