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I know that this is not my forum section but.
A good rule of thumb with intake plenums is that you want at least as much internal volume as engine displacment. Which means that most OEM plenums are way too small.
The reason you want plenty of internal volume as it acts as a reserve of air so that on each intake stroke there is a full charge in the plenum for the cylinder to draw from.
Having too small of a plenum hurts top end power as there is less time to fill that plenum no matter what the size of it. This causes the manifold preasure to drop higher in the rev range costing you power.
I know that this may seem accedemic with a forced induction vehicle. But it will still cost you power at higher revs just not as noticable as a N/A car.
I guestimated that your plenum at about 16 inches long would be about 1.25 litres in volume which would be about the standard plenum volume on a 1.5 litre car.
Ever driven a 1.5 litre car with a vacuum gauge and noticed the needle move off of 0 VAC at WOT me too. It costs the engine power. Now imagine that on a 2.5 or 3 litre engine.
I would keep maximising internal volume in mind if I were you. Though I realise there are of lot of other deign considerations too.
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