If its winding over slow that's a good indication of bent things, unfortunately you won't know what you need to replace until you do a bit more investigation, a leak down test will show if the cylinders are still sealing, but if you've got to pull the head off anyway to change rods+ whatever else is damaged you may as well inspect the valvetrain while it's off

The engine dosent sound too fucked up (no knocking or scraping) just sounds like it has a dead battery. So if the rods are bent could I still run a leakdown test to see if the valves are still good? Im just thinking if just the rods are bent can I just get some used rods and just slap it back together?

Compression test will show if the engine is mechanically damaged, all cylinders within 5-8% of the same reading should still run smoothly, a bent rod or valves will show lower compression on that cylinder, cylinder walls can crack from hydrolocking but you would most likely bend rods firts (and possibly damaged the skirt on a few pistons if the rod hes bent far enough to let the piston hit the crank, alternatively if the rod twisted it could have bent far enough the valve reliefs dont line up any more, in that case you would have bent valves too) and if the cylinder wall was cracked you would have water in the sump, main bearings will be totally fine unless you've managed to bend the crank but it would probably not run if you managed to do that, e series rods are shorter than au-fgx except el hybrid engines they are the same length as au-fgx, the bigger problem for swapping rods on anything but a factory turbo 4.0 is they use a press fit gudgeon pin and are quite difficult to change without the proper equipment,.you also can't put sohc pistons in a barra as the valve reliefs are in the wrong position for the barra head

The oil has no water in it as I got to the other side before it died. Yeah flooded road with overgrown depth markers, water was only about 30 cm deep but this car is too low for almost any water and so it went over the bonnet and flooded the intake. Im worried if maybe the main bearing caps would have been damaged, if the block and head are salvagable ill put some new rods in it and do it up to xr6t spec. Also might be a dumb question but would el rods fit? I got an el bottom end in the scrap heap and it would be nice to get something out of it

How did that happen - a flooded road?   Yeah, water doesn't compress, although I wouldn't have thought bent conrods, would make the engine run poorly?   An inspection camera down each bore, could be helpful maybe? A leakdown test, would show any bent valves. Plus as @deankxf says, measuring each cylinder stroke with a plastic stick or similar, would be useful!   Did you replace the oil too - I imagine that would be full of water also?  

That probably would be the go, although if most of the guts are ok I was maybe thinking of doing a turbo build (nothing too crazy just put some stock xr6t parts on it if i can find them cheap)

depends i guess.  if it was mine i'd probably chuck another engine in.  @Thom may know if a bore could crack from it, i'd expect it possible, so therefore having the engine block crack tested might be something to do. 
i wouldn't expect anything else, but i also don't know heaps about this issue. 

Ill give that a try. If con rods are fked what else needs doing or could I get some rods and go?

bent con rods are likely.  there might be a way to check by dipping something down the spark plug holes and rotating the engine by hand.
if one or 2 are lower on each top of the stroke position  then bent conrod is confirmed. 

 

3 days ago I learned the meaning of if its flooded forget it and my fg falcon got the engine flooded with water. I pulled the plugs out and got the water out of the cylenders but it wont run now and the engine sounds very lumpy. Im planning on running a compression test but what kinda damage am I gonna be in for. Im already suspecting bent con rods or bent valves but I dont know how to check that apart from just pulling the head and looking at it. Any help would be much appreciated!