355 Fabspeed Headers + Capristo= F1 Shriek?

You can hear the flat spot in the 3,500-4,000 RPM range--which is typical of the 4-1 arrangement where the 4-2-1 arrangement avoids this flat spot and start doing its header thing just above 2,600 RPMs.

And it is at least 10 DBa louder than my tinnitus allows.

 

It only does; when you have the option of not having a dead spot at all.

That is the lore.

My CFD simulations indicate that if one tries, one can make a 4-2-1 header that loses absolutely nothing on the high end, has no flat spot, and "comes on song" earlier in the RPM band. If one tries !
This simulation also showed lower gas consumption than the 4-1 header {not by enough to be notices, but one of those funny statistics someone using that simulator might want to know sometime.}

The 4-1 header is certainly cheaper to construct than the 4-2-1 header--and I suspect it is this reason that most aftermarket headers are 4-1--
However, that lack-of-dead-spot in the lower midrange is exactly what the length of the header pipes between the 4-2 and the 2-1 allow you to tune out !!! So, instead of having a just-under-1/2 of the RPM band boosted by header waveflow--one can have just-under-2/3rds of the RPM band boosted by header waveflow. .....