just as a basic rule of thumb, its not always limited to your displacement, N/A OHC engines tend to need more backpressure for optimal running. pushrods with open heads tend to run a bit better. with proper tuning when you open the headers what you're going to do isn't necessarily LOSE a ton of tq but you're going to watch your tq lines shift wayyyy higher in the RPM range. however without any tune work your computer may become a bit befuddled by the sudden lack of backpressure, you'll probably start running lean due to the sudden increase in airflow. it can also throw off the valves and stroke timing in your engine, your tuned to have bad air moving out at a certain time and clean air in at a certain time... changin the rate at which air leaves the engine will change that tune..
as a former camaro owner this is a really helpful explanation of what is going on with open headers it gets a bit words so here are some key points:
http://www.fbody.com/anything/950661
When the exhaust pulse exits the cylinder and
meets a negative pressure zone, it will effectively
get sucked out (scavenged) if the timing is right.
This is why RPM is a factor because it is variable
whereas the pipe length, and valve timing is not.
In the graph below, you can see that at the point
of exhaust stroke and overlap, the pressure wave
is below 0 PSI which helps draw out the gas from
the cylinder.
Getting on the topic of backpressure. Those who
claim you need mufflers to make power often say
this because the engine feels weaker when they
remove the secondary length of pipe from their
system.
Most of the time, people will drop the exhaust
right after the collector and the bottom end
power is lost.
To justify this, they say "I've lost backpressure
and therefore my engine is weaker."
In effect, what really happened is, you have
shortened the tuned length and thrown off the
timing of the reflected pulse back to the exhaust
port.
So...instead of getting a negative peak, you
may get a positive peak when the exhaust valve
opens which prevents some of the gas from leaving
the cylinder.
What does that do? It leaves dirty exhaust gas
in the cylinder and it mixes with the intake
charge on the proceeding intake cycle.
Now instead of having nice cool, fresh air and
fuel, you also have hot, inert exhaust gas mixed
in.
This acts like "EGR" and reduces the power
potential of the power stroke.
...still wondering what stock pipe size is...
