In the quest for ever more speed and power, eventually you come to the point of ordering up a built engine block, or even having one custom built to your specifications. This is an incredibly exciting day, as you'll no longer be held back by the limitations of the stock bottom end and can explore the upper limits of what your car (and in some cases you) are capable of. Similar to swapping for another stock engine, you'll need to get a custom tune in order to keep your engine running safe and in optimal condition. An added reason for this is that when using aftermarket parts that are forged or billet, there are differences in how much expansion the materials will undergo when they start to absorb the heat of the engine running. As a result when attempting to run a stock tune with a built engine, you'll probably see a lot of knock numbers from piston noise. It's important to tune the car appropriately for that so that these false knock readings don't mask the ones which are real, just like brakes can help you avoid an accident and keep your car safe, a tune can help avoid detonation or running lean, and keep your new engine safe.