I know how low lift flow increases with the use of larger valves and would thus make the valve timing and overlap more critical. I see how a cam has to match with valves sizes (and the rest...).

To my knowlegde, the exhaust port is a limitting factor when it comes to performace. In order to increase exhaust flow, one could increase lift a little but, it wouldn't help much. More duration would be better. Too much duration results in an engine not suitable for daily driving, so you would want to increase flow, without (overly) increasing duration. This can be done by opening the valves quicker, so the valve spends more time at higher lifts, or (here it comes) fit a larger valve. Like you say, especially at the lower lifts, flow will benefit from a larger valve. I would think that this would improve flow even without doing anything to the port (except matching the valve seat offcourse). However, perhaps this would be at the cost of flow at max lift, I don't know.

Right now I'm running a K-cam, which is almost simular to the D-cam. It runs ok, but when looking at lobe shape and valve motion, I figured that a quicker opening and closing of the valve would be a good thing. (more 'area under the curve') A different cam profile would help, however, there are limits to this by means of lobe-lifter contact (Cam-theory thread). 1.6 ratio rockers could help some perhaps, but that would also increase max lift (which isn't neccesary bad, but not the goal). An other way to sort of 'simulate' a quicker opening and closing valve, would be to fit a larger valve. That would increase flow when the valve is in between closed and maxlift position.

So I have/had two reasons to fit a larger valve:
-make up for the bad exhaust port
-make up for the slow opening/closing cam

Perhaps there is only one way to find out....

I will also contact KG, but John do you have any (own) flow numbers showing the flow comparing 35mm, 37mm, and 38.5 mm valves, or only combined with radical porting, (so the numbers would not directly relate to valve size)?

Cheers! Ben


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P131, '65, B20B+M47. P131, '69, B20E+AW71L+LSD. (www.tinustechniek.tk)