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Old 03-06-2008, 11:18 AM
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i searched for factory compression on a stock 305,and found this
"Factory new compression pressure is 180#."but that thread was a ford thread,i searched thru 10 pages,so bear with me if its been answered already,would the same pertain to a chevy motor,and when is a chevy motor consider tired,at what psi reading,in other words,my highest reading was 145,and the lowest was 136,well within the 10 % diff that people speak of,but is this a descent reading,and what is the lowest psi reading you should even think of drivin on,to make things simple for me,plz jus fill in the following
excellent condition___psi to ___ psi
good " " "" ""
fair " " "" "" """
pooched "" """ """ """"
thank you kindly

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Old 03-06-2008, 12:04 PM
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its not the PSI you need to be worried about its the diffraction between cylinders that lead to problems...

As far as when the SBC is tired? When its burning more oil than gas
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Old 03-06-2008, 12:21 PM
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A compression reading of 165 all around would be about the limit for pump gas. I've had a bunch of daily drivers in the past that checked as low as 110 and still cranked and ran.

Warm the motor and remove the valve covers. Idle it while you reset all the valves. Replace the covers and remove all spark plugs. Disconnect the coil. Wire the throttle blades wide open. Do your compression check again.
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Old 03-06-2008, 12:54 PM
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Running on low compression is not a good indication of whether you have enough or not. I picked up a 350 last week that you could grab the dampner and turn the motor over very easily, you could hear the compression leaking down with no resistance at all.

It was an emergency and we threw it into an Ice racer, it fired right up and would turn 6500 no problem...for about 5 laps...then it snapped a rod in half and that was the end of that.

I am willing to bet it would have had less than 50 psi on most of the cylinders.
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Old 03-06-2008, 01:49 PM
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cyl don't fire with less then 90psi
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Old 03-06-2008, 03:44 PM
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As far as when the SBC is tired? When its burning more oil than gas[/QUOTE]

Thats classic .................and I'm using it.
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Old 03-06-2008, 04:02 PM
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you need to take the guage to a industrial instrument's shop and have it calibrated for correct actual reading....(or atleast go borrow 2 more comp testers and do it 3 times to average the results)

you need to know the corrections factor for your altitude,,,ex: a motor that tests 180psi(=approx 10/1 at sea level) at sea level will only test about 160psi(=approx 9/1 at sea level) at 5,000ft altitude

your camshaft grind and/or how it's installed (retarded/advanced) can show a low static CR test results (due to cranking so slow) but make fantastic actual power due to a very high dynamic CR at the actual operating rpms...

your test told you what you want to know about the rotating assembly:
it's in good shape!
because..... some cylinders do work harder and some cylinders run hotter or colder.......the motor develops a wear pattern which show up as some test high, some test low=time for a rebuild

ps: redo the comp test following techinspectors instructions
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