Turbo header design

[Jon Fedock]

This clears up alot of questions. THANKS
Jon Fedock

-----Original Message-----
From: Greg Hermann <bearbvd at sni.net>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Thursday, January 07, 1999 3:52 PM
Subject: Re: Turbo header design


>
>
>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
>
>
>