Frederic Breitwieser/turbo discussion, design Q's

Gary Derian gderian at oh.verio.com
Wed Mar 17 21:39:53 GMT 1999


Adding a turbo to an engine raises the pressure on both sides, intake and
exhaust so a turbo really doesn't see much different conditions than an atmo
engine.  A really good setup can generate more boost than backpressure but
in most practical situations, the exhaust pressure is higher.  This is true
even for atmo engines.

Unlike a V-6, a V-8 does have uneven exhaust events in each bank, unless it
has a flat crank like Ferrari V-8s.  A 4-2-1 header (bi-Y or tri-Y) still
works as long as you take the exhaust pattern into account.  Different V-8s
have different cylinder numbering and firing orders but I am pretty sure
they all have the same exhaust pattern.  The one I know at the top of my
head is Chevy so I will use it.  Its firing order is 18436572.  Take it and
write it like this:

1843
6572

Ideally cylinders 1-6, 8-5, 4-7, and 3-2 should be paired in the first merge
but what a basket of snakes this creates!  By shifting over one, you can
pair 1-5, 8-6, 4-2, 3-7.  This keeps each bank separate but still keeps the
exhaust pulses from stepping on each other.

High performance V-8s from BMW and Lexus use this type of setup with the
factory exhaust.

For low to mid rpm use smaller diameter tubes.  Of course for a turbo setup
you will want to get some nice mandrel bend 321 stainless steel ($$$)!
Every installation is a series of compromises.  It sound like your setup is
reasonably good but a nice tri-Y each side would be better for power but not
so good for cost or time.

The only reason I can figure that your H pipe after the turbots helped is it
balanced the flow on each exhaust pipe reducing backpressure  I would not
have expected it to help.  H pipes work by providing a spot for the pressure
waves to reflect.  They don't have to actually flow gas to work.  The same
thing can be accomplished by building an expansion chamber in each pipe.
The other h pipe benefit is higher frequency noise which is easier to muffle
so a lower backpressure exhaust system can be built for the same noise
level.

Putting the second merge of a tri-Y at the h pipe point also gets the job
done.  You don't need the h pipe in that case, although it may still provide
better tuning.

I am interested to learn what exhaust configuration you used and maybe an
explanation to go along with it.

Gary Derian <gderian at oh.verio.com>

>
>What would you recommend for low to mid end then?  I am curious, for I'm
>building a 383 Mopar B block stroked to 431 cubes (or more if I can figure
it
>out), using the EEC stuff to manage it.





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