Turbo header design

Pat Ford pford at qnx.com
Tue Aug 10 18:29:22 GMT 1999

Hi Greg:
 Is there any simular design rules for intakes ??

Previously, you (Greg Hermann) wrote:
> 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

Pat Ford                           email: pford at qnx.com
QNX Software Systems, Ltd.           WWW: http://www.qnx.com
(613) 591-0931      (voice)         mail: 175 Terrence Matthews          
(613) 591-3579      (fax)                 Kanata, Ontario, Canada K2M 1W8

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