I have flowed many of the designs on my flow bench with a head mounted backwards so the headers can be mounted and flowed through the exhaust port into the header. The headers get so hot on the flow bench that you can't touch them for several minutes after flowing them. If the exhuast port is a good flowing port without a tube, any size header will help pick up the flow unless it has a sharp bend immediately after the port opening. Every angle that is welded into the tube will cut flow some, and turns/bends that are smooth do not. You can build engines to make close to 800hp with 1 3/4 header primaries if the collector is merged correctly. Stepped headers should have the primary tube volumn for first step of at least the cubic inch of that cylinder before the next step up in tube size, and so should the second step, etc. A street car will always run better on the street with a smaller header primary tube size than most folks think is appropriate.
Factory exhausts had the exhaust pipes after the mufflers smaller for a reason. It has been a proven fact that the exhaust cools enough back to the mufflers under normal driving conditions to allow the use of a smaller exhaust after the muffler and keep velocity up in the pipes on the street. Also, think of the jet engines with the afterburner, for them to speed up the exhuast and make more power, they squeeze the exhaust down considerable to aid in acceleration. And as the power comes up, they open in stages to keep the velocity at its peak. Automobiles don't have adjustable exhaust pipes, but squeezing the pipes down after the primaries with a merge collector does the same thing. Smaller exhaust tubing after the muffler is a similiar affect. Joe-JDC
Joe, I agree you CAN use smaller pipes out back, but I have not found where
cooling was a "proven fact" and certainly haven't seen any proof that large tailpipes hurt other than noise control. Primaries being too big, yes, collectors being too short or too big with open exhaust, yes, but not diameter further back.
Matter of fact, I would say the cooling theory is
only theory based on interpretation of what we both agree we see with smaller pipes. My personal opinion is that it isn't based on cooling, if it was, eventually you could have an exhaust long enough, maybe an ice cooled 5/8 heater hose, but no matter how cool it was, I don't see that 5/8 tube flowing enough.
That being said, I spent a ton of time researching it, with no solid answer....but we agree that you CAN go smaller. Knowing there is a relatively violent exhaust pressure spike and then negative spike that decays over distance, my interpretation is that is why the tailpipes can be smaller because that spike and recovery minimizes. After that it is just an area calculation, however even then the exhaust experts don't agree completely on anything and I haven't found a definitive answer.
I'll also go one step farther and agree with the exact words you used..."in normal driving" I can probably buy into the fact at light throttle there isn't much heat back there, but when your foot is in it for a continuous period, racing or pulling a hill, I think things stay pretty hot all the way to the tail pipe exit.
However, I agree on what you said about sizing and hp requirements and the fact that many people use too big of a primary pipe.
hrtatk1 - I run my pipes all the way back and turn down in basically stock location at the back valance
