First off don't laugh at me and second please answer my questions with gas mileage in mind (I don't want mine to suffer that much).

Anyways I'm rebuilding my D15 and have a few questions. The bottom end will be stock as of now but is if I bolt on a beefier head will I lose mileage or gain mileage. I'm looking at a head right now thats been ported and polished, 5 angle valve job, milled .010", lightened rockers and retainers, and stock cam. Also should I invest in a I/H/E? Will that improve my gas mileage?

 

I was getting 43mpg stock (with EX trans and 4.7 FD) on my Y7. I put on my custom SMSP header and I'm lucky if I get 110km's out of $10 of 87 octane (2.95/gal).

You'll need to get it tuned if you want good economy. Even shelf headers like DC will throw off your A/F.

 

But how will this head affect my mpg?

 

You'll need to be tuned since you'll need more fuel.