[W126 Coupe] Tri-Y headers

[mramay at att.biz]

I have always wondered why AMG came up with something as strange as a Tri-Y
header arrangement. It had to be for power, but why? I was reading an
article on LS1Tech.com to educate me a bit on headers and exhaust systems
for the FrankenBenz when I came across this bit. If you want to read the
whole article, it's buried in a discussion aimed at the Camaro/Firebird
crowd, but it's good stuff anyway since the LS1/LS6 engines are 350 cu. in
(5.7L), very close to our 5.6L engines.

Enjoy,
Mike R.
Freezing in DC, home to Vegas on Saturday!!! (70+ degrees)

http://www.ls1tech.com/forums/showthread.php?t=520845

In the case of V-8 firing order, the five pulses fire alternately back and
forth from left to right collector, giving the ideal 180 degree firing
cycle. Then it fires two in succession into the left collector, then two in
succession into the right collector. If the proper collector outlet
diameter is being used (two sizes larger than primaries) the two pulses in
succession load up the collector with more air than it can flow. This
results in a very strong midrange torque, but causes the engine to "sign
off" early, not reaching its potential peek R.P.M. The improper firing
order on a V-8 engine results in the need to use large diameter collectors
so the engine will perform well at high R.P.M.s. Unfortunately the large
diameter collectors cause a tremendous drop in air velocity, resulting in
less scavenging through the entire R.P.M. range.

Often cams are used with extended valve timing to help the exhaust cycling.
This results in valve timing overlap (Intake and Exhaust valves both open
at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this
happens, exhaust actually backs up into the cylinder causing intake air to
be pushed back out the intake. This reversion causes "Standoff" (fuel
blowing out of the Intake) at low R.P.M.s. This whole improper cycling has
resulted in a number of "Cure Alls" to help stop this reversion and
standoff.

The plentum intake was created to stop the fuel "Standoff". Then came "Anti
Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the
header, extended collector lengths and even plentums in the exhaust tubes.

In this chain of events beginning with improper firing order, a series of
cures has developed, each one causing a new problem. 

The optimum cure to this whole problem is to correct the exhaust firing
cycle. The two cylinders that fire in succession into each collector have
to be separated. This can be done partially by a "Tri-Y" header, where the
four primary tubes from each bank merge into two secondary tubes
(separating the two pulses firing in succession) and finally collect into a
single collector. This type of header helps, but the two pulses are still
coming back together at the collector. 

The second optimum cure is to cross the two center tubes from each bank,
across the engine running them into the collector on the opposite side.
This makes the firing cycle in each collector 180 degrees apart, the same
as a four cylinder engine. Once this firing order is achieved, the small
collector outlet diameter can be used and the "High Velocity Scavenging" at
low R.P.M.s cures the reversion problems and eliminates the need for
extreme cam duration.


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