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I am informed by my engineering friends and family that the general rule for exhaust tubes is that they should have the same cross sectional area as the port at max valve lift. As I understand it for most of the exhaust cycle the port area varies with the valve lift, and changes with respect to time. Therefore for most of the exhaust strock the major empedament to flow is the exhaust port. so that at low rpm there is relatively lots of time for the gases to get out, even if the tube is smaller diameter. I agree with stone bracker that a larger tube diamerter is best, however the more important issue is how much differance will it make to a FE engine at low rpm? not much, unless the tubes are cosiderably smaller than the port area at max lift. If you really want to know what is best you will have to do a lot of testing to find out. Most "high performance" stuff is to maximize toque or power at a specified rpm so that the vehical will accelerate faster... what we are looking for is minimal fuel consumption. what we share with the HP folks is a desire to reduce pumping loses and to improve over all engine efficiency, so that we can burn less fuel, fast acceleration uses lots of fuel!
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