Downpipe sizes

[filthyjohn]

Hi just a quick one,

Does anyone know the internal and external diameters of the williams downpipe? i.e. what size pipe should one use when one is constructing one's custom exhaust system? I need to know so I can order some bits.

Thanks in advance, I promise to pay for this advice with pictures of my car when it's done.
:D

[MatBrown]

I wouldn't use the same diameter as the williams manifold/downpipe, they are very weedy!


Mat.

[filthyjohn]

Hmm, how weedy exactly? I wouldn't want to have too much of a difference between the size of the DP outlet and the rest of the exhaust, as a sudden increase in bore size will stall the gas flow and create turbulence, increasing the back pressure.
i was thinking of 2" diameter pipe, should flow well enough for 200hp in simple terms, and I'm not aiming for that much power anyway, more like 150. I really don't want a loud system, as the car is a sleeper.