Explanation of Merge Collector Theory

[Bob Hollinshead]

Can anyone here explaint the theory behind Merge Collectors? I'm thinking these designs are an effort to increase a negative pulse in the primary pipes

[rookie]

From CobbTuning.com - Exhaust Design

The collector merges all of the primary pipes together. There are designs ranging from cheap and simple to incredibly complex and costly. If you just joined the pipes in the simplest possible way you would have something of a dead space in the middle of all of the pipes .

The dead space in the middle of all of the pipes would cause a lot of turbulence and hinder flow. Eliminating the dead space is the main advantage of the merge collector. The simpelest way is to form the pipes to make a cross pattern in the center. This is a more cost effective way to make the pipes join smoothly. Not quite as elegant as the merge collector, but still very good.

Here you can see in the upper right hand photo that the dead space in the center is virtually eliminated.
http://www.performanceweldingheaders.com/compmerg.htm

Basically you want the least amount of abrupt changes as possible. This mostly applies to the collector where it necks down to the diameter the exhaust will be. You do not want an abrupt angle as it will hinder flow.

[BlackCat13]

Thanks, that is very interesting.

I note that he disagrees with DV and everybody else in that he states that a longer collector moves the power band to a higher rpm range.

Good reading.

[BlackCat13]

Help me here, so that I know that I understand this correctly.

The merge collectors contain a flow cone to smooth the transition between the primary pipes (as some upscale straight collector headers do).

The convergence of the collector reduces the collector volume to help maintain exhaust velocity to allow each primary pipe to interact with the the other 3 at all rpms. A lot like the 4-2-1 headers sold by Flowmaster. But I can see that the merge collector would more easily allow any of the 4 primaries to interact with any of the 4 others.

The divergence of the collector functions like the exhaust termination box that DV explains in his no loss exhaust systems.

The merge collector functions similar to a 4-2-1 with a termination box, but in a very compact packaging space, but tuned to a broader and higher rpm range.

[rookie]

Originally Posted by BlackCat13

Thanks, that is very interesting.

I note that he disagrees with DV and everybody else in that he states that a longer collector moves the power band to a higher rpm range.

Good reading.

Very interesting, I did not catch that on my first read, maybe DV can expand on what set up could cause this to happen, if any.

Also on DV's termination box you need an abrupt change, ie: the end of the straight cut pipe extending in to the box an inch or two, not a smooth transition.

Like This.
http://images.superchevy.com/technic...r_exh_20_z.jpg

[Devious]

A merge collector is simply a smooth transition that can keep velocity higher. At most, it will add a bit to the torque curve. In my experience, to get the most out of a merge collector, your lengths and diameters need to be correct, and using a diverging cone that starts at the correct diameter for your usage can help considerably.

It will still not offer the broad power of a tri-y (4-2-1) system, but it can be a nice improvement if it all comes together correctly. If not, there is nothing gained but more expensive headers.

[rookie]

Interesting header test or sales pitch you decide.

Exhaust Header Dyno Test - Circle Track Magazine

[Devious]

Rookie,
I've read that. A useless article IMHO. The engine is very mild, and there is no info as to whether or not the different headers even matched the rest of the engine. There is more useful info left out than the article actually contains.

[seattle smitty]

On the Cobb Tuning site, he says that (on a 4-cyl engine) a 4-1 header will make more torque than a 4-2-1 header (Tri-Y). This disagrees with other sources, who assert the opposite.

On the Performance Welding site, when they make a collector (look at the 2-1 collector especially), rather than simply angle-cut each tube and join them, they cut out a little plate and weld it into the middle of the joint. I'm trying to guess why. The plate would have the effect of putting a significant restriction into the end of each primary pipe immediately before they enter the secondary pipe, which by itself would seem undesirable. But in reading elsewhere about collectors, I have seen it said that the idea of a pulse from one pipe drawing the pulse from the other pipe is less likely than that the first pulse would tend to pressurize and back up the second pipe. So, maybe this plate helps keep everything headed downstream, even though it is a restriction.

What am I not getting?

[Devious]

A 4-1 can make more peak torque than a 4-2-1. The 4-2-1 torque can be significantly broader.