Originally Posted by Stu @ M Developments

Typically, anything big power with large injectors (EG: 850cc) takes a week to do correctly due to the driveability and cold start maps taking so long to perfect. Making them fast is actually quite simple... making them drive properly and run nicely when stone cold is the tricky bit that requires us to have the car for a full week, and even then, we only have 5 attempts at getting it perfect when cold, cos once shes hot, shes hot.

Hope that clarifies.