Originally Posted by blackhand
I find this confusing. Back pressure assists to hold the exhaust valve closed, so I can't see why less back pressure on a dead engine would affect cylinder pressure.
The valve spring holds the valve closed. Back pressure in fact acts against the spring, trying to push the valve into the cylinder (i e open).
Less back pressure means the piston will have to work less as it empties the compression chamber on the exhaust stroke. This translates to lower pressure in the combustion chamber all through the exhaust stroke as it vents easier into the exhaust manifold.
I imagine there would still be some back pressure even with a dead engine.
Yes. But less than had the engine been running.