cont.
"Longer tubes will create more torque at the rpms below peak torque.
How do they do this?
Longer tubes will speed up air flow velocity. The flow velocity of 240 ft/sec and peak torque will occur at an earlier rpm compared to a shorter tube. Changing the length of the header primary tubes does not increase the value of peak torque like diameter does. Instead length changes the behaviour of the torque around peak torque along the rpm band.
If you imagine the torque vs rpm curve from a dyno to be like a see-saw: then, on a see-saw there is a point where the plank sits to allow it to rock up and down. This is usually in the middle of the see saw and is also called the fulcrum. On our torque vs rpm curve, imagine the peak torque to be the fulcrum, although this fulcrum doesn’t necessarily have to be in the middle like the see-saw…it can be moved. Changing length “rocks” the torque curve about the peak torque.
If you have a longer primary header tube, the torque curve will “rock” in such a way that the left side is higher than the right side. There is higher torque at earlier rpms before peak torque. There is less torque at later rpms after peak torque.
If you shorten the length of the primary tube, the torque curve will will have the see-saw with the right side higher than the left. So there is more torque at later rpms after peak torque.
- Merge Collector Diameter, Length, Angle, and Layout:
In terms of header layout, merge collectors are the portions of the header where the tubes join.
So in a 4-2-1 header, the 4 primary tubes are first joined at a collector into 2 tubes. The 2 tubes are then joined by a second collector into 1 tube.
In a 4-1, the 4 primaries are joined at only 1 collector into 1 tube.
In some cases, the collectors are in a box shape where 2 tubes are stacked directly on top of the other 2 tubes. In other cases, the collectors have the top 2 tubes offset from the bottom 2 tubes. This is called a tri-Y collector. The box collectors give less header ground clearance than tri-Y collectors.
The collectors join the tubes and co-ordinate the 4 exhaust pulses leaving the primaries.
Shorter, large diameter collectors have more peak power.
Longer , smaller diameter collectors have more power in the midrange.
The angle of the merge collector tubes should not be steep or sharp, in order to keep the energy or speed of the merging pulses coming from the tubes at a high level.
For example, the stock ITR header has a less steep merge collector angle than the stock GSR header (see SurferX’s article on the features of the ITR). So, the diameter of the collector affects the flow volume or how much exhaust gas can be removed and how much peak hp can be achieved. The bigger the collector diameter, the higher the peak hp you can achieve. This is why the better headers have larger 2.5 in. collectors instead of the usual 2 in. collectors in some aftermarket headers made to match up to the stock catalytic converter 2 in. flange.
- How the Header Primaries Are Paired -> Sequentially versus Non-sequentially:
the ignition firing order determines which exhaust pulses leave in a particular order. In integras it’s cylinder # 1,3,4,2. How we pair the header’s 4 primary tubes together at the first header collector determines the horsepower vs rpm curve’s characteristics or shape. Sequential pairing allows for a broader powerband and better acceleration properties from an engine.
You can look at your header and see which tubes are paired together: Is it sequential: 1 with 2, and 3 with 4? Or is it non-sequential? 1-4, 2-3?
A 4-1 header layout will have peak torque occurring at later rpms compared to a 4-2-1.
Newer hybrid headers of the 21st century are a fusion of the old 4-1’s extra length with the 4-2-1 layout, have stepped diameters, and have large diameter collectors. So you have low end peak torque with enough breathing capacity to support more peak gains (the best of both worlds).
CONCLUSION
So the old adage that 4-1 = more peak hp with a loss in midrange torque and 4-2-1 = more midrange torque with less peak hp is an obsolete idea.
"
again credit is greatly given to Michael Delaney at team-integra for this info.
)