[s-cars] Re: tubular exhaust manifold and turbo options

[MLP]

Trevor -

WRG to the "pretty sophisticated" software program for header design
etc., that you mentioned Burns Stainless uses for making their
recommendations on tubular header design, do you know what adjustments
or assumptions, or changes made by the software, or the Burns operator,
to take into account use in a turbocharged application?  Do they size
the runners down at all from what a NA recommendation would be, or go
with the same size?

I ask in light of the following two points on small vs. larger PRE-TURBO
tube diameters:   

FIRST, there's the following comments, attributed to a Jay Kavanaugh, a
turbosystems engineer at Garret, responding to a thread on
www.impreza.net on design and exhaust theory that have made their way to
postings in several places:

“Howdy, This thread was brought to my attention by a friend of mine in
hopes of shedding some light on the issue of exhaust size selection for
turbocharged vehicles. Most of the facts have been covered already. FWIW
I'm an turbocharger development engineer for Garrett Engine Boosting
Systems.

N/A cars: As most of you know, the design of turbo exhaust systems runs
counter to exhaust design for n/a vehicles. N/A cars utilize exhaust
velocity (not backpressure) in the collector to aid in scavenging other
cylinders during the blowdown process. It just so happens that to get
the appropriate velocity, you have to squeeze down the diameter of the
discharge of the collector (aka the exhaust), which also induces
backpressure. The backpressure is an undesirable byproduct of the desire
to have a certain degree of exhaust velocity. Go too big, and you lose
velocity and its associated beneficial scavenging effect. Too small and
the backpressure skyrockets, more than offsetting any gain made by
scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window. You want the exhaust
velocity to be high upstream of the turbine (i.e. in the header). You'll
notice that primaries of turbo headers are smaller diameter than those
of an n/a car of two-thirds the horsepower. The idea is to get the
exhaust velocity up quickly, to get the turbo spooling as early as
possible. Here, getting the boost up early is a much more effective way
to torque than playing with tuned primary lengths and scavenging. The
scavenging effects are small compared to what you'd get if you just got
boost sooner instead. You have a turbo; you want boost. Just don't go so
small on the header's primary diameter that you choke off the high end.
....."  there is more on the theoretical optimization of the post turbo
exhaust system, including FWIW the observation "... As for 2.5" vs.
3.0", the "best" turboback exhaust depends on the amount of flow, or
horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level
won't get you much, if anything, other than a louder exhaust note. 300
hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3"
is on the small side.”

SECOND, vis-à-vis Audi's 2.7L twin turbo EM design and implementation,
(a) if you have ever looked at the motors two cast & cased EM's, while
the exhaust ports in the heads are "regular" sized, the three in each
manifold squeeze into an extraordinarily small common exit/feed into the
KO3/O4 hot side scroll.  (b) I believe the EM is actually enclosed in a
second, probably insulating clam shell.  Are these just a packaging
design problem for the twin turbo engines or a performance design goals
as well?

Mike