compression test

[64 200 ranchero]

#1 - 150
#2 - 150
#3 - 150
#4 - 145
#5 - 155
#6 - 160

are these numbers acceptable, i checked out the classic inlines spec sheet and it said that normal cranking compression should be around 175, if i degree my cam incorrectly would that lower my numbers? i think i have around 9.5:1 compression but not 100%

my 250 head has been milled to 48 cc's
i have 30 over pistons dished
victor head gasket im told it has a .044 crush thickness
stock deck height

 

[62Ranchero200]

@64 200 Ranchero,

Your cranking compression is generally acceptable.

You probably covered it in another thread, but my two questions are:

Was the bottom end just rebuilt? If so, I would expect the numbers to be a little more consistent, but I'm no expert. Assuming that your 250 head had a three-angle valve job while it was at the machine shop, is that right?

What cam are you running? Although your signature gives a good idea of your setup, your cam information isn't in there unless I just missed it. More duration decreases dynamic compression, which I believe would lower your cranking compression readings.

Thanks
Bob the Builder

 

[64 200 ranchero]

The head has had a 3 angle vale job done 3 years ago im running a 264 cam @ 112 lc from classic inlines

 

[62Ranchero200]

The 264 cam does have a bit more duration than a stock cam, so would reduce your dynamic compression slightly. Was your bottom end just rebuilt?

Thanks
Bob

 

[64 200 ranchero]

I did not rebuild the bottom end

 

[62Ranchero200]

The variance you mention in cranking compression is acceptable for a seasoned engine with some wear. My guess would be that the variation comes from the rings and/or bores being worn more in some cylinders than in others. One way that you could test this (if you haven't already done so) is to squirt about a teaspoonful of oil into each cylinder just before you check the cranking compression in that cylinder. If the cranking compression increases significantly in a cylinder, it's because the oil is helping the rings to seal to the cylinder wall more effectively.

Having said that, with the setup you describe I wouldn't worry about rebuilding the bottom end just because of the compression readings you found.

Thanks
Bob