Frederic Breitwieser/turbo discussion, design Q's

[Gary Derian]

Turbo engines respond to all the same tuning tricks as atmo engines.  Tuned
headers included.  Conventional wisdom states that a 4-2-1 header gives
better mid range and a 4-1 gives better top end.  There is lots of racing
engines that use 4-2-1 designs  With largish tube diameters and appropriate
lengths, I thing a 4-2-1 is better up beyond 7000 rpm.  Heck, even the
Ferrari F355 V-8 (peak power at 8500 rpm) uses a 4-2-1 design.

Firing order definitely dictates the merging.  The first merge joins
opposites in the firing order.  All 4s have a 1-3-4-2 order so join 1-4 and
2-3 together.

I suspect that some of Porsche's reasoning is packaging the system, but it
also allows enough room to fit a tuned exhaust.  Back in the old days when I
used to build carbureted turbo engines with draw thru carbs, putting the
turbo close to the exhaust resulted in a long and tortuous intake path that
results in really poor performance.  With EFI your have more choices.

Gary Derian <gderian at oh.verio.com>

>HI Gary,
> Is a 4-into-2-into-1 turbo header superior  to say,
> 4 tubes converging right at the turbine inlet?
>I'm trying to establish some rules as to when one design works
>better than another (while lag is a fairly big concern).   Is there a
>rule of thumb, say with 4 vs 6 vs 8 cylinders?   Mine is a 4cyl.
>The info on the Porche design is very  interesting, but
>does anyone know why they chose that design over others?
>With the 4-2-1 setup, does the firing order dictate which tubes
>get paired together?  For example, should the first merge join
>two cylinders that are farthest away from each other in the firing
>order?
>Thx
>Mike