We can clearly see with the headers aligned that the stock primaries work their way down to a very crushed section with a very sharp downward angle. This is far from ideal for optimal gas flow. While some may think the cross sectional area is maintained it is not. The Tubular headers have a perfect radius and have a larger inner diameter.
Here we can again see the stock primary crushing down and the difference in the pipe size.
Again we can see how the stock primaries all come together and are crushed. We can also see that there is no real design behind the collector as BMW were forced to go into the CAT. Essentially the collector has no 'pulling' effect like on a true high flow merge collector.
Showing the overall length and smoother larger flow of the tubular headers
Another image showing the pipe crushing and the multiple bends in the stock primary setup vs the much smoother bigger direct path of the tubular headers
So is it just this that allows the consistently tested +35-50hp from going tubular headers from stock headers with CATs OR the +15-20hp over a set of decatted stock primaries (Evolve RACE R)?
In way yes but also no.
Once you go for a header of this type you not only improve the gas flow and scavenging effect from the merge collectors but you also reduce back pressure and combustion temperatures / pressure. The DME will normally not come close to hitting the set ignition targets set by BMW with the stock headers/CATS. Once the combustion process is improved the level of knock will also be drastically reduced and the DME will push much closer to the set ignition targets.
So, you get the gains from a much more free flowing path for the gasses, a larger cross section area of the tubular header primaries and then also the effect of the DME adapting to a higher ignition timing.
Some engine will gain more power straight away than others and this is mostly down to the ignition adaption taking place faster whereas other engines will take their time as they may have a long term adaption for timing which is retarded.
Showing a tubular header
Showing a complete stock header.
Dyno graph showing gain from tubular headers fully adapted and on a very low mileage engine:
Again, here we see the stock headers flow well enough put around 5000rpm after which they start becoming restrictive. The larger pipe diameter, reduction in back pressure, free flowing design and higher achieved ignition timing give substantial gains in performance.
Dyno graphs of engine now quite fully adapted dyno tested straight away (MAHA)
As mentioned this is a brief explanation so everyone can understand it. We hope this comes across as useful.