Originally Posted by zippy

most blowers and turbo's have a compressor map to read. getting out of a blowers efficiency range is very common and will lead to big numbers on a dyno, but not so for track use. throwing around hot air just doesn't get the job done.

Originally Posted by natural1

When you up the turbo size you are not exactly increasing efficiency, a smaller turbo can be more efficient than a larger turbo, turbochargers are only so efficient anyway (70-77% at best). Efificiency ratings vary from turbo to turbo through the manufacturer(s), it is not determined by what engine the turbo is mated to as the manufacturer almost never knows exactly what their product will end up on. However, the manufacturer does supply an efficiency rating as well as a compressor map that tells you what the operating range of the turbo is. When you choose one, make sure that your engine can keep the turbo in that range. Matching a turbo to an engine is more than a guessing game, it takes careful consideration, there are many variables that should be addressed. A properly sized single turbo will orerate much the same as twins, if set up properly. With turbo size, often times it is better to copy what someone else has already done, this takes the guess work out of the whole matter if you are new to the turbo scene (don't use an STS setup as an example unless you plan to use a remote configuration). If you understand about volumetric efficiency, the air flow demands/characteristics of your engine at given rpm points, and can read compressor maps then you should be able to size your own turbo. As vanillagorilla just stated the 67mm should be a minimum. But, remember there are many variations of each turbo in the form of compressor and turbine a/r. We looked into running a Precision 76mm on a 5.0L mustang before switching to a seriously built 351 with a Precision 88mm. Either way the 67mm will be good but the 76mm would be better...IMO.