Quote:
Originally Posted by Frank Lee
One thing that has stopped me from pursuing this further is not wanting to have the system "neck down" as IMHO that would be bad. Does that mean the ports in the heads are too big? Exhaust valves too? Probably.
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The bigger the exhaust valves, the faster the cylinder can blow down, decreasing pumping losses. In other words, the faster the exhaust blows out of the cylinder, the less work the engine has to do pumping it out of the cylinder into the exhaust system.
In a performance situation, you worry about filling the cylinder as much as you do about emptying it, so you make both valves as big as possible, with the exhaust valve ideally flowing about 75% of the intake flow.
In a hypermiling situation, the intake valve is always going to be plenty big. So if I were building an engine from scratch to increase FE, my focus would be on making the exhaust flow as much as possible at low lift numbers. That means as big an exhaust valve as I can fit in the head, with regard to other parameters such as shrouding.