I have found no rpm limits yet. There are 2 engines currently being built tht are going to turn some major rpm. I will let you know how they do. The reason we came up with these springs is that the pressure on the nose of the cam is too low on stock and 60lbers to control high-rpm, high-lift profiles. And the fact that people are having to spend a lot of money on machine work to make adequate small block triples work. Here is a chart of the lift vs. pressure on stock, 60lb, and HD175's
Zach
lift Stock 60lb HD175
1.940 88 150 150
1.900 98 156 165
1.800 125 170 200
1.700 150 187 240
1.600 182 210 285
1.500 210 225 327
1.400 245 245 365
the most interesting thing is that at 1.400 the 60lbers and the stock spring have the exact same pressure! I can't believe that there has not been a better spring made to date.
Zach Hamilton