I rekcon with an exhaust (I was only running a almost stock exahust) and tune all the little simple mods you can go 13's in a manual 5 litre no problem. Little bit harder in a auto because you can't pull drastic 4000 rpm launches and plus you are down 10rwkw everywhere before you even drive it and another 6+rwkw for the irs and it's getting a bit hard. My 60 foot time got down to 2.06's at one stage, pretty good for a auto. Far as I know no ones gone under 2.0 in any kind of unopened 5 litre ever, even in a manual but no doubt it's possible given my auto time. But I don't think you can do more than about a 13.9 without playing manifolds. I wouldn't have been able to break a 14.2 in mine, it just didnt have the top end on that exhaust. You need to be able to make a touch over 100 mph to be on the a 13 and I've never seen anyone get even close to that in a auto 5 litre. I might have been up for 97-98 but that's still along way short (about a manual box and exhaust) of the power you need