Hi, what compression readings should you get with a stock LS5 engine (454cu - 365hp) when cold.
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Re: big block engine compression test
At sea level, atmospheric air pressure is 14.7 psi. Multiply by your engine's specified compression ratio and you'll get the MAX reading you should expect. Don't be alarmed if observed readings fall below MAX since engine wear and resulting leakage is normal over time/use...
The rule of thumb is you want to see similar readings on each cylinder and red flag those that fall 15 psi or more below adjacent cylinders. When/if you find individual cylinders that are out of line compared to the others, it's time to consider going beyond a simple compression check and execute leak-down tests on low reading cylinders.
When done properly, leak-down tests can pin point whether the problem is caused by piston ring wear, intake valve and/or exhaust valve seating failure. That'll give you a picture of what level of engine overhaul you're looking at... -
Re: big block engine compression test
Engines will never achieve atmospheric pressure times CR because there is almost always reversion at cranking speed.
For example the listed cranking compression pressure for the 10.5:1 CR 300 HP engine in 1963 is 160 psi, and the 11.25:1 CR SHP/FI engines are speced at 150 because of the later closing inlet valve of the Duntov cam.
The applicable service manual will list the cranking compression pressure, and the test should be done with the engine hot. Also, the throttle should be blocked open and remove all the plugs for the test - not just one at a time. The usual acceptable variation recommended by GM is 20 psi.
Often the readings will be higher than the specified range. The key is to look at consistentcy, so variation is more important than the absolute readings.
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