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Which such a small plenum the air will want to go directly into the #1 cylinder, therefore making the ITB's virtually useless. When using a wider/larger plenum and putting the inlet closer to the middle of the runners, it makes for a substantial increase in air flow throughout all 4 cylinders, rather than having it all cram into 1 cylinder. Plus, with the larger plenum and placement of the inlet it makes the flow fairly even in the cylinders making it not such a complete bitch when it comes time to tune. I can go on and on, but I hope that sums it up for you. It's all about flow.
This Gen2 MR2 IM from Ross Machine Racing is kind of the "ideal design" as far as the inlet placement:
Which such a small plenum the air will want to go directly into the #1 cylinder, therefore making the ITB's virtually useless. When using a wider/larger plenum and putting the inlet closer to the middle of the runners, it makes for a substantial increase in air flow throughout all 4 cylinders, rather than having it all cram into 1 cylinder. Plus, with the larger plenum and placement of the inlet it makes the flow fairly even in the cyliders making it not such a complete bitch when it comes time to tune. I can go on and on, but I hope that sums it up for you. It's all about flow.
I didn't catch this the first time around. What you're saying is true, but it's true for plenums in general and doesn't have anything to do with ITBs specifically. Also, what you're saying only applies to a plenum that's substantially undersized, because the engine will still draw air in through all cylinders. You're not filling a bucket with water here, you're letting air into a pump. You're only changing the pumping efficiency of the individual cylinders, and the first cylinder is more efficient in a log-type plenum, etc.
As for the MR2 plenum, it isn't that ideal either, as the air has to make a double 90 degree turn to go into the cylinders. Shouldn't be a problem until you get well into the flow range, but the point is that it's not ideal either. It would also be a lot better if it wasn't so geometric, hard angles like that on all the plenum sides isn't that great. Air is a fluid, and likes smooth radii instead of stagnant pockets at all the corners. Eddies tend to build in corners and edges like that, not especially efficient either.