# Compression



## olde-goat (Feb 1, 2009)

I am going to run a compression test on a stock late 1965 389 engine with the 093 heads, this is a tired engine with no previous work done on it. What is an acceptable compression ratio? Is their an upper and lower value that I should watch for. Thanks


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## BearGFR (Aug 25, 2008)

It's more about having all the cylinders close to being the same than it is absolute pressure numbers, but I'd think somewhere in the general vicinity of 150's would be reasonably healthy. Make sure you remove all the spark plugs and also block the throttle plates wide open. If you have one or more that are significantly lower than the others and suspect worn rings, one thing you can do is to take a "dry" reading first, then measure the same cylinder after squirting a little oil in through the spark plug hole (not a ton.. just enough to put a thin coat on the cylinder walls). If the pressure comes up significantly, chances are the rings in that cylinder are worn. If it doesn't change anything, then it's probably leaking past the valves.

Bear


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## olde-goat (Feb 1, 2009)

Bear, thanks for the reply. Sorry;I should have had my coffee first before I posted. I should have mentioned that this engine has a lot of blow-by. If and I believe that I will get some bad numbers obviously this is going to trigger a complete engine rebuild. Haven't done one of these in years and I have to think that this is going to need some significant cash. In my neck of the woods my last rebuild on my other 389 ran about 7000 and change.


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## BearGFR (Aug 25, 2008)

Well then, if you already know it's tired (like you said) why not just pull it and rebuild it yourself?

Bear


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