Would using a VX ecu in an EX yield better FE?
 

Ive got a 96 civic ex, so i know there is the obdI vs obdII difference, but would using a vx ecu help make a d16y8 run more efficiently than it already does?

No, that's not the way it works.

If you changed heads, possibly, but there would be some wiring work involved in swapping from OBD-II to OBD-I.

It would be easier to just swap in the HX head (96-00 VX equivalent) since it is the same as the Y7 and Y8.

I've actually got a Y8 head on a y7 block... Is the HX head a vtec head? What is the dif between the Hx head and the EX head? Are the cams different? Wouldnt i have to use the hx ecu to get the same vtec settings?

They are both VTEC, but they use it in different ways.
Yeah it's just cam profiling, the VX/HX or VTEC-E system operates sort of in reverse of conventional VTEC. Below 3000 rpm, the engine is running a very sedate cam profile, each secondary inlet valve barely opens. When VTEC "kicks in" the engine basically just switches to the cam profile of the Y7.

For a better explanation,
http://www.youtube.com/watch?v=Bzfeeajm07g#t=4m10

Well crap, that link didn't provide the jump I wanted. The explanation is at 4m10s

As thebrad has already said, it'll just run a whole lot worse.

so then using a hx head WITH the Hx ecu would improve the FE..
What about the HX cat conv situation? would i need to switch to an oem hx one as well?
also, the transmission i am using right now is from a 93 dx.

It will only improve your FE if you drive it within the engineered parameters. It's a simple job for anyone who's remotely mechanically inclined, but I'm sure you can find an HX in your area for a reasonable price, they aren't exactly flying off the shelves.

HX also requires special spark plugs and oxygen sensors, not something you have to replace everyday but when you do they are pretty costly items.

The cat is part of the exhaust manifold assembly like on the CX if I remember right.

Gearing probably isn't ideal, but work with what you have, I guess if you are that determined.

TheBRAD, is the civic on the salt flats in your profile your daily driver? Does it go fast as well as get good FE?

Honestly, i've always wanted a civic vx, but also i want a Bseries setup, too. The Vx milage would be even better in a sedan, but i know it would score less mpg than a lighter hatchback... My y8heady7block gets me around 30mpg average AND that is delivering pizza, and i drive it pretty hard daily, pushin my rpms up to 7k often <- still getting 30mpgs...

IF i had the money i'd have a sedan with a b for me, and sedan with a hx or vx maybe for my wifey.

No, not even close to my car. My car is the exact opposite. It's pretty much a hoopty.
http://i19.photobucket.com/albums/b1...e/IMG_0856.jpg
It goes slow and delivers good fuel economy. That image was included in the profile because it's a pretty good baseline for aero mods for that generation Civic.

If you want better fuel economy, take it easier on the pedal dude.