RE: exhaust diameter

["Earl H. " <earlh@xxxxxxxxxxxxxx>]

Exhaust size is a relative thing. Remember, the more horse power your 
making the more gas your putting out of the exhaust. The size of the pipes 
should be in proportion to the horse power output, and how you wish the 
exhaust to assist in that horse power development. This changes with volume 
at a given RPM. In a perfect world, you want the exhaust to assist with
combustion 
chamber scavenging. This is not a perfect world, it is an RPM world. With 
any exhaust, the scavenging effect will occur within a narrow RPM range. We
do 
custom programs for several teams that help them determine how to build
exhaust 
to fit track and engine/car specifications. Race teams (NASCAR) will have
several exhaust 
systems for different combos. The real key is velocity @ RPM.

For a street car, start your thinking at your manifolds. If you have stock
manifolds they 
have a relatively small tube size compared to a set of race headers. They
also lack 
correct individual tube length and a collector to even make scavenging
possible. So if 
you put a big exhaust pipe on a small outlet stock manifold, you kill the
velocity of 
the exhaust gas flow. It gets all confused and breaks up, this is when you
will hear 
people say "The big pipes killed my torque". Not really (but possibly), you
just changed 
the RPM/velocity range to perhaps a scale that is beyond being effective for
your 
motor. Next, think where your do most of your driving and where you really
want 
your power. Most street cars do better with more torque than horse power.
Give me 
a 440 with 650 FtLbs of torque and 500 HP over the reverse any day. It will
win 
the street race. So in general for a street car, if you have headers under
2" in diameter, 
and a 3.5 inch collector, or a 3" collector, somewhere in the 2.25 to 2.5
inch exhaust 
will have  better overall seat of the pants performance than the 3 to 3.5
inch range. 
You won't be killing the velocity. To big an exhaust will have an adverse
effect on 
overall engine performance. It is best to error on the smaller side.. 1 7/8
to 1 3/4 is 
a good size for a street header.

Exhaust flow is a very complicated subject. The software to figure out
exhaust flow is 
very expensive, and it can take several days just to set up one study, at
one RPM.

If your running older, less efficient carbs, you will see more gains from
headers and 
exhaust. If your running EFI, you will see less effect from exhaust headers,
and they 
are more costly to produce small gains with EFI. 

Earl


I though the general rule was (as stated by somebody, already) torque
increases with larger diameter pipes and HP decreases.

Mike


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