|Merged collectors work wonders making for wider powerbands if the design is optimized to exploit their inherent advantages. We will get into this more in our next installment of the series. Many companies make merged collectors but we really prefer the ones made by Burns Stainless. The Burns parts have exquisite construction quality, correct interior aero and proportions and the biggest advantage: Burns' X-Design. X-Design is Burns computer modeling program that helps provide the correct primary configuration. Burns offers X-Design consulting when you purchase one of their collectors and we have found in our experience that it outpowers what we have been able to come up with on our own saving a bunch of expensive cutting and trying, not to mention dyno and tuning time!|
The way the primary pipes that come from each cylinder gather together is important also. This area of convergence or the collector as it is called is critical for proper header function. The collecter must be a larger diameter than the primary tubes because it has to acoustically represent the end of the pipe for tuning reasons and it must be big enough to support the flow from all the cylinders without creating excessive backpressure. Usually the collector is just a junction where all of the pipes are stuffed and welded into a larger pipe that may or may not neck down into the final size of the exhaust pipe. A well designed collector, pairs cylinders opposite each other in the firing order so an exiting pressure pulse from one cylinder will not hamper the evacuation of the next cylinder which is on the overlap part of the power stroke. In a typical inline 4 cylinder, that would mean paring cylinders 1 and 4 and 2 and 3.
|A CAD solid model of the interior of a Burns collector showing the smooth aerodynamics of the interior.|
The best collectors are called merged collectors. This is a collector where the primary tubes are paired together in a smooth taper. Merged collectors usually produce a wider powerband and sometimes more top end power. The wider powerband is due to the more gradual propagation of waves from the smoothly joined tube ends. With smoothly merged tube ends the flow in the pipes is less turbulent creating less backpressure and flow velocity is increased. Not too many production headers are merged due to the difficulty in fabrication but most headers found on race cars are.
|Tri-Y headers often can have wider powerband than 4-1 headers. The second Y acts like an interference resonator which creates extension pulses to the reflected wave. Even though conventional wisdom says Tri-Y's have a wider powerband at the expense of top end power, we have found that many times the opposite is true depending on where the second Y is placed. This is more often true where the Y is shorter like on these DC headers.|
Many headers presently available for popular sport compacts are of the Tri-Y design. For street cars Tri-Y’s are usually the best as they are forgiving to differences in camshaft design and other tuning factors that the header builder has no control over, unlike a real race car designer who know exactly what is in his engine. Tri-Y’s also promote a wide power band. A Tri-Y design pairs the opposite cylinders in the firing order together in a short “Y” and then brings the two pairs of “Y’s” into a single collector, hence the name Tri-Y.
When a pulse travels down the primary of a Tri-Y header to the collector, it mostly goes down the main branch of the primary. When it reaches the collector, the reflected wave also travels back up the main primary to the exhaust valve and back out again. However in a Tri-Y, the branch that goes up to the opposite cylinder, acts like an interference branch, since the exhaust valve is closed for that cylinder, creating a pulse and an assisting negative pressure wave of its own, slightly out of phase with the main pulse. This widens the bandwidth of rpm that the additional scavenging is effective by making the pipe less sensitive to rpm induced changes in pitch.
|Having longer Y's with the split down near the collector seem to work really well with nice wide powerbands. Some OEM manifolds that make good power are designed like this so the OEM's must be on to something!|
The pipe becomes “in-tune” for a longer band of rpm, widening the engine's powerband at the expense of slightly reducing peak power over a 4-1 design. This is because some of the pulse's energy is dissipated in the interference branch, the main pulse is not as strong and the scavenge effect is not as total for the Tri-Y. Peak scavenging efficiency is compromised for having good scavenge over a wider range of rpm. That is why many full race engines where peak power is important use 4-1 designs, while many headers that are designed for a wide powerband or for applications where the final cam and engine specifications are unknown like street engines or rally engines use Tri-Y headers.