Jerry:
All I can comment on is what is have read. To wit;
1. Reduced back pressure is good; it means that the engine needs
to do less work expelling the gases, which means more power available to
move the car. Headers reduce the back pressure by providing large
diameter, smoothly bending exhaust passages.
2. Utilizing the inertia of a moving column of gas permits a
vacuum to be applied to the exhaust port, reducing the work in expelling
the spent gasses still further. This vacuum has a short lifetime, and so
the length of the header pipe is tuned to a length that causes the
vacuum to be present at the exhaust valve when it opens. This means that
headers produce this extra scavenging effect only in a certain RPM
range, when the length of the pipe and the engine speed coincide to
deliver a low pressure 'pulse' when the exhaust valve is open.
3. The narrower the pipe, stronger the vacuum pulse, until the
pipe becomes so narrow that it restricts the flow of the gas. So we now
have a trade off, narrow pipes favor lower RPMS with lower gas flows,
fatter pipes favor higher RPMs because of the lower restriction.
4. The longer the pipe, the lower the RPM that the vacuum pulse
arrives back at the valve. So longer pipes favor lower RPM and torque,
but take up more room and are more difficult to get to fit in the engine
compartment.
5. None of this works very well, unless downstream of the header
has a low back pressure. Contrary to popular belief, it is not the pipe
diameter that affects this in any meaningful way, rather it is the
muffler (on street legal cars). A low restriction muffler will deliver
more power with a given header than a higher restriction muffler. The
pipe is of almost no significance compared to the muffler, so it doesn't
really enter in to the discussion, except in very arcane ways or in
extreme cases.
After that, everything goes to hell; Some manufacturers claim
stepped pipes are best (JBA, for example); some claim that multiple pipe
sizes work best (Edelbrock), still others claim anti-reversion designs
are best. Then you get into the 6 into 1, 6 into 2 into 1, tri-y
designs, etc. I saw one manufacturer claim that the pipes needed to
enter the collector in rotational firing order, so that "swirl" was
increased in the collector, which gave more power. Ack! It all turns
into snake oil pretty quickly.
The thing is, ALL of these claims are probably true under some
circumstances. With the right cam, muffler, at a certain RPM, etc there
is probably some truth in just about everyone's claim. It is enough to
drive you to drink (and I need very little incentive to drink =;-o)
Things change dramatically with no muffler, so even the length
of the exhaust pipe can have a measurable effect according to David
Vizard. Remember that Kastner ran with no muffler, so his claims about
pipe size, length etc will pretty much be moot for the street.
So what would I do? First, I would replace the stock muffler
with a good, low restriction item. Not an ANSA or a Monza, but something
I had numbers on. Top of my list is a Cyclone Sonic Turbo, followed
closely by a Walker Dyno-Max muffler. Some of this data is pretty old,
but it is all I have seen with hard numbers. These mufflers actually
muffle, too, so your significant other will ride in the car with you for
more than 5 minutes.
After I had a nice exhaust system, I would probably just choose
my header based on fitment, quality, and lastly price. All the stuff
about 6-2-1, 6-1, etc is just gooblygook after a while. If it is the hot
ticket but I can't make it fit, it doesn't matter much.
Also bear in mind; Headers are much more rust prone unless you
get them coated or use stainless, and are noisy compared to manifolds,
especially with a longer period cam. The stock exhaust manifold on our
cars is pretty good, especially on the later cars. You don't get the
scavenging that headers give, but they flow well with adequate sized
pipes and sweeping curves. The later system was good for 150HP on the PI
models, so it had to be at least reasonable.
All IMHO, of course...
Vance
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Cogito Ergo Zoom
(I think, therefore I go fast)
TR6 Web page: http://home.comcast.net/~v.navarrette/
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