the question on my mind is..
A. how much MpG a tank will you gain from the GDI because maybe in a year that part alone would pay for all the extra work ?
You tell me. You're still going to burn roughly 20 HP worth of gas to move it down the road at legal highway speeds.
B. Honestly do you think hot rodders back in the day said hey.. that mod is only going to get me a extra 5hp why bother?? NOOO
They said HELL YA Extra 5 hp.. and honestly a extra 30 hp is a nice bump add that with a few other mods and suddenly things are imporving.
The hot rodders back in the day were also working with carbs and points on an engine that was incredibly inefficient, so just about anything they did was sure to realize gains. They weren't working on engines with millions of dollars worth of R&D sunk into them run by miles of wires and computers as powerful as any commercially available PC.
No doubt, a bunch of little things add up, but there are far larger gains to be found elsewhere for far less monetary and time investment. Since the donor cars are at most two years old, you're going to be paying a premium for parts; this isn't a $50 head from a '91 Civic we're talking here... expect to pay thousands for the hard parts. Being a high pressure fuel system, you may have to change everything from the tank forward, depending on where Hyundai/ Kia decided to put the high pressure pump. And since you're going to be taking parts from a different car, nothing will line up perfectly in that regard, so you're going to need custom lines made. Then you have the electrical nightmare. Have you ever even looked at a wiring diagram for a modern drive by wire engine management system? Have fun with that mess. So then there's other smaller issues that won't pop up 'till you really get into it. Like cam to deck height... what if the cams on the GDI head are a different height from the deck of the block than the EFI cams? Time for a custom cam chain/ belt to make it work, and adjustable cam gears to make up for any changes in timing the height difference causes. Since the GDI has both a spark plug and injector in the combustion chamber, the valve angles and valve diameters are most likely different than the EFI head, so you're looking at at the very least, cutting the valve reliefs in the top of your existing pistons, and quite possibly, will need entirely new pistons to match the GDI head for this and combustion dynamic reasons (every little bit counts, right? Remember that when you go have custom pistons machined). Other stuff that may pop up is that exhaust and intake ports may be of different sizes and shapes, or that the GDI intake manifold interferes with an accessory or the radiator or hood, or that the GDI throttle body won't line up with your intake...
Kinda get where I'm coming from now? It's not as easy as slapping a head on and going, "OMG, hybrid just like a Honda!"
a person might be able to do this GDI mod alittle cheaper if he / she has the know how to do the work
If the head physically bolts to the block, you're about as close to having the swap done as you are to landing on the moon by jumping on a trampoline. I've been working on and modifying cars for 23 years, and I can tell you from personal experience that there is no practical reason to go through the immense undertaking of swapping over to GDI for a measly 30 HP. Assuming you even get that much out of it. Remember: the engine is a system, so the GDI engine's bottom end most likely has different internals that contribute to the gain in power. Even something as insignificant as a different pressure spring in the oil pump bypass valve will have an effect. Remember: Ever little bit counts.
So let's go back to the whole "mileage increase will pay for the mod" idea you had back there...
A solid estimate for this mod would be in the neighborhood of $4000 at this point in time, assuming you did ALL the work yourself (including various unknown machining/ welding tasks that will pop up). If you think this number is unrealistic, go price everything out and let me know how far off I was. Let's also figure that a realistic mileage increase would be 2 MPG, that you drive 12,000 miles a year and your combined mileage is 32 MPG. You ready for some math? Here we go...
12,000 miles @ 32 MPG= 375 gallons * $3.75/ gallon = $1406.25 (stock engine)
12,000 miles @ 34 MPG= 353 gallons * $3.75/ gallon = $1323.53 (GDI head swap)
That's an awesome savings of $82.72. Or, almost two tanks of gas. Over the course of a year. So how long would that head swap have to be driven to pay for itself?
$4000/ $82.72 = 48.36 years.
So, to reference my previous statement about doing something for the sake of doing something... Have fun with your sex change operation. I'll stick with turbos.