At what point do RPMs become an issue? Provided I have HD valve springs, how high can i safely rev?

 

4100rpm is the terminal speed for the stock crank... Keeping it there for very long or trying to rev past it will severely shorten the fatigue life of the crank and probably leave you walking home after a few runs.... 128# crank that isn't exactly balanced on the worlds most accurate machines... Yeah you see where I'm going with this. A good balancing and shaving a bunch of weight off the fly weights would show a lot of improvement. You would be able to rev higher, and due to the lighter crank, faster than with a stock crank.

 

to the moon!!!!!!

 

Oh gawd

Might I suggest a crank damper to help keep those rotational vibrations in check

 

Diesel has a much slower flame front than gasoline so higher RPM's gives the fuel less time to burn and results in lower efficiency, at some point, these inefficiencies will overcome any of the RPM benefits. Now, you could just mix a little nitro methane in there and be just fine, rev to the moon. My last nitro RC car had a motor that red lines around 50,000 rpm's. Nitro burns fast!
By the way, at those R's, the rod and such are designed to maximize internal aerodynamics and actually sports a knife sharp edge to cut the air fuel mix inside the crank case.

So if you want to rev to the moon, you better design some more aerodynamic rods, crank, and cam assembly!!!