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Performance Exhaust Systems
Performance Exhaust Manifolds
Sports Cats
Header/Exhaust Manifold Design
Tune Your Own Custom Made
Header/Exhaust Manifold
If you are serious about your all-motor power, then a custom
header/exhaust manifold is going to be mandatory for your project
vehicle. By this point your motor is built to the limit or near-limit.
You have high compression, a ported head, different intake manifold and
perhaps a different combination of bore and stroke. You have camshafts
with an aggressive profile. The last of these modifications, the
camshafts, have the largest affect on header/exhaust manifold design. A
camshaft's duration and lobe centers have a direct effect on overlap and
the optimal primary length, diameter and collector type of the header.
If you have a highly modified engine then regular pre-made
headers/exhaust manifolds probably won't cut the mustard.
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Header/Exhaust Manifold Design
Tune Your Own Custom Made Header/Exhaust Manifold
continued...
Generally, the off-the-shelf headers/exhaust manifolds on the market
have been designed around stock camshafts. The worst headers are going
to be copies of something or just simply made using the designer's
'experience' in making headers. This can be as bad as having some
fabricator stuff some tubes into the engine compartment with no
consideration toward tuning. Motors with stock camshafts are less
sensitive to variations in header design but once you start upping the
duration and increasing overlap, the engine becomes increasingly
sensitive to header and exhaust design and thus the more important
optimizing the header/exhaust manifold becomes. Creating harmony between
a built engine and the header's tuning is one of the main reasons why
custom headers can extract surprising amounts of power from heavily
modified motors over off-the-shelf offerings. I have personally seen a
custom header make 30 hp on a heavily modified street motor over the
best off-the-shelf header design. The next time you go to a drag racing
event and walk through the pits, note that none of the top all-motor
cars are using off-the-shelf headers.
Design your own header/exhaust manifold
If you want to try your own hand at header/exhaust manifold design, I
have reduced down some of the math involved to these simple equations
and tables that should get you in the ballpark. The actual formulas are
much more complex, but the complex stuff is reduced down to constants
here. These simple formulas are not the end all solution for the
ultimate in header design, but they are much better than a 'WAG' and
they will get you in the ballpark. This information has helped me many
times in my career to date, and it has never resulted in a header that
sucked. Even if you don't want to design your header, you can use this
information to sort through the design specs of a bunch of off-the-shelf
headers to help pick one that is the most likely to work well on your
motor.
The first step is to calculate the length of the primary tube. The
formula for Primary length is:
Where:
P = Primary Length
ED = 180 degrees plus the amount of degrees before bottom dead center
that the exhaust valve opens
RPM = the RPM that the header is tuned to work best at.
You can roughly calculate primary internal diameter with this formula:
ID = (The square root of cc/(P+3) x 25) x 2.1
Where:
ID = Inside Diameter
cc = Cylinder Volume in cc
P = Primary Length
Having a tube with a slightly larger cross sectional area than the
exhaust port is a decent starting point as well.
If you wanted to design a Tri Y or an interference branch style header,
you first determine the best overall primary length by using the above
equation or my handy dandy table. Make the length to the first Y
junction from 13-16 inches. Subtract this from the overall primary
length to determine how long to make the tube from the first junction to
the main collector.
To find the inside diameter of the first junction use the equation we
last used to determine the ID of the primary pipe. From this diameter we
can determine the diameter of the next branch using this equation:
Where:
ID2 = the inside diameter of the secondary primary
ID = the inside diameter of the first part of the primary
The collector should ideally be a merged collector with an included
merging angle of 14-20 degrees.
To find the diameter of the collector, this formula can be used to get
you in the ballpark:
Collector ID = (the square root of cc x 2/ (P+3) x 25) x 2
Where:
cc = cylinder volume in cc
P = primary length in inches
When designing a header for low-end power and street use, you typically
want to tune the header for the rpm of the estimated torque peak. For
forms of racing that need a useful powerband like road racing or short
circle tracks or perhaps a serious streetcar, you may want to tune the
header for somewhere between the torque and power peak. For all out drag
racing with a close ratio gear box in a light car you might tune for the
rpm of the power peak.
For a fun mental exercise, try calculating what should work with a stock
engine with a stock cam at various streetable rpm ranges, and then
compare your findings with typical off-the-shelf headers. Afterwards,
"design" some headers for the same engine with available performance and
racing camshafts at higher but realistic RPM ranges. See the big
differences? Now do you wonder why you rarely see any market headers on
the cars in the all-motor class?
Header/Exhaust Manifold Design Trends
Although these equations are what many engineers use when designing a
header/exhaust manifold, they don't take into account many of the recent
design trends for header design that are being proven to work quite
well. Many of these latest trends cost a lot more to make and are not
likely to be found in an off-the-shelf production header. These trends
are proven power adders or powerband wideners which makes designing a
custom header incorporating these features more and more worthwhile.
Some of the latest design trends are: stepped primary tube diameters,
anti-reversion chambers, merged collectors and venturi collectors. A
stepped primary diameter steps up in primary diameter two to three times
over the length of the primary. Measurements like 1.75 inches to 1.875
inches to 2 inches are common in high revving import motors. Usually
these steps go in lengths of 7 inches or so. By making the propagation
of waves and refractions as discussed last month more gradual, stepped
primaries generally give a wider powerband with no loss of top end
power. Most engines with larger camshafts respond well to stepped
primaries.
Anti-reversion chambers are controversial. These chambers are areas in
the primary tube with a larger ID over a short distance usually about
5-7 inches away from the head flange. The chambers sort of look like
goiter bulges in the primary pipe. They are supposed to damp out the
return of the reflected acoustic wave to prevent the short-term spike in
primary tube pressure around the exhaust valve on overlap. Whether they
actually do anything is a fierce source of debate among header
designers.
As discussed last month, merged collectors are the best for power
production and width of powerband. They are exceeding difficult to
fabricate, however. Burns Stainless sells many variations of merged
collectors of exquisite quality, which can greatly aid in fabrication of
your custom header. The majority of fast all-motor racers in this
country use Burns Collectors. Additionally, many of the best fabricators
use Burns collectors as a labor-reducing component in there own custom
headers since no one does it better.
A venturi collector has a necked down area just past where the primary
tubes merge. Generally this is a cone shaped neck down with a 7-10
degree taper with a megaphone with a similar taper stepping the
collector back up to the full diameter of the exhaust pipe. For most
compact cars, the venturi goes from 3 inches in diameter at the merge
down to a 2 3/8-inch venturi, back up to 3 or more inches to the exhaust
pipe. Sometimes a short reverse cone is added to the end of this
megaphone before the exhaust pipe starts to add yet another back pulse
to help broaden the powerband further. The purpose of this venturi is to
speed velocity and create a stronger low-pressure rarefaction at the
exhaust valve without reducing flow much. Some header builders use a
short primary tube for good top-end and use the venturi collector to
help maintain a broader powerband. Burns Stainless offers prefabricated
venturi collectors, some with removable and tunable venture sections.
Header/Exhaust Manifold Tuning
One of the best ways to design a custom header is to let a professional
engineer do it. Burns Stainless offers such a service. If you can give
them some detailed information about your engine, they can do all of the
calculations for a nominal fee to spec out a header's primary length,
diameter, step sizes, collector taper, megaphone length and diameter.
The fee is applied towards your purchase of a merged collector for your
project. It's a little known fact that a majority of the successful
naturally aspirated racing motor headers in this country have had design
input from Burns. Even if you don't go custom, you can take Burns'
suggestions and use them when shopping to buy a shelf header that comes
closest to what Burns's calculations ended up being.
Even with the best equations and calculations, the header created
usually still is not the optimal for your engine. Even a change in cam
timing done on the dyno can change the optimal tuned length for the
header's primaries. If you have the budget, dyno testing is the best way
to fully optimize your header to your combination. Burns Stainless sells
slip fit collectors that are held to the header primaries with springs.
This enables you to make a test header where you can alter the tuned
length of a header during testing in short order to find what works
best. Burns also makes a megaphone merged collector with slip in
venturis which can be exchanged with different sizes to determine which
works the best during dyno testing. This also allows the header to be
tuned for different track conditions as well.
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