Found this on DirtracingForum
One thing that I don't see anymore is the addition of exhaust tubing to the headers. At one time you would see some guys with 18" to 36" extensions! Do you remember when you tuned the exhaust length by painting a stripe on the exhaust and cutting it where the stripe didn't burn off!
Technology has came a longs way!
This was a old way to help tune the exhaust system before header technology and extensive dyno testing was being used by (for the most part) non-professional racers.
With most engines coming from reputable engine builders, they are researched out enough that this isn't necesary. However if you have the wrong headers/cam for your combo you can at times see a fair gain in torque by doing this
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Things have advanced & there is definitely a science to it.
The length should be mathematically determined to maximize engine breathing but I don't know the formulas involved.
Since an engine is really an air pump anything to improve breathing volume increases output.
The best arrangement creates a pulse in the exhaust pipe where the low pressure behind each "puff" actually helps pull the next "breath" along & helps scaveng spent fuel out of combustion chambers.
Here is an depth article covering this subject by respected exhaust guru Jack Burns.