[The Rooster]

All things being equal, bigger engines burn more fuel, and the Teg's 1.8 has no trouble moving the car as it is, mine anyway never strains. I can usually accelerate up hills in 5th gear, or at least maintain speed on steep hills in 5th. To make a larger engine save fuel, you have to spin it at a lower RPM, which means you could have better luck sticking with the stock engine, and doing something about the transmission to make your gears taller, rather than putting in a larger engine. A M/T swap would be cheaper, easier and more effective in your case.

If you drive your car modestly and aren't doing a lot of hill driving or towing, the small opening should be fine, the AT isn't going to generate a ton of heat unless its working, aka the torque converter is never locked up.