I don't think dropping in the d15z1 is worth it, your d15b6 is already good enough for that car. The thing about the federal HF that I don't like is the gearing, they just seem too high LOL. It's good for gas but it's a hassle to drive around. It would be nice if you can find a CA HF transmission for it. I used to have a federal HF that got great gas 50-54mpg which I swapped the engine out 250,000 miles for a CA HF engine 80,000 miles. The CA HF was much more friendly to drive due to its shorter gear, but it averaged a little less gas 47-51mpg. I would take the CA HF over the federal any day because the federal version is just to hard to drive around. I now have a VX because the HF two seaters just wasn't cutting in for me. I miss my HF though.