Some time back I bought a used 42mm Clubman exhaust off the bay.
Its been kicking around under the workbench (along with a 48mm clubman) waiting for the time to fit it .
This afternoon I was sorting out my new copper exhaust gaskets and noticed that the 2x 42mm copper gaskets that I had, appeared to be different . Lining up the holes sure enough the centre cutouts were completely different .Intrigued... I thought that I would offer them up against the 42mm clubman manifold under the bench to see how well they fit.
It was then that I noticed that the used clubman had come with an old gasket fitted . I peeled it off and noticed immediately that the gasket was much too small for the manifold . Ummmmmm
I pulled down an old 150 block and offered up the small silver gasket which fitted perfectly !?? The difference in size between the 150 engine standard 38mm port and the 42mm manifold is quite a bit.
To make things easy lets assume that the engine port and manifold flange are both circular
Area of 38mm port + 1133mm2 Area of 42mm manifold = 1384mm2.... difference approx 250mm2 which works out about 20% restriction.( please correct my maths if it is wrong)
So we assume that the previous user fitted a 42mm Clubman exhaust to his scoot to improve the exhaust flow and increase power . I imagine he used the original 38mm gasket from the old 38mm manifold as it was all he had to hand. He must have been surprised at the very modest power increase from the clubman Exhaust not realising he was restricting the exhaust gasses .
If you look at the size difference between the 150cc standard port and the 42mm clubman exhaust flange it gives an idea how big a mismatch there is and presumably quite a bit of material would need to be removed to blend the port with the manifold for the exhaust upgrade to give a worthwhile power increase.
