[aerohead]

In Todd Howard's,HOT ROD Magazine' article,'HEADER SCHOOL', he mentions:
*primary tubes determine peak torque (mpg)
*primary diameter affects the rpm at which peak torque occurs
*small primaries offer a low end peak
*large primaries offer a high end peak
*"In most applications you should always opt for the smaller primaries."
*"... header length is a second factor in torque peak:
.......long = low rpm
....... short = high rpm"
*equal-length primaries if you can
*collectors must be tuned for diameter and length
*up to a point,the larger the collector the more low end torque
*Tri-Y's are good but they're not for 3-banger
*primaries need to be paired with cylinders 180-degrees out of firing order,so one's rarifaction can help scavenge the others' flow as it passes.Makes for steadier pulse train as well.Good for mid-range
*If the engine has much valve overlap,anti-reversion pipe stepping needs to be incorprated,otherwise exhaust will flow into the open exhaust valve during overlap event,spoiling the new charge.(Mr. Gasket's Cyclone and Blackjack headers can be studied for this feature.)
*EFI and CPU will automatically recalibrate ignition timing and mixture,which headers will tend to lean out.....