Are there any NA street car engines that can idle and have a reasonably wide power band that are over 90%.VE. I really don’t know, I just sort of remember how my dad calculated Cubic feet per minute. He also had a homemade flow bench in his shop.

 

over 90% VE on a street engine is pretty much any post 2000 dohc engine. minus the mod motor that thing was crap stock.

 

That's why I said "most" of these calculations are rubbish. Not everyone has access to Ricardo wave software and the ability to accurately come to a definite answer. And even then, I would put my money on race teams doing the tried and trued method when all said and done, and doing the final testing on the dyno to confirm the softwares conclusions.

Regarding scavenging - Your statements are a bit broad. The exhaust needs to step up from what point? Every application is going to be different in this regard and also dependent on the desired powerband. A lot of scavenging is produced by an increase in exhaust velocity due to decreasing diameter (such as at the collector). Now by stepping up the diameter of the piping at a certain point (by using megaphone or such) you can help to broaden the scavenging affect (making it less peaky but also slightly less powerful ultimately) by tuning the reflected wave. With this said, the proof is in the pudding, and it's quite obvious that it's not neccesary to have an exhaust that constantly steps up in size to reek the benefits of scavenging. If this was the case, then I suppose you assume that a 3" single exhaust commonly used on these cars doesn't provide any scavenging effect since it commonly comes off a 2.5" dual Y-pipe (which has much greater overall diameter)?

As far as pipe length/diameter past the collector. I agree to an extent, although as long as there are no major restrictions in the exhaust system, it's quite easy to produce an exhaust system with little to zero loss on our cars (even if the exhaust piping is the same size as the header collector). A good reason for this is due to exhaust temps (and mass) significantly dropping as they travel down the exhaust pipe.

Looking at the bigger picture, and putting the mumbo jumbo to the side...I don't understand why the OP or a lot of others in this thread are trying to over complicate things so much. I mean FFS, why get so caught up in your cat back design when you run a utterly crap DC header to begin with, and likely an intake system far from optimized. The writing is already on the wall for the most part, and for 300whp a well flowing 3" single or even dual 2 1/4" (with nice x-pipe) will do the job just fine....with very minimal losses if any at all! Back to reality people