Some thoughts on exhaust headers from an old Motorhead:
Our 260/289 engines were designed for mundane passenger car usage, not
with high-performance in mind, so an untuned cast iron manifold was all
that was required. The designers were really after cheap reliable
manifolding that kept engine bay temperatures reasonable, and in the RPM
range these engines were designed for a little backpressure was not a
problem.
The hypo cast manifolds of the time (289 Hypo, Tri-Y) were really just
larger, less restrictive versions of the stock manifolds. These allowed
the engine to breath a little better but did not really provide a
substantial boost in power. For that you need equal-length long tube
headers. Your engine is basically a pump, the faster you rev it, the
higher the mass flow rate of the gases being expelled. The tubular
performance header not only allows for good gas flow, it improves power
output by helping to scavenge the exhaust out of each cylinder. The tube
must be long enough and of the correct diameter to allow the exhaust
pulse, putzing along at 250 fps, to travel without restriction into the
collector. The gases then expands inside the collector but late enough
that they do not interfere with the next exhaust pulse like what happens
in the stock cast manifold. This allows faster and a more complete
evacuation of the next firing cylinder. Header manufacturers balance
header tube diameter (larger raises the torque peak, smaller lowers it)
and tube length (short tube for low RPM torque, long tube for high RPM
power) depending on engine configuration. Collector volume is also key,
too small and you lose low-end torque.
A good set of headers can add 15% to 20% more horsepower depending on
how high you are willing to rev your engine. However, there are a few
downsides to using tubular headers. They are by nature tuned to make
power at higher RPM, so if you are running a significant amount of cam
overlap you will lose torque at normal street engine RPMs. Also, headers
seldom fit well, they are noisier (thin wall steel), prone to cracking,
and transmit a lot more convective and radiant heat into the engine bay,
which is why ceramic coatings are popular options for headers. Some
headers also restrict ground clearance because the collectors are so
low.
So, if you really need to make serious power, go for the tubular header
system. But for day-to-day practicality the original cast manifolds will
be the better choice for most folks. I have used both and the last 289 I
built for B382000991 uses the stock manifolds, and still over 250HP at
the rear wheels.
Bugz
_______________________________________________
Tigers@autox.team.net
Donate: http://www.team.net/donate.html
Archive: http://www.team.net/archive
Forums: http://www.team.net/forums
|