So to sum up the advantages in practical terms, if I understand you correctly, the Y5 has more power and pollutes less. I would certainly expect an engine thats 7% larger to have more power and torque. On the other hand, I would then expect the fuel economy to be within 7%, in order to qualify as "similar mileage" However comparing the 96 HX to the 95 VX, the fuel economy is 20% less. If power is what you want, I agree the HX is the better choice, although IMO there are better, but if fuel economy is the overriding consideration, nothing touches the VX. Just my opinion.
Loserkidwac, your swap/project is awesome and I wouldn't change to a Y5 because then you'd have to convert to OBD2 (I would only recommend this to people with a 96+ Del Sol). IMO you'd achieve a better result by pursuing those aero mods you mentioned in your build thread which I always keep an eye on.
The only similarity between the Y5 and Z1 heads is that they use the same cam. All of the Y5's superiority is built into the head design. The only exception is that the CKF is mounted at the crank. In order to use a Y5 ECU, you'd need this sensor so you need the Y5 block.
Originally Posted by nowhhs
comparing the 96 HX to the 95 VX, the fuel economy is 20% less.
That is not a fair comparison. You didn't read my post closely because I mentioned that the HX is in a heavier, less aerodynamic chassis with larger wheels and with more accessories driven off the crank (ie power steering, air conditioning was more common than the VX). Take these out of the equation and the D16Y5 gets similar if not better mpg than the D15Z1.
Since we are talking about the D15Z1 vs the D16Y5, you'd have to compare a stock VX to a D16Y5 swapped VX. In that case the Y5 VX would get better mpg as well as all the other benefits I listed to the Y5 including more power and lower emissions. The Y5 is also more reliable. For instance if you have a small vacuum leak on a VX, you get bucking and all kinds of driveability problems. On an HX the OBD2 system is smart enough to back off the mixture and timing so you wont notice any driveability problems; just a drop in mpg and a CEL.
My car is a pretty good example of this. I've already taken this thread way off base and I apologize for that. But I will say that when I'm not tuning the car or driving the piss out of it, I get similar mpg to the stock engine using an engine that is very similar to the Y5.