There is always constant discussion about what is the proper diameter wastegate to get. Some say, "38's always work." Some say,"44 is the shit." And even some just say, "Hell, toss a 60mm on and you'll never have to worry!" Regardless of what they say, there's a reason behind it all. The proper wastegate size is all determined by how well your manifold is build, where your wastegate is located, and how it is placed.

First off, the point of a wastegate is to take exhaust flow away from the turbine. When you have a wastegate at a straight 90* angle, the exhaust flow tends to pass it by, and very little exhaust gases actually flow towards your wastegate. There are manifolds with 90* wastegate angles, but they have made it so there is more flow towards the wastegate by "tear dropping" the entry. Here is a Full-Race manifold for example:


The exhaust flow gets brought into the wastegate in a very smooth flow. Without this, the exhaust gases just pass by and create boost creep. The following photos, thanks to Full-Race, show this perfectly:




Even Precision knows their stuff (although their journal bearing turbos leak like a siv ). Here is what they have in their manual:


Hope this information was helpful