Quote:
Originally posted by RarlyL8
Goran - What is the purpose of the long intake runners? Someone went to a lot of trouble to fab that design.
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The reason for long runners is to lower the resonant frequency of runners to more sensible revs. We found out that early stubby runners were tuned for 10000 RPM and now they are down in rev rannge where they can offer substantial help, both on- and off boost.
When it comes to plenum, turbocharged car works exactly as N/A one. It openns and closes the valves and breathes in "gulps". If runners are carefully designed (length-wise) to resonate in certain rev-range, they will help fill the cylinders with air to greater degree.
We are talking about volumetric efficiency, VE. VE of 90% means that cylinder fills with 90% of air that could theoretically fit inside. The faster you rev the less time air has to fill the cylinder. Some highly-optimized N/A engines (F1 etc.) can obtain VE of over 100% in certain rev range.
Air accellerates down the runner and is "rammed" into the cylinder just before intake valve closes, thus force-feeding the engine w/o turbo.
This is seen as a "sweet spot" on dyno where torque peaks slightly. If this happends just before turbocharger start spooling, it will shorten the turbo lag and provide more power.
Same thing happends even when on boost, only difference is that air is more dense.
With other words even if you are force-feeding air into cylinders by some type of supercharging, there are still gains to be made by optimizing the runners.
At WOT, throttles are wide open and just in a way for air entering the cylinders. So, my opinion is that there is more to be won by careful intake runner design than by installing ITB's on turbocharged car.