Guess it depends what sort of engine you're trying to do some datalogging on...

Single cylinder lawnmower engine - could probably get away with a 12F683
Datalogging on a B-36H, well, 168 cylinders, 336 spark plugs, 4x J47 jet engine, now you need something a bit more substantial...

Voltage inputs - It's all in your chosen PIC's 'Electrical Specifications' section of the datasheet.