Turbo header design

[Greg Hermann]

FOR THIS PARTICULAR V-8> (IT HAS A 90 degree crank)
>                Firing order
>          1  8  4  3  6  5  7  2

With this firing order, take every other cylinder in the firing order if
you want  180 degree (crank rotation) spaced pulses into a pair of 4 into 1
collectors:

1, 4, 6, & 7 go into one collector (two end cylinders from one bank, and
the two center cylinders from the other bank.

8, 3, 5, & 2 go into the other collector. (the two end cylinders on the
other side, and the two middles from the first side.

If you want PROPERLY done tri-wyes, go 1/2 way around the firing order, so
that the pulses in the pairs of tubes which combine at the first wye are
spaced evenly, 360 (crank) degrees apart:

1 & 6; 8 &5; 4 & 7; 3 & 2. As you can see, each pair takes one tube from
each bank of the motor.

Then, at the second wye, pair 1-6 with 4-7 and pair 8-5 with 3-2 so as to
get evenly spaced pulses at the second wye in each header.

If you wanna go with REALLY long tri-wye branches, (surprising how
fantastically good this is for street, high torque, & 4x4 type apps.) start
out with a regular set of 4 into one headers (of the smallest tube size you
can find), cut them off a bit before the collectors, and do the snaking
back and forth under the bell housing/tranny to get the correct first wye
pairings. The second branches want to be the same length as the first ones,
but there are no crossovers if you plan ahead. After the four second
branches combine into two (tertiary) pipes, you want a length of pipe of
length equal to all the first and second branches. At the end of that pipe,
you need a gap (no diameter change of offset in the pipe) in the pipe about
3/4 inch long. Put an empty plenum chamber around that gap. The volume of
the plenum needs to be maybe double the internal volume of the last
(single) length of pipe feeding it. Just run a full size tail pipe and low
restriction muffler behind the plenum (Not much vehicle length left after
this, anyway!)
The headers will effectively see the plenum as an open ended (to
atmosphere) pipe. DO NOT omit the plenum, it is worth quite a bit of HP and
response!
For street stuff, usually, primary tubes 1 trade size SMALLER than the size
which can be swedged square to fit into a flange and match the port
properly work out to be the correct size. (If a 1-7/8" tube can be squared
at one end and fitted into a flange so that the inside of that tube matches
the port shape, then run the primary tubes with 1-3/4 " tubing.) You want
the internal AREA of your primary tubes to be about equal to the
cros-sectional AREA of the exhaust port. The above is what usually works
out right. (Especially if the engine designer did his homework!) The extra
work involved in getting down to the smaller tube size is why most off the
shelf headers use too big a tube size!  Figure the length of the primary
tubes so that each of their internal volumes is about 140% of the
displacement of an individual cylinder. (For instance, for a 350 cid V-8,
using 1-3/4" tubes (which are about 1-5/8" inside diameter in 16 gauge
tube, this would mean you want (at least) 24 inch long primary tubes.) (And
if you are using lighter than 16 gauge tubing, don't bother!)
If the primaries are 1-3/4", then 2" is usually about right for the
secondaries, and 2-1/4" for the next ones. (A quarter inch increase in tube
size at each successive wye is usually about right.

1-3/4" diameter primary tube size was only an example. It is prolly BIG for
a street 350, 1-5/8" or 1-1/2" is more likely to be right (but with more
tube length so as to get to the same internal tube volume!)

If you build a set of tri-wyes this way, they will sound and run like
nothing else! The throttle response will be astounding! But it is a ROYAL
pain in the #@$% to do it!

All my experience says that the biggest gain from headers is from giving as
much of the exhaust gas as possible somewhere to go freely during the
"blowdown" part of the exhaust stroke--before the piston starts back up
significantly. Yes, it's nice to have a low pressure pulse in the exhaust
port at overlap, and that pulse helps to scavenge clearance gasses if you
have it there at the right time, but the longer you can keep the blowdown
stage flow sonic, the less work the engine will have to waste pumping
exhaust gas out of its cylinders, and the more sonic energy there will be
available for creation of a scavenging pulse!

Regards, Greg