Stock Engine Compression Question

[farmtruckman]

Starting to diagnose engine problems in my 400 ci in my '71 F100 and after running compression check in all cylinders, had a question. Entire truck is a mystery and the only history I have is from someone's brother-in-law that owed someone money or something like that..... but, the seller told me that the 400 is out of a 80 Bronco. I am getting a consistant 90 PSI compression check with or without oil. Seems a little low, but my brother mentioned that the late 70's, early 80's motors were notoriously low compression engines to comply with emission standards. Can't find anything in my manuals about what the stock compression would be, but I am consistant on every cylinder. Does anyone remember what the stock value was?

[My427stang]

That's pretty low, however, be sure you are checking with the throttle held wide open.

Your brother is correct though, a combination of retarded cam timing and 8:1-ish compression kills cranking compression numbers. In addition, if your motor has a worn timing chain, that will retard the cam even more, lowering cranking compression even more.

I think stock is 8.4:1 but as you can see, that really isnt the only factor.

[farmtruckman]

Thanks for the answer. Any idea how to convert that number to PSI? I know there is one using "swept area values" but can't remember how to do it.
Still thinking heads are not in the greatest shape also. Easy to pull heads. Carb is an old Quadrajet which seems to be missing the choke hardware. Like I said, a mystery in the making. Have a borescope and plan to inspect each cylinder this weekend just to peek inside. One plug was banged up so I have a feeling that something went down the carb and maybe banged up the plug. Maybe it was the missing choke hardware?

I did get a good deal on it so I tell my wife that I am still ahead of the game. Plus, it keeps me out of the house (out of her way)!