Please stop saying thi, it's making me crazy. An engine can be tuned, a chassi cannot See definition below
I rest my case
chas·sis
ˈCHasē,ˈSHasē/Submit
noun
noun: chassis; plural noun: chassis
the base frame of a motor vehicle or other wheeled conveyance.
synonyms: framework, frame, structure, substructure, shell, casing
"the chassis of the car is in mint condition"