Good information! Now I need to plan for
head CFM improvements and matching header
flow. Very comprehensive Bob, thanks very
much!
George
>>>>>>>>>>>>>>>>>>>
From: Bob Tom <tigers@bserv.com>
Subject:
At 09:45 PM 2/1/02 -0500, you wrote:
>Long tube headers will not cost you low end provided they are made of EQUAL
>length primary tubes and the primary tubes are not too large. There is a lot
>of bad information out there about header design and what works and why. I
>refer you to Headers by ED at <www.headersbyed.com>. Here you will find
>sound info. on what works and why. Andrew
Andrew and George.
Andrew, you are correct about long tubes need not cost low end torque
reduction.
Due to the sloppiness of my mind, I wrote that to George when I really meant
was that the selection of the primary tube (to a lesser extent) and
secondary tube
(collector) diameters/lengths (to a greater extent) are important
considerations as
to where the power band ends up. I also agree with Andrew that there are a
lot of
different and conflicting exhaust advocacies.
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-----------------------------------
Here's one that I came across quite awhile ago that seems contrary to that
from Headers by ED.
Research shows most street engine are not sensitive to the shape of the exhaust
gas passages after leaving the head ports. Nor are they sensitive to
header pipe
length within a fairly broad range (varying length between as little as 22"
and as much
as 48" produces similar power curve) on high performance street machine meaning
equal-length headers are not a great advantage UNLESS everything else is
working
together as on a full-race engine. In fact "Tri-Y" headers take advantage
on this street
vehicle length and sensitivity by joining parts of headers pipes with a
Y-joint, then
joining those pipes together in a Y. Such 4-into-2-into-1 design are cheap
and work
fine on lower speed engines with milder cams (duration below
260-270). Although on
longer cams, they can make considerably less power. When you consider the
terrible
exhaust manifolds "headers" delivered as original equipment and many
vehicle Tri-Y
headers can be a good step up from a restrictive cast-iron manifold.
Street engines should be designed to optimize low and mid-range torque in
order to
be fun to drive, and headers should be selected with this in mind-meaning
long-length
headers with relatively small diameter tubing, designed to maintain high
gas velocity
through the engines of heavier vehicles at low and medium
rpm. Unfortunately, such
headers will offer restriction to high-end torque and power. Large
diameter header
tubes offer unrestricted high-end velocities, but low-end performance is
compromised.
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-----------------------------------------
There is an article on the whole exhaust system is written by David
Vizard. I can't find
it now so I'm not able to list the background or exact expertise of the
author. I tend
to use it as a guide from which to evaluate articles on headers. This part
of the
articles deals with his conclusions regarding the headers.
HEADER PRIMARY PIPE DIAMETERS
If pipes are too big or too small, scavenging obviously suffers. Here
is a way to optimize primary pipe size that does not involve the
normally-used dyno trial and error method.
The underlying principle behind this short cut is that the rate a cylinder
dumps exhaust is strongly determined by the port's flow capacity.
This has led to the development of the following table which predicts,
with excellent accuracy, primary pipe sizing.
Exhaust Pipe Inside Diameter Selection
Exhaust Port CFM Exhaust Pipe Diameter
at 28" H2O (inches)
60 1
80 1 1/8
100 1 1/4
120 1 3/8
140 1 1/2
160 1 5/8
180 ~ 1.70
200 ~ 1.80
220 ~ 1.87
240 ~ 1.98
260 ~ 2.05
280 2 1/8
300 ~ 2.20
~ means approximately
Example: If heads flow at 190 cfm, then an ID pipe of 1.75"
is needed.
OR this formula can be used:
Pipe Diameter = square root of ((cfm x 1.27)/FD)
where FD is flow density.
On a street/strip application, use an FD of 85
On a pure strip application, use an FD of 90
HEADER SECONDARY (COLLECTOR) PIPE DIAMETER
Often overlooked, the collector diameter is often as crucial
as the primary. A good approximation is to take the
needed primary pipe diameter and multiply by 1.75 .
PRIMARY PIPE LENGTHS
Because an individual cylinder/pipe length can scavenge
over a 4000 rpm bandwidth, having equal length headers
is not that important. Most race engines use an rpm
bandwidth of 3000 or less. If the primary pipe scavenging
effect of each cylinder overlaps by 3000 rpm, then the
fact that one pipe tunes as much as 1000 rpm from
another matters little. Hence pipe lengths varying
by almost a foot have little effect on performance.
(I found this to be a brain buster!)
Dyno tests with headers with primary lengths adjustable
by 3" increments show that lengths between 24" and 36"
have only a minor effect on the power curve, although
the longer the pipes do marginally favour the low end.
SECONDARY PIPE (COLLECTOR) LENGTHS
Although primary pipe length is far less critical than often
supposed, the collector length has far more influence on
the power curve if the primaries are REASONABLY close. In
practice, collector length and diameter can have more effect
on the power curve than the primary length.
A basic rule on collectors is that short pipes with large
diameters favour the top end while long pipes with small
diameters favour the low end. If a vehicle has a relatively
tight converter (low stall) and launches at lower engine
rpms, then a longer secondary pipe can cut ETs by
getting the car underway quicker.
- ------------------------------------------------------------------------------
------
Sorry for the length. Hope that you find something useful
in the above.
George, if I remember correctly, I added the Leach headers
along with a K&N 14x2.25 open air cleaner at the same time.
Previous mods were the Gibson dual exit cat-back, BFG 28"
drag radials and a K&N replacement filter. The order of
my mods were dictated by backorders from MP and F&B.
On the street, it FELT like my power band had been moved
up a few hundred rpms but, on the strip, my ET dropped
about 15-20/100 of a second. I believe, at that time,
my previous pb was 15.6s and it fell to the 15.4s.
Bob
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