In the vein of my last thread, here's another entry on Exhaust Manifolds:
Velocity, Collectors, pipe length, pipe diameter, displacement... TQ and HP.
Low end torque is facilitated by high velocity and smooth transitions. Big power at high RPM is facilitated by wide open free flow. By manipulating the variables you can create or choose a header best suited to your goals for the engine.
Want a drag car? 4 pipes open to atmosphere will net the most gain. Highest TQ and HP at the highest RPM. If you want something more pedestrian, start adding pipe length, a collector, and exhaust to flatten the TQ curve and bring the peak lower in the RPM range.
At a certain point you get diminishing returns in pulling the TQ curve back by manipulating header design.
This may explain why our V has the choke collar at the collector. Instead of properly using pipe length to pull the curve down, they used a shorty collector with a collar to increase velocity exiting the manifold. It's not very efficient, but it is cheap. And as we're all aware, neither the HR nor the MR used in the V was ever meant to be a performance motor.
4-1 Designs (MR 18) by OBX and DC simply remove the collar, which (in theory) will improve torque, but will have a greater influence on the shape and peak location (RPM) for torque. The result is less torque lower, but more torque higher. As stated in the article linked below, it would be a much better solution to use a collector that had equal measurements for diameter and length.
4-2-1 Vs 4-1
This then really becomes a question of preference, space, and engineering. One of the articles linked does point to a 4-2-1 being quieter in general, but other than that, with proper engineering, there is little difference between the 2 styles. A 4-2-1 benefits from longer pipe and smaller collectors, so velocity is generally increased and trasnitions are relatively smooth (depending on the angle of the collectors). A 4-1 is simpler, has a single larger collector and shorter piping.
You may look at the Dyno test article and be confused then, that the 4-1 showed lower RPM for peak TQ. Note the length of the collector on the 4-1, as well as the narrow angle, and compare that to the collectors on the 4-2-1. Note also that peak HP on the 4-1 is pushed to a higher RPM, and the 4-2-1 doesn't change.
In this case both improve on stock; the 4-2-1 narrows the power band slightly, where the 4-1 opens it up by almost 1000RPM, both moving peak torque earlier and peak HP later.
Closing
The example above is one example, and every header design will perform differently. There is no guarantee or law that says one style is better than another, they are just different. The engineering principles are same, and can be used to make any style just as efficient as any other.
This is the link to the Dyno Article, very well laid out:Header Comparison Test
This is an article of EXCELLENT general information and includes a few calculations you can use to start understanding the relationships:Understanding Header Design - Car Craft Magazine Page 2
Velocity, Collectors, pipe length, pipe diameter, displacement... TQ and HP.
Low end torque is facilitated by high velocity and smooth transitions. Big power at high RPM is facilitated by wide open free flow. By manipulating the variables you can create or choose a header best suited to your goals for the engine.
Want a drag car? 4 pipes open to atmosphere will net the most gain. Highest TQ and HP at the highest RPM. If you want something more pedestrian, start adding pipe length, a collector, and exhaust to flatten the TQ curve and bring the peak lower in the RPM range.
At a certain point you get diminishing returns in pulling the TQ curve back by manipulating header design.
This may explain why our V has the choke collar at the collector. Instead of properly using pipe length to pull the curve down, they used a shorty collector with a collar to increase velocity exiting the manifold. It's not very efficient, but it is cheap. And as we're all aware, neither the HR nor the MR used in the V was ever meant to be a performance motor.
4-1 Designs (MR 18) by OBX and DC simply remove the collar, which (in theory) will improve torque, but will have a greater influence on the shape and peak location (RPM) for torque. The result is less torque lower, but more torque higher. As stated in the article linked below, it would be a much better solution to use a collector that had equal measurements for diameter and length.
4-2-1 Vs 4-1
This then really becomes a question of preference, space, and engineering. One of the articles linked does point to a 4-2-1 being quieter in general, but other than that, with proper engineering, there is little difference between the 2 styles. A 4-2-1 benefits from longer pipe and smaller collectors, so velocity is generally increased and trasnitions are relatively smooth (depending on the angle of the collectors). A 4-1 is simpler, has a single larger collector and shorter piping.
You may look at the Dyno test article and be confused then, that the 4-1 showed lower RPM for peak TQ. Note the length of the collector on the 4-1, as well as the narrow angle, and compare that to the collectors on the 4-2-1. Note also that peak HP on the 4-1 is pushed to a higher RPM, and the 4-2-1 doesn't change.
In this case both improve on stock; the 4-2-1 narrows the power band slightly, where the 4-1 opens it up by almost 1000RPM, both moving peak torque earlier and peak HP later.
Closing
The example above is one example, and every header design will perform differently. There is no guarantee or law that says one style is better than another, they are just different. The engineering principles are same, and can be used to make any style just as efficient as any other.
This is the link to the Dyno Article, very well laid out:Header Comparison Test
This is an article of EXCELLENT general information and includes a few calculations you can use to start understanding the relationships:Understanding Header Design - Car Craft Magazine Page 2
