By "car computer" , do you mean for Electronic Fuel Igntion system..

If so , its very do-able with an 8051.

Early Bosch Motronic systems were made utilising the 80c515 and Lucas Cav produced an 8032 based sequential fuel ignition system that ran the Metro 6R4 V6 rally engine (400bhp at 9000rpm).

The following may help

http://www.diy-efi.org/diy_efi/m...jects.html
http://www.donsautopages.co.nz/efi.htm
http://www.bgsoflex.com/megasquirt.html