Following on from the article on exhaust manifold sizing this article considers the ideal diameter of exhaust system to be non restrictive for a given power output. In this case it makes no difference if the engine is normally aspirated or forced induction, how many cylinders or what the engine capacity is as long as a single exhaust system is fed by all cylinders. In the case of dual pipe exhaust systems obviously you need to halve the bhp limits in the table to determine the optimum pipe diameter.
The table assumes that the silencers are high efficiency items that reduce noise by virtue of good internal design rather than restrictive baffling which sadly is far too often the case. The table also assumes imperial tubing with 1/16" (1.6mm) wall thickness so the I/D in all cases will be 1/8" (3.2mm) smaller than the O/D. Obviously what the engine sees is the I/D so if you're using non standard tubing then you'll have to work back to I/D to find out what size you need.
|Max Power Output
Please don't assume that the maximum power output figure represents an absolute bottleneck bhp for that size of pipe. What will happen in practice is that a system that is too small in diameter will become progressively more restrictive but there should be no restriction up to the bhp limits set out in the table.
While I'm on the subject of exhaust systems there still seems to be a small but diehard contingent of the hard of thinking out there who continue to believe that exhaust systems need some backpressure to make an engine run at its best. Why this myth persists I couldn't say although I suspect it's most prevalent amongst people whose house still has the "Wide Load" sign on the back wall, who can change the oil on their truck engine without bending down to get under it, keep a can of Raid on the kitchen table at all times and think book lernin' is bad juju.
Any back pressure in an exhaust system means that energy is being lost evacuating the exhaust gases and that always hurts engine power. Obviously fuel and ignition need to be recalibrated properly if any part of an exhaust system is changed which is why sometimes putting a better system on doesn't lead to the expected gains if no recalibration is done.