I'm confused by the mileage figures between the Civic Hatchback CX and the VX. Here are the engines on wikipedia:Honda D engine - Wikipedia, the free encyclopedia And this is the comparison between the two cars: 1995 Honda Civic Specs and Features - MSN Autos The VX has D15Z7 engine which gets 47/56MPG City/Highway while the CX has the D15B8 engine which gets 40/45mpg City/Highway. The CX has 70hp while the VX has 92HP. The largest difference I noticed was that the CX has 2 valves per cylinder while the VX has 4 valves. The VX also has VTEC, so when under I think 3K RPM it runs on 12 valves and above that on 16 valves. In 12 valve mode, the engine keeps the "unused valves" slightly open as to not have fuel collect behind it but otherwise it's effectively shut. The CX is running on 8 valves all the time, which I thought would actually yield better fuel economy since it wouldn't be able to consume so much fuel at once. I believe this means that on the D15Z7 you've got 1 intake and 2 exhaust at under 3K RPM which is most driving. I can sort of rationalize why the VX would get better highway MPG since it does have more HP, though in my experience, vehicles with more HP always get less MPG compared to their lower powered brethren. (Of same engine type/block/litre) So everything that has been mentioned, could the reason why the VX gets better mileage than the CX is because it has twice as many exhaust ports as intake ports? The NonVTEC version of this engine gets worse mileage than the one with VTEC-E which is what the D15Z7 engine has and what is subsequently in the VX model. If this is true, wouldn't this imply that performance is hampered by exhaust restriction? And that improving exhaust flow would improve performance somewhat while improving fuel economy considerably? It would seem to make sense since it would rob the engine of momentum by having to force the exhaust out of the chamber. So then why would people make the impression that the exhaust system on the civics are adequate?