by lord13 » Wed Aug 21, 2019 7:20 am
I had the Janspeed style one fitted to my HB back in the day, the complex design is to provide equal length primaries and secondaries to enable better extraction. The Idea/science behind it is that the exhaust pulse from cyl 1 will cause a depression that will 'pull' the exhaust pulse from cyl 4, and vice versa, the same for cyl 2 and 3, which is why they are paired 1-4 and 2-3 as firing (and therefore the exhaust) order is 1, 3, 4, 2, if they were paired in firing order the exhaust pulses would not be timed correctly. The size of the primaries and secondaries are also important as too large a primary would cause less of a depression, which is why the convoluted deign has smaller primaries than the more straight forward DTV one. This is also why it works best at a high rpm, it is tuned to extract the exhaust pulses at a specified RPM, for race use, where a vehicle will be 'on cam' as much as possible, i.e. flat out. It's pretty much useless for a road car apart from looking good in the engine bay. You should see the Chevette one, that is a wildly twisted piece of kit, more so than this.
The DTV style one on the other hand is a 'quick and dirty' style of extraction, going on the idea that the bigger the better to get the exhaust out as quick as possible. You'll notice it too has the 1-4, 2-3 primary configuration, again to aid extraction, but they are not of equal length. The larger bore of the primaries mean it is a more forgiving manifold and will be more versatile across the rev range but will not give a noticable 'punch' when the engine comes 'on cam' as the pulse timing is not so essential.
In my humble opinion, the DTV style one is the better for road use, and for maintenance, that janspeed one is a total *Bleep* to fit correctly, if you get it wrong its too low on the exit, the best bet is to fit the complete system ,front to back, loosely , and tighten it all up when it's exactly in the position you need it.