Starter turns but no compression - 2006 Pontiac G6

A little more info would help. Is this a rebuild situation? R U trying to start a fresh engine? If so, if you can turn the assembly by hand but not with the starter, I would look at increasing the ground connection. Starter should turn it especially if you can turn it by hand. Use a heavier cable for the ground.

Do a compression check on all cylinders with all plugs out. Low compression will always make it fail to start unless you get it spinning about 1500 rpm with starter. Not gonna happen.

Charles: Maybe if you watch these videos, they will help: https://www.youtube.com/watch?v=fwdtfOAwGcQ or this one: https://www.youtube.com/watch?v=MAjZ7X7NbwA According to OnAllCylindars.com:

Problem: Starter failed shortly after installation.
Solution #1: Confirm that you purchased a starter designed to work with your engine's compression ratio. Engines with 11:1 or higher compression generally require a high-torque, gear-reduction starter. Otherwise, the engine will turn over slowly.
Solution #2: Check the position of your starter in relation to your headers. If your starter is continuously heat soaked from exhaust heat, chances are it will fail early. You'll either need a different starter design, or you'll have to install a starter heat shield with your next starter.
Solution #3: Clean the starter mounting surface. Most starters are grounded through the mounting block, and if there is excessive oil or paint on the block, the starter will have a faulty ground. This will cause the starter to pull excessive amps, making it overheat and wear out.
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I hope this helps!

Nope, the starter relay controls the starter, if the starter is turning the engine over then it is working and you have another problem. Either you aren't getting fuel or you aren't getting spark. Or possibly compression, but I would think it is one of the other items before it would loose compression.

When it "whines" is the engine actually turning over? If not, your starter is not engageing the flywheel and is just spinning. could be some missing teeth on either the starter gear or flywheel.

sounds like,yes, Might try pulling the spark plugs and try it then,
if the intake gasket leaked or even fuel leaked into a cyl. when
the compression stroke comes up liquid won't compress so the
engine won't turn over.

This would be normal for an engine with compression. If it does this with a starter, then the compression may be too much for the starter.

Also if the timing were overadvanced, it would bind against the compression.

Sounds like a Timing chain or gear issue to me too. But check the Starter drive gear and Flywheel teeth. What bothers me is that all cylinders are dead without 1 cylinder creating drag on the compression or power stroke. It seems a little funny that 1 cylinder does not have all valves closed out of 4 cylinders.

I envision the Starter spinning without the motor turning and this can happen if the Starter Bendix stripped or the Flywheel stripped. You would have zero compression. You can put a probe into the cylinder and turn the Crankshaft pulley to determine if the pistons are moving.

Would like to know the outcome of this.

Pull the distributor cap off and watch the rotor as you crank the motor. If the rotor doesn't turn your timing belt is broken. If the Engine is actually firing while you have the starter engaged but dies immediately when you let off even when you press the accelerator check to make sure the starter isn't staying engaged in the flywheel as that will kill the motor and burn your starter out. If all is good so far do a compression check. If you don't have good compression on all cylinders your engine maybe firing but with no compression will not run.

Check for power at the starter from the solenoid when the key is in the start position.. if you have power there it is the starter .. if not you probably have a safety switch malfunction . You can bypass them for testing purposes.