There you go . That's 1 reason the 180 was developed.
Port configuration is what made the Hemi famous, not so much as the hemispherical head but both high flow port & head, with only 2 valves per cylinder. There were many atempts made to correct the flat spot ( actualy more of a strangeness phenomenom ). Ford made tunnel port heads that realy only worked at high rpms, tunnel port intakes were built with huge plenums ( high rpms only, but had cushion ) and today with fuel injectors at the intake valve this still happens.
As Don points out this is a hardware problem. At one time I was told that the flat head was not as prone to having a flat spot but who knows. Maybe someone with a 48 Merc could put it on a dyno......

 

How did you see my cfm?


Don't tell me.... there's pictures on the internet

 

Are you kidding.....EVERYONE has seen your cfm .

 

Well then you know it's actually CFM

 

There is a difference in my exhaust manifold “guess” and the one on the Precision MINI site. As I see it the port extension would be a sleeve within the primary; it need only be as short as a half inch or so in length and have a smaller OD as the ID of the primary pipe to be effective. It really isn’t a guess, it’s an established design that has yet to show up on any widely available (non custom) header for the MINI. A well executed anti-reversion step in the exhaust header can forgive some sins in a designed by committee intake.

 

Are you saying that the sleeve's OD should fit snug in the primary's ID, or should there be a small gap?

 

A gap, more on the short side (bottom), and some on the top, just like stock; a little on the sides is ok but it to get that may require a larger primary pipe than needed (not ok).

 

The Internal Combustion Engine in Theory and Practice by John Haywood is an excellent source for those interested in studying the issue in detail.

 

Does the gap start right at the flange? How are you able to weld a sleeve in the primary if there is a gap all the way around? or do you make it snug on the sides, and tack it there?
Sorry man, I haven't had my header off yet, I'm just trying to get a visual.

 

Long ago, I tried to improve some Pontiac exhaust ports (those familiar with such ports know there is a sharp right angle turn right off the valve seat). I applied the then modern thinking by leaving a bump in the floor of the short-side radius while lowering the sides. This central fin worked as expected to help the exhaust turn the corner and CFM was greatly improved... at high valve lift. The problem was that at lower valve lift, the airflow began pulsing (loud enough to hear on the flowbench) and the resulting turbulence destroyed the flow there, even at steady state operation. Add the dynamics of abruptly closing valves of varying frquency (rpm) + a throttle and it's pretty easy to expect reversion to happen sometimes.

Sometimes you can actually see reversion. Folks from the old-school should remember seeing clouds of vapor hovering over the carburetors of engines equipped with long-duration camshafts. See, carburetors are simple devices that happily meter fuel into the airstream no matter which direction it is moving. Those huge, lumpy-idling cams caused so much reversion that fuel was metered three times as the air went down, then back up and down again to cause an excessively rich condition... and a big flat spot. While EFI can take care of the too rich part, when conditions are just right (wrong?) and reversion back into the intake port occurs right before the intake valve closes, a sort of reverse supercharging effect can also cause a flat spot.

It is well known that tuning plenum size and runner length can be used to produce a supercharging effect at particular RPMs, so we have plenty of manufacturers offering variable runner lengths like Mazda or plenum size like Yamaha (SHO) to extend this range. But the number one priority for the MINI engine was undoubtedly packaging so we kind of have to expect things to be less than optimal.

You know the hollow plastic chambers in the intake tract of some cars that everyone removes? They're not always just for controlling unpleasant sounding resonances, and sometimes actually are intended to absorb reversion so eg. an AFM doesn't keep slamming open and shut, so I wouldn't be surprised if something as far away from the intake ports as a DFIC actually could affect some things.

An anti-reversion step is merely a chamfer on the port of the outgoing side: the intake ports on the head or the header flange.

Anti-reversion cones actually reduce the ID of the port and may cause power loss across the powerband unless the header is grossly oversized. http://www.nrhsperformance.com/tech_arcones.shtml
http://www3.bc.sympatico.ca/Volvo_Books/arcone.gif

Anti-reversion chambers are generally placed a few inches past the header flange: http://www.stefanoparis.com/crx/ias/images/52.jpg
http://www.stefanoparis.com/crx/ias/images/53.jpg
http://www.eastcoastseaplanes.com/antireversion.jpg

 

There you go. Nice post

With a heavy enough cam, on a 4 cylinder, with some DOECs, you can watch raw gas spitting right out. Some years ago I built an open class Briggs motor for my son & it would spit out almost as much meth as it burned at idle. Idle being a relative term 1600rpm.

 

Been there; done that!

 

maybe the cooler air is giving a leaner mix ;giving more torque lower down. i got more torque much lower with GIAC and it pulls from way down now . good on you .

 

well his flat spot went away with dfic so put that in your bicycle bag .

 

grasshopper ...if the tree falls with no one present... did the tree fall ? (passes blunt to left).

 

and there it is ....no flow bench either . if it feels better ;it must be better.