Couldn't resist.Lets keep free flow terminology outside for a few minutes... and also consider cams and valves into the picture.

An Exhaust system is designed to match your cams , your cam influences how u design your exhaust, You cant just say "Dude lets bolt on a "X" cam with "Y" duration and then bolt on a "A" header and a "Z" muffler because the dyno chart in some mag said they make "X" makes 8 BHP and "A" makes 12 bhp and "Z" makes 5 BHP" so do the math : 8+12+4= 25 BHP

Without understanding how they work with each other..
think like this--> is everything they have is same as yours ? does the low end torque type of cam match with your "super Hi Flo ultralo back pressure exhaust system" match ?????
and lets back up a bit to the basic question...what do you want ? (a)low end, (b)mid range or (c)top end performance?

Any of these 3 can be obtained by a free flow exhaust system and combination of your cams.In simpler words, your cylinder has explosive mixture which explodes at a optimum pressure and spark timing.It works best a particular rpm band, you get good torque . But as you increase your revs you need to evacuate this gas at a higher rate.
this is where your exhaust velocity and back pressure can be explained.
Now question is, do you want power at low or mid range ? if yes then make sure the valves do not overlap to much i.e the duration the intake valve is open along with the exhaust valve.. as per "textbook" definition in a 4stroke the exhaust remains closed when the intake is open and vice versa... but in engine these valves remain open for a certain amt of degrees of the camshaft together which is overlapping with each other... this overlap creates a scavenging effect pulling some of your intake charge in to the exhaust.
And what happened ? intake charge going into exhaust... ???

Again in simple words, this helps in maintaining your exhaust velocity, you see the gas is flowing much faster thru your system at higher revs due to this valve overlap... now if the exhaust system(headers, pipes,cats and mufflers) are optimised for this they will allow for low backpresure and quick flow of gasses thru your system.

But now the low end torques is lost as the intake charge is being lost due to scavenging but u get good BHP gains at higher revs..(I.E,your example of the Toy getting more hp)

If you want low end torquey engine choose a exhaust system which matches your cams with lesser overlap and more exhaust backpressure and exhaust velocity matching this setup.
So if u have a high rev power requiremenst go for cams with greater ovelap and a exhaust sytem which allows for greater velocity of the gasses to flow, consider this .. a narrower pipe would increase the velocity of the gas as compared to a pipe which is larger in dia for the asme pressure . e.g water flowing thru a narrow pipe, then coonnect a larger dia pipe and see the flow of water which is faster? .. the narrower one. so this doesnt mean you should use narrow pipes, what Im saying is "the dia of the pipe should be such that it creates best exhaust velocity for providing scavenging matching your cams ovelap"

 

Heres a good link on a TEST done on a converter to see how the VELOCITY is effected by it.If you notice it really remains the same through the cat,and mind you,this is with a stock converter......

http://www.fluent.us/direct/dieselnet-listing/ex185.pdf


But as I keep saying charts and diagrams and links to posts mean little compared to REAL FIRST HAND KNOWLEDGE.

 

Here are some good links/info about Back pressure and cats.
http://www.dakota-durango.com/forum/...t=backpressure
http://www.longbeachmuffler.com/faq.html

I'm done arguing though. Your just dragging it out. I said my point and i will post my Theory when its done. I always do...

Either way i would much rather have high end power pulling me those last inches down the track rather then off the line, we make enough torque we dont have to worry about bottom end loss, we need the high end more, and a HFC wont help that! you want less BP

HFC use it if you want to increase the flow a tad and not loose much bottom end, but like i said above, in our trucks the 344#tq created stock is VERY high for a 250hp motor, we can take the loss, where as we need the high end anyways to make up for the low HP to pull us through the last few inches of the track instead of ******* out, and getting beat by a 200hp 4cyl with no torque.

In my opinion, i would go cat free and not use the HFC.

 

oh boy, this thread made me read more than i read all of 2008 combined so far

thanks for the input though, i will have to consider my options on this one, i may just hold of until i have the exhaust ready and i have a converter waiting, then do them both together and be even happier.

 

Im not arguing and how am I dragging it out?Im only posting in response to your posts.

But as for needing top end for running down the track,it's actually low and mid range power you need for quarter mile times,not top end.Your 250hp motor is not putting out enough exhaust flow to benefit from no back pressure.As Ive posted,they work hand in hand and the vacuum created by your exhaust is what gives you the velocity you need. Not enough of this and you are just hurting your torque.

But I will step away quietly and wait for your posts after you have tested it.

Im sorry if I came across like im arguing.Only trying to post info that I have first hand knowledge on.Like I said before,you can post links as well as I or anyone else with claims to support both of our points,but the real truth is in trial and error.