The last paragraph of the exhaust chapter of the "Performance tuning in theory and practice, Four strokes" by A. Graham Bell:
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In conclusion, let me make one final point that may save you a good deal of money. Whilst we have had an in-depth look at header tuned lengths and sizes, do not get too carried away applying this science to a street machine. Any reasonable header will work well with a road cam as pulse tuning is limited to the small valve overlap of this type of cam. In fact I have found on the dyno, using cams up to 290° duration and 70° overlap, that headers 10% 'out of tune' resulted in a power decrease of not more than 3% on the same motor with 'in tune' headers. However, I have also determinded that once the valve overlap increased to 100°-130° the tuned length became very critical, to the point that a slightly out of tune header dropped power by 10--12%.
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