Leak down 500 EFI ???
 

How much leak down should a 390hr 500efi have and how far from a total tear down are they?

It really depends on how hard it was run. The bottom end should be fine, but you never know. A leak down will tell you where any issues are.
If they have never been apart, they definately need to have the top ends freshened with springs, valve job, seals and head gaskets.

exactly right and it will make a big diffrence if they can be started or not. a motor that's been sitting unrun for any perior of time builds up a bit of rust on the valve seats and won't seal up particulary well cold. if it can be run then you run it for a bit, warm it up and then do the test... anything under 10 % is fine 10 to 15 is ok depnding on what you hear for escaping air and where, 20 and above is a sign of some meaningful wear or damage.

this is one of those deals where , if you are looking at the motors on the floor to buy for re power or something, then the price tells you what to do. if you are paying retail, then you have the pan and valve covers off and do a borescope and the whole deal because there isn't any money left in the deal to fix them if you are wrong. in the other extreme if you are paying " core value " then you look at and smell the oil, cut a filter, do a leakdown and assume that you might do rings, bearings and a valve job before you install them and pay accordingly.

And make sure the leak down is done with the engines warm

If the leakdown is around 10-15 percent warm you are fine, but a lot of people will say to pull the heads anyway because of the head gaskets. I pulled my 350 hour engines down for too much leakdown and also checked the bottom end while apart. The heads and valves did need some cleaning up and I replaced springs and lifters, but everything else was almost perfect. The bottom end on these is hand assembled and blue printed so they wear very well. I know of 2 that went 750 hours in a 40SS Formula and were still completely intact and sealed.

Thanks guys exactly what I wanted to know

But why warm leak downs vs cold?

The numbers will be better warm. Although the main reason is to make sure the engine has not sit dormant for a while. As was said before rust build up on the valves will increase leakdown and can show as bad. As an example I have leak checked a motor that was not run in 5 or more months. A couple of cylinders showed leakage past the valves. Took a hammer and smacked the valve tips several times and the leakage got better, I had knocked some of the rust loose and they sealed better. A more important barometer is how even the leakage is. Let's say you have a cold engine that shows 20% across every cylinder. That would probably be fine. I would be more concerned with a warm engine that went 10% on 7 and 20% on one, that is an issue.