Trust me, I completely understand this equation and accept it for intake flow. This is basically the same equation that I used in my previous post except that I excluded volumetric efficiency for simplicity's sake.

What I am interested in is the flow of the exhaust at the header which I imagine that takes alot more complex analysis than just 2.2 cfm per hp rule. I believe this is for muffler selection only. You can't tell me that the exhaust flow is uniform and steady state throughout the entire exhuast system. That is just not realistic.

 

In case you hadn't seen it yet, there are some calcs for primary pipe diameter and length in the dcr calculator lines 59 and 60. These aren't derived directly from hp numbers either, but the do come from discplacement, scr and dcr numbers, which should get you pretty close.

I agree, exhaust flow would not be uniform at all. Faster out of the head and slowing rapidly as it cools and reaches the collector. I think this is one the reasons merge collectors work as well as they do - as they neck down to increase velocity at the point where all four pipes from that bank merge. Hopefully, this design leads to a stronger scavenge of the next cylinder to be exhausted.

Velocity and flow calcs would be interesting to see though.

 

Just historical sidenote. I was subscribing to all the car mags from about 1978 into the late 1980s.

When Vizard was publishing his stuff one article at a time he decided carburetor guys were more likely to to mess with exhaust systems then cylinder head porters. And carb flow benches were just about a dime a dozen, while a flow bench that could handle a cylinder head was pretty uncommon outsidethe big three R&D labs.

So he messed around measuring the flow capacity of various off the shelf mufflers and straight pipe sections with a carburetor flow bench. Using a measurement standard lots of people could duplicate, he dyno tested exhaust systems of known, repeatable flow capacities and came up with 2.2 cfm per flywheel HP as the sweetspot, at 28" H2O suction.

Just figured I would throw it in thread if someone wondered where it came from.

In my shade tree experience, if you can get your exhaust system flowing within 10% of your paper target you have spent a lot of time on it and your are going to be pretty happy with it.