Believe me, don't over stress or over think yourself. R18 is not a good idea for this chassis. L15A7 will definitely require a lot of changes in the bay area as well. I currently have an L15A V-TEC swapped with Engine Wiring, ECU Wiring, and Dashboard Wiring, and believe me it was a tough job to do. My motto is if you want to swap things, Do it! but on the other scale do remember, the best and peaceful swaps are only those swaps in these family cars which are offered in other domestic markets.
Honda Fit comes in three variants : 1.2 DSI, 1.4 DSI, 1.5 VTEC. You can swap any of it with any other easily. Everything stays bolt-on. If you plan on fitting a R-18 into your City, it's gonna ruin the weight-body ratio, you are gonna have to cut so much, there's no Honda Aria that comes with Prosmatec Tech, it either comes in CVT or Manual Tranny. The only Prosmatec Fits are for Canada and America.
I had to fit the immobilizer unit, I had to alter some connections of the new dashboard wiring because it supported a climate control console while mine was simple, I had to figure out the genuine wireless entry key fob. Do you know how long did the engine swap took with the bay wiring and ECU? 4.5 Hours.
But how long did the dashboard wiring swap took? 3 days and some dope electrical knowledge.
Now I can easily do 180 kph in 7 Speed Mode, I Allhamdullilah get 18+ km/l on highway (when hyper-miling) and 13~14 km/l in city driving. My weight body ratio didn't change. I didn't have to put a new compressor nor a radiator everything was just so perfect. I did put a new Speed-o-meter cluster as the old one was a piece of crap but whatever I needed one that could display 7-speed mode info. And yes, I did swap the steering wheel as well for the buttons.
Every thing costed about 75K. Which is a fairly cheap swap. The only thing I dislike in this setup which I did before too as I had a Vario, is that this car utilizes a start clutch rather than a torque converter so the launch is kinda sluggish but once clutch is totally engaged, I love the torque, I love the acceleration and the best of all, I love the sound it makes when between 5K~6K RPMs. This is the smallest VTEC in the whole family but still I love it because it's the perfect motor believe me.
I just got a Aspire 1.5 Pros, like a couple of weeks ago? Car is insanely smooth, launch if amazing, absolutely amazing, it easily revs to 6.5K in the first gear, but I never hear that transition from 12 valves to a 16 valves engine because it's i-VTEC so the valve timings are continuously varying but in my L15A, when not under load it acts as 12 valve engine but 16 valve on higher rpm and loads. The transition point is around 3.5K as I hooked up my scanner with it and was reading live data, the VTEC solenoid was activated every time I reached 3.5 RPMs.
So what I am saying my man is that please don't use a tank or a F22 to kill a sparrow, just get a good riffle.Changing my Honda City Vario to Honda Fit Aria (Vtec Steermatic PKDM) was one of the best investments as well as the decisions of my life. The whole family uses my car without any problems! no one knows I did this lol.
If you really want a sports or a snappy car and love swaps go for a RX-8, rebuild it's rotary, put a 2JZ in it (although this is gonna mess up it's handling but again it's not a family car!).
P.S If you don't like you dashboard, go for a black one! It's absolutely delicious. I hate beige one myself but I bought beige seats and now I regret it lol. But black dashboard looks so cool man you'll forget GM's dashboard.
To deal with the body roll although I still think it's too much. My car is a bit lowered and I have 15" wheels with 195/60/R15. This helps sticking to the road at higher RPMs plus my tires are spec for 200 km/h but I had never sped more than 140 Km/h, 180 km/h just this one time to test it's limits
