hi, I'm looking for a jumpstarter for a 5.3l petrol. The old Mechpro 1200amp used to start a 5.7l petrol, but new ones say a 1200 won't cut it. The AA was no help at all. Any advice on what size I need and is there a best time to buy one. (Also any insights on why the new 1200a won't start 5.3l when the old one started 5.7l with a totally flat battery). TIA.