i found this when i had the same question..
Log manifolds do have their benefits though: they’re cheap, they’re easy to make, they’re durable (especially if cast and not welded), and they allow the turbo to spool very quickly due to its stubby primaries (an exhaust primary is the first tube through which exhaust gas from the cylinder flows; there is one for each cylinder) and overall smaller volume. If you go with a log manifold, make sure that the flange that bolts to the head is cut between each runner. The resulting spaces between the flanges of each runner will allow them to expand and contract independently as they’re heated up and cooled down, preventing the chance of cracking. Log manifolds are usually made of thick, mild steel--again, there's no point in using expensive and hard-to-work-with stainless steel if the main selling point of a log manifold is price.
Tubular manifolds are more expensive than log manifolds, but they do have their benefits. The design is far more efficient and high flowing, and while 350 whp can easily be produced using a log manifold, if you are looking to run a lot of boost and plan on making a lot of power (400+ whp), a tubular manifold will get you to your goal quicker. And while the longer tubes might increase the time it takes the turbo to spool up, a tubular manifold will never restrict flow (and therefore power) to the turbo in the upper RPM ranges. In fact, if you have the money, you can have the quick spool time of a log manifold with the high flowing characteristics of a tubular manifold:
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