Quote:
Originally Posted by Madact
Based on this, I think I'll keep my original plan of an anti-reversion step at 14-15" to capture the first rarification pulse reflection (especially useful at high RPM), and a merge at 32"... between harmonics and the fact the the flow-rate-based formula and 1/2 total length rule agree on ~32", it can't be too terrible  |
I still think this is wrong. I've just had a look at the calculations I did when I changed my header and the result is 31.49 inches which agrees with your figures, however I'm using 16" primaries so why does practice not coincide with the calculations? The headers that are on sale tend to be either 16" primaries & 16" secondaries or 24" & 8" both designs giving 32" total, I've not come across 32" primaries for sale.
I think the reason is that almost all of the formulae documented on the web are for V8 engines while we have straight 4 engines which use a different firing order. The V8 calculations time the pulses for a cylinder which fires 360? degrees later but we need to time them for the other cylinder in the pair which fires 180 degrees later thus we want primaries half the length. Or something like that, I'm not too sure about the V8 firing orders and angles.
I do know that on my straight 4 engine, a tri-y header with 16" primaries give a substantial torque increase centred on 2300 rpm when compared to the OEM header which has 4 short primaries into a pre-cat. I assume that a torque increase corresponds to an efficiency increase although I haven't managed to measure efficiency accurately enough to be able to tell.