I feel like I have to write this. These cards are cards you should absolutely not fuse if you want to free up inventory.
This includes Initial Tech, Early Tech, Mid Tech, Advanced Tech and bike-only tech cards like Common Tech, Rare Tech and Pro Tech. One time I got a crap ton of Initial Tech cards and when I bought the Donkey I lacked the Tech cards needed to PRO it because I already fused some of them.
Engine cards are almost as valuable as tokens. Be it the common ones (i4, V6, V8, etc), the rare ones (Electric, Hybrid, SRE, etc) (with the exception of Hot Wheels engine, since HW cars are useless anyway), or the legendary ones (FiV8, F6, V10, V12, W16), engine cards should not be thrown to trash at all costs, including bike engines. Even the F12 engines which now only Ferrari Testarossa uses could be a benefit in the future should GL release another F12-engined car.
This is important if you like bikes in A8 like I do. Bike parts are rare despite their rarity being stated as Common like D, C, and B class bike parts.
BPs of useless cars such as Mazda 6, Cadillac ATS-V, Mercedes-Benz SLK 55 AMG, Sbarro Alcador and many other useless cars are OK to be fused unless you're a collector.