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Reply to "Chesil Speedster - my planned winter overhaul of '98 build"

Martin wrote: "Why would the PO or original engine builder fit a 40mm ID exhaust when the exhaust manifold is only 34mm ID?"

Good question.    If it were a 2-stroke motorcycle or snowmobile, I would think it was a poor man's attempt at an expansion chamber but in your case it might simply be that that's what was available and........   It fit.

The 40mm will allow a bit better flow, but if you have 34mm carb venturiis and 34mm head ports then the additional flow will amount to mice nuts.  I don't think it will hurt it or decrease HP - I think it will increase your torque band very slightly in RPM but I'm not certain of that.  

It won't add anything, either, IMHO, unless you increase the flow elsewhere (bigger carb venturiis plus bigger head ports and maybe increase the runner diameters on your intake manifolds - All this starts to be a lot of work and £££).

But, let's get a few comments from our North American motor-head neighbors, like @ALB and @Stan Galat and @DannyP.  I no longer have access to my flow analysis programs in "Pro Engineer" (I retired and couldn't afford the Sun server they were running on)    but these guys are more familiar than me with what works.

Last edited by Gordon Nichols
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