[T3] Aftermarket exhaust effecting low end performance

[bob diefendorf <bobsail5@yahoo.com>]

Chris:
  An astute observation, and one that has lead to many
people scratching their heads over why a "performance"
exhaust doesn't deliver the performance they are
looking for. 
  It all has to do with effective exhaust scavenging
from the combustion chamber, and especially, where you
want the increase in power and or torque. (Actually
torque is measured, then hp is mathematically figured,
but that's another can of worms.)
  In my work at Honda Motors North America, Gale Banks
Engineering, and my own performance works, the subject
of this header/exhaust working "better" has always
been debated. It boils down to where you want the
power: high rpm or at low rpm. That's why tuning is
called tuning.
 As a rule, when selecting an exhaust system, the
longer the exhaust runners, the higher up in the rpm
band will you see an increase in power. This has to do
with exhaust velocity being effected by the exhaust
pulses, and how they in turn effect the overall
scavenging in the cylinders.
  Conversely, if you want low-end power, look for an
exhaust system with short runners. The short runners
allow for more effective scavenging at low rpm, and
thus an increase in power. And this is just the
exhaust side of things. You also have to take in
consideration the intake side.
  In your case, Chris, you have an effective exhaust
system that is good for improved power at higher rpm.
Which is what you have seen. The downfall, which
you're also seeing, is that, when at lower rpm, or
when the engine is not working hard--like you cruising
down the 
highway, the exhaust system does hinder the scavenging

of the combustion chambers. When the exhaust velocity
increases, your stumble/vibration goes away, and you
have the increase in power. 
  You also mentioned you have a fuel injected engine.
Given the age and primitiveness of the injection
system, make sure to have the injectors rebuilt or
replaced. They DO WEAR OUT, and you'd be amazed at
what 
a 2 micron size particles flowing out of the nozzles
can score the injector, and thus lead to an overly
rich or lean mixture; both of which can cause the
stumble/vibration.
   Also, check the air mixture/altitude sensors. On
the early mechanical injection system you have, the
sensors were a bit sensitive, so check them out. If it
(they-depends on the year, model, and if it was
replaced) is not properly adjusted, it can cause or
lead to the problems you are having. 
  Sounds, though, like a little bit of tuning of the
air fuel mixture off idle, and when just coming up on
load is in order. Get your hands on an exhaust gas
sensor to check the levels; some of the better tune up
shops have them, and you can check the levels.
  Had fun messing with the old type 3s. Good vehicles.
Hope this has helped. 
Sincerely,
Bob D.

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