Im not a lawyer or anything,but i would bet a good some of money that if you mod your engine 10k+ and are not paying a higher premium and so on the insurance isn't going to cover the car (lets say its 15000 value without motor mods) + the 10 grand in mods if it is a your fault accident. They lose money due to your rate being based off your risk as a driver and the vehicle. All kinds of cool calculators for it. Now i will say that a stock 4.6 sohc brand new from ford is like 6k, and yet you can buy a fully built one for 8k easly. So some cars it is close either way. Where is gets shady is when they try to get you a used 4.6 with approximentlly the same miles and say its fair, rarly would every occur because a collision of that much energy would total the car, but you get my point.