When we are speaking about engine tuning... is there somebody who has more than 150PS from 2,2 engine?
I plan to get there with my ported cylinder head etc.
You can compare it with resonance.
It *is* a resonance effect. It's like an organ pipe or flute only wanting to play one note - at that note, the air flows through more easily than at others, where the waves die out and energy is lost.
It pays to consider this resonance effect because it's there and it makes a considerable difference whether you want it or not. And if you think of it, a muffler far down in the exhaust system is probably not "felt" by the engine as even if it restricts the exhaust a little, it also opens up the exhaust and allows the pressure waves to die out instead of coming out of the exhaust outlet. That applies to the inlet as well. So a well designed and damped muffler can be seen as an open outlet from the engine.
So exhaust resonance tuning is a good tool to increase and/or moves the max torque point of the engine to the most RPM.
The other effect is that of the vacuum on the back of the exhaust bubble. To benefit from that you need vacuum, which you won't have in a completely open or restrictionless exhaust system. So there's a point where there will be too little restriction! Now, the vacuum on the back of the exhaust bubble is distributed back to the other cylinders from the place where the runners are joined, where the (low pressure) wave will travel (at the speed of sound) back. Depending on the exhaust design and engine RPM, it will either aid the scavenging of the cylinder (as in a short manifold where cylinders are joined by pair, as in the Murena 2.2 cast-iron manifold), or starting the early suction of air/fuel mixture into the cylinder during the cam overlap when both inlet and exhaust valves are open (as on the 4-in-1 manifold with its longer runners). Or both!
All that said, I don't think there's a design rationale behind the 4-in-1 manifold on Murena - and I don't think anyone will have the inclination or money to spend on trying out different designs. Except GP perhaps?
However, it looks and performs well!
/Anders