How much compression
Safety Car
Joined: Oct 2007
Posts: 3,729
Likes: 267
From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
the actual psi # will vary depending upon the engine and the condition of the motor,
you are looking for constant #'s across all eight cylinders.
ball park #'s .......60 ready for rebuild.............. 125 good to go.
My bet for 79-350 78k miles is about 85 psi
if any 1 cylinder is way lower than the average that's a problem. 69VETT
you are looking for constant #'s across all eight cylinders.
ball park #'s .......60 ready for rebuild.............. 125 good to go.
My bet for 79-350 78k miles is about 85 psi
if any 1 cylinder is way lower than the average that's a problem. 69VETT
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Look at (and record) each cylinder's first stroke pressure and the maximum pressure (5 or 6 strokes). All should be within 10-15% of each other and no 2 adjacent cylinders should match (1-3, 3-5, 5-7). A cylinder leak test is a good thing too. Pressure in the cylinder at TDC and you can hear where (or if) there's a leak.
ED
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
It's KYBB-- Keeping Your Bride Buffaloed.
As long as you can make it LOOK like you know EXACTLY what your'e doing, she'll never say a word. I do it all the time. Approach a job you've never done before like is an everyday thing. Confidence is the key. Or at least looking like it. Be careful- coming back all bloody is points off. Don't ask how I know that one.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
With all 8 sparkplugs out of the heads and testing just one of them for max. compression reading after 4-5 strokes of the piston [with carb throttle & choke is 'blocked' open state], you should get around 150-160 psi with a 'tight' engine. If you get over 140 psi on all of them and they are all within 15 psi (or so) of each other, you shouldn't need to do any work on the pistons/rings. If lower than that, you should squirt some oil in each cylinder before a repeat test to see if the numbers jump up. If so, you have worn or broken ring(s) in that cylinder. If the numbers don't increase when you add the oil, your valves/heads need to be rebuilt.
