A car can onli ever have at the most 85% - 90% volumetric efficiency its to do with the method of intake and how the strokes over lap, if you understand what i mean, as exhaust gases are expelled in comes new air but some will escape with the exhaust emissions. that cannot be prevented.
u can onli ever have 90% max of thermal efficiency, or rarely you will be able to increase that, if you have the money to spend 
then you have forced induction kits, they are good, cheap for the slight performance that they increase
You could change the manifolds, bore it out, change the pistons, skim the head, and this may improve the performance,
then obn other hand you could supercharge or turbo, this would improve performance, efficiency and a lot more...
however nah that spiralmax is a load of ****e
