Kris01 wrote on 01/29/16 at 11:29:45: I'm not going against what's been said already. If you open the exhaust port you are allowing the exhaust gases to exit faster. They don't build up behind the port as easy. This will effectively lower the cylinder pressure (not compression -- I'm picky about terminology.). There's absolutely nothing you can do to change the compression ratio without changing the "squish area" above the piston.
Once the piston is moving fast enough and bringing in more air faster than the newly opened up exhaust port can expel the exhaust gas you start making up the loss in cylinder pressure. That's why opening the exhaust up moves your power higher in RPMs. The engine has to rev higher in order to make the same cylinder pressure.
Terminology or not, if there are still pressured gasses in the cylinder before the intake valve opens up, you end up with more gas in the cylinder on the compression stroke than if the cylinder would completely empty out of gas.
Cold air fuel mixes with more hot gasses, causing the cold gasses to expand,
Thus, increasing compression as well.
Well, technicallyyou're right in that the compression ratio hasn't changed, since the stroke and chamber remain the same size,
But pressure on the compression stroke has; so if you had a pressure gauge, it would show higher compression.
Opening the exhaust port will exit more hot gasses, thus allowing more cold gas to enter in the cylinder, and less expansion of the cold gas through mixing with hot gasses, essentially lowering compression due to lower air temperature, and less gas in the cylinder.
There! I think I said it twice in one sentence!
This results in higher efficiency, as the cold gas only starts expanding once it ignites, rather than as soon as it enters the chamber.
This also should result in lower vibrations, as there is less resistance at the exhaust, and less resistance at the compression stroke, again causing higher efficiency.
But that'd be my take on it all...
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