I have 2005 Sonata sirius -II engine(based on 4g63 mitsubushi engine). What are the possible engine replacements or swaps with high Fuel efficiency ?
Replacement of this engine is best done with either of the 2 engines which came in the Sonata in 2005 - the 4 cylinder you now have (highest fuel efficiency), and a 2.7 liter V6 (more power, lower fuel mileage). Replacing your existing 4 cylinder with the same engine; whether a rebuilt/remanufactured engine, or a used engine that comes with an acceptable warranty, will best meet your fuel efficiency requirement, and, in the most straightforward, inexpensive manner. Engine swaps are an invitation to death by a thousand cuts - lots of hidden "gotchas". If you decide to go through with it, consider contacting a certified technician, such as one from YourMechanic, to inspect the car and suggest your best options for you.
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