I always thought for turbo about 1.5 times capicity, but i know some one who made their own with volumes from a program, mind you its nissan ca18, but it has 2.5lt plenum, about 120mm runners, proper bellmouths. doesn't sound massive and i guess it doesn't need to be, It uses an 80mm t body to get the required amount of air and makes
475hp@wheels.
So don't go over the top in size.
cheers
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