they start around 2800 and go up from there, the extrudabody kits are modular kits and parts are really easy to replace and easy to get from the factory, the Harrop manifold is documented making 40hp over the LS6 manifold. but frankly, with an open runner and no plenum to speak of, no wonder, on top of all that, with velocity stacks you can tune your runner length to fit your rpm range and build an awesome air delivery system tailored to your cam and heads