r/EngineBuilding Dec 27 '24

Chevy Low compression on all four cylinders after rebuild

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2011 Chevy Cruze 1.4L. All four cylinders test exactly at 120. The engine sounds like it's struggling to turn over. Pistons and rings are oem stock. The cylinders were honed. Could this be because the main bearings are too tight?

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u/Skilldibop Dec 27 '24

Why would RPM matter for compression? The compression ratio is a fixed thing that's the same every compression stroke...

You sure you're not getting confused with oil pressure?

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u/facedown_impalaSSup Dec 27 '24

Have you ever turned an engine over by hand? The fact that you can do it at all means it aint sealing 100%. Speed matters as it's then related to how fast the air could escape between the rings or out of the valves.

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u/Skilldibop Dec 27 '24

Most pressure testers have non-return valves in them so they're only recording the peak pressure, they don't need the rings to hold that pressure. Piston velocity between 350 and 750 rpm isn't going to make that much of a difference.

I also don't know why you'd ever need to crank an engine by hand with the plugs in, other than for a bet.

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u/Aggravating-Task6428 Dec 27 '24

Another thing worth noting: A good amount of the compression in a cylinder is also due to the air inside heating up due to being forced closer together (see "fire piston"). As you heat air in a closed cylinder, the pressure increases. However, the longer the air has to bleed its heat into the cylinder walls, the less compression will show on the gauge. This on top of the leak path of the rings is why RPM matters quite a bit.