Header / Extractor design questions

Hey all, I'm looking to build my first set of 4-2-1 headers for my KA24DE 2.4L Inline 4 cylinder petrol street engine

I'm basing my info on A. Graham Bells book "4 Stroke Performance tuning" and he gives info / formulas for selecting primary/secondary diameters and the total length of the tubes and says the primaries should be "At least 15" shorter primary's boost top end power and longer primary's boost low end power"

But he doesn't say what percentages of primary - secondary correlates to what rpm?

So basically my question is if my total length of the header is 44"... The primary has to be at least 15" and being a street engine and a work ute im more interested in low to mid range power.... say 3-4000rpm what length should the primary's be?

Reading elsewhere online and some people say the primary's should be the same length as the secondaries, others say the primaries should be twice the length of the secondaries.

Any input is appreciated

Haven't sat down and worked it out, AFAIR, but there are two effective lengths in play here, they diverge at the primary "Y", and my thoughts about them.

The first, which is what most seem to be concerned about, is the peak power length, and there are numerous formulae on-line for this, taking into account the "VTEC' high rpm camshaft timing, if used, and the rpm peak power is expected to be - if you're using a relativly stock engine the peak power, and torque, should be good estimation points.

For the primary length, instead of the power peak, use the torque rpm, and use that and the low speed cam' (if that's where the torque peak lies) timing - but with the added complication that the "end" of the pipe is CLOSED, which means a POSITIVE reflection back - IIRC (been a while). If I recall correctly - might not, so check - in practice this reflection is in phase - but it's been many decades...

You might find these to be good starting points for reflecting on the pressure waves - they're music theory based, but the exact same principles apply.

http://labman.phys.utk.edu/phys221core/modules/m12/Standing%20sound%20waves.html

https://www.physicsclassroom.com/class/sound/Lesson-5/Closed-End-Air-Columns

Also Helmholtz resonators may be worth a read up on, as they use a closed end, and same principle of operation.