Im running synthetic in a new 350 zz4 shortblock.  The oil is prematurely getiting black, I suspect some piston blow by, but not really sure.  Anyone have any input on a similiar problem, and what be the cause and solution. 

Oil that dirty ups quickly indicates that either the engine is dirty with sludge and the detergents are stripping it away. Or the engine has excessive blow-by and lots of carbon and chemically reactive combustion products are getting around the rings.

You said this is a new short block, but what about heads, intake manifold, pan, etc. Were these items clean when installed?

Even a new engine is subject to manufacturing and assembly errors. A leakdown test will identify rings that arn't sealing.

What about break-in oil, it's not a good idea to run synthetics for break in. They provide too good of a surface for the rings to run on and, therefore, they tend not to "break-in". This is a big problem with engines running moly rings and smoothly honed bores. Then the rings just tend to skate on the oil rather than seat to the cylinder wall. There is a school of thought that it is no longer necessary to break in with a mineral oil, but this is highly dependent on the ring face material and cylinder wall finish. You can get away with a synthetic on break-in if the cylinder wall finish is rough. But these days most the theory is a smoother wall.

If the maker happened to combine a smooth wall with chrome rings, then you've got an even bigger break in problem, as these hard surface rings just won't seat against a smooth wall.

If you started the engine with a synthetic, drain it out and replace the filter; then refill with a simple mineral based oil, a new filter; then run the engine for a thousand miles or so. It should settle down, then change the oil back to a synthetic.

Bogie

Thanks bogie.  The block is a short block new, with 195 trick flock heads.  We did the initial break in of the cam and block, and maybe ran 500 to 1000 miles, but then switched over to synthetic, since then the no 7 spark plug has ben getting somewhat fouled, and well as the oil getting prematurely black. The short block is a gmpp block zz 350.  I've always heard when you go synthetic that you can go back to standard oil.  what do you think, the motor maybe has 2000 to 2500 miles on her now.  Let me know.  lcamino1

You can switch back and forth and even mix the two types.

The story on #7 dosen't sound good, I do a leak down test to see how fast compression goes away. Similar to a leak down, with the piston at BDC power stroke so all valves are closed, with a suitable attachmnet in the spark plug hole, pressurize the cylinder and listen for where air escapes. Into the crankcase from around the piston could isolate a problem with rings, bore finish, piston damage.

Bogie

Ill do the leak down test, and see what I can hear if any air in the crankcase.  Im gonna switch back and get off the synthetic, and see what happens.  Maybe she'll settle down a little, and get a better piston ring seal.  tks   

I hope so.

Bogie