School me on some exhaust questions

[jlew]

I remember my grandfather growing up telling me 'build the exhaust to your application; small rpm small pipes - big rpm big pipes'

was he talking back pressure back then? wasn't the back pressure to ensure the lead in the gas coated the valve seats?
do bikes now with non leaded fuels need back pressure?

I ask because my exhaust has slowly rusted away and is getting replaced. the header is still in good shape and I had glossed over with a spot welder and generally cleaned up.

so now i sit with my 4-1 header and a 6" bullet race muffler. I put a reducer on to take the collector from 2.25 to the 3" diameter of the bullet.

my question is how would this change my power band? (95% of riding is 8k+ RPM on the freeway)

I rode to work this morning and I seem to have a little more power in the upper range, loss of some pull off the line (who cares) and some popping on deceleration. It's quite smooth throughout the RPM range sans midrange; goes from a deep throaty hum to a dirtbike-ish reverb , then back to the deep hum.

edit: oh yeah 1980 CB750 DOHC

 

[Sonreir]

Back pressure is a common myth to see around the Interweb.

You don't actually want pressure in your exhaust system. What you want is gas velocity, and that's usually only achievable by
introducing pressure, so I can see where the misconception comes from...

For the more in-depth response, check this out:
http://www.dotheton.com/forum/index.php?topic=39814.msg471764#msg471764

 

[jlew]

excellent!