area of a circle = pi r squared

therefore
single systems;
1-2.25 " pipe =3.97 sq inches
1-2.5" pipe = 4.9 sq inches
1 -2.75 inch pipe =5.93 sq inches
1- 3" =7.06 sq inches

dual systems;
2- 2" = 6.28 sq inches
2-2.25 " =7.94 sq inches
2- 2.5 = 9.82 sq inches

so if my stock 5.4 litre system seems to be about 2 3/8 which is about 4.5 sq inches, wouldnt a slightly upsized set of duals have to be prettty small to avoid having too much flow and sacrifice low end tq for meager high end power gains?
even a 2" dual is gonna flow 40% more than stock....

 

If you are still running the cats and stock Y-pipe it makes no difference what you have cat back. You won't flow any more than the cats and Y-pipe will flow even with a 6" exhaust system.

 

i probly wont be able to use either in its present form, they will both need heavy modifications or be completely replaced

more specifically the y pipe will be replaced and the cats will be modified if possible to not be a restriction unless of course they r plugged with carbon buildup or something

 

how much would hi flow cats help?

 

im no expert but if my stock cats are still good and the smaller ypipes attached to them are removed, they shud not present a restriction and less likely to set off a cel than hi flow cats that dont get hot enough

 

so I should just keep the stock cats and make a new Y pipe?

 

if you can see through your cats still and there seems to be no plugged up areas, i cant see why not.

in most cases the pipes feeding into the ypipe will flow alot more than the single pipe that makes up the single exit of the ypipe.
for eg if you have 2 - 2" pipes feeding into one 2.5 inch pipe, those 2 two inch pipes feeding two seperate/dual exhaust will flow 40 % more than the the single 2.5 " pipe.
so in those cases the stock cats will NOT likely represent any restriction in a dual muffle design.

even if the pipes feeding the y pipe did represent a restriction such as 2 -1.75 inch pipes feeding a 2.5 inch y pipe, then im sure the pipe could be removed and enough of the cat housing could also be removed to fit/weld on a 2" pipe w/o the former 1.75 inch bottle neck as long as you are not actually cutting into the catylitic material but now you will be pushing the flow limits of the cats themselves and this would only be good for otherwise stock engines.

these scenarios are all assuming fairly stock breathing engines that have no power adders such as hiflow heads, stroker kits, supercharger or nitrous have been added in which cases larger exhaust pipes would be needed and possibly cats too

 

Just keep in mind that if you increase the flow dramatically thru the cats &/or Y-pipe you will need a programmer with custom tune to account for the extra flow or you'll set off a CEL and may have driveability issues. My experience has been that in the rpm range these engines are designed to operate in the higher flow of the exhaust would be of little gain versus the expense. Most of us rarely see more than 2500 rpm so a high flow exhaust and even intake is of no value.

 

so this engine is not the old ones that when you open the air in and out you gain a lot more HP and MPG? Now I am looking into getting a custom from PHP but I have to call them to see what the big benefit over price would be. MPG / HP gain per $

 

Most programmers will give you 10 to 30 hp on a gas engine with the accompanying modifications to add power. As far as fuel mileage gains you'll get maybe 2 mpg max. You will get significant improvements in driveability with the programmer and custom tunes.

 

correction; most programmers will CLAIM to give you 10-30 hp. and what they dont even mention is that the gains will be in the upper rpm bands where you will never notice them unless you goto the track and race the 1/4 very often.

-not to mention any gains are only realized with higher octane more expensive fuel.
-also the minor gains in the upper rpm are almost always sacrifcing equal amts or more of lowend tq, which is what you really need and feel in a truck 99.99% of the time.

i know all this first hand since i have modified the intakes exhaust and/or computer of 3 different trucks ; 97 f150, 2003 dodge hemi, 2010 harley f150

i concluded
1. the larger exhaust made awesome bassy sounds but less tq, less seat of the pants acceleration that outweighed any hi rpm gains
2. the intakes did nothing but add more full throttle sound and empty my wallet
3. the computers/chips i had were a mixed bag since they all had different functions but for the most part didnt notice any significant gains.
the one noteable exception was the transmission shift enhancer i had for the 97 f150. it noticeably firmed up the auto tranny shifts so much in race mode it literalls snapped your head
back when shifting 1-2

all of these after market manufacturers of cai, programmers, exhaust systems throttle bodys, headers etc etc would have you believe the pistons, cranks, valves, heads , etc etc are all designed to flow more air than the intakes throttle bodies, exhausts etc etc can flow....
-maybe in some rare cases this is true but it is by far NOT an across the board premise one can apply to all gasoline engines.
-especially in this new age of high efficient engines im sure most piston, heads valves, cams and other internalls are all very well mated for equal efficiencies and flows to the intakes, exhausts, headers etc.

 

"all of these after market manufacturers of cai, programmers, exhaust systems throttle bodys, headers etc etc would have you believe the pistons, cranks, valves, heads , etc etc are all designed to flow more air than the intakes throttle bodies, exhausts etc etc can flow....
-maybe in some rare cases this is true but it is by far NOT an across the board premise one can apply to all gasoline engines.
-especially in this new age of high efficient engines im sure most piston, heads valves, cams and other internalls are all very well mated for equal efficiencies and flows to the intakes, exhausts, headers etc."

Exactly right

 

Your exhaust is only as good as the smallest diameter within the system...hence the use of mandrel bends in aftermarket exhaust. Factory piped are crimped way down wherever they are bent. If you take a look at the tubing exiting the cats, there is room for improvement in that area. Duplicate the tubing using mandrel bends and a better 2 into one merge where it connects to the tailpipe...and if you have a 3" tailpipe, you can eliminate restriction by using the proper 2 into 1 collector.

 

I was thinking on replacing the pipes from the back of the cat to where the meet then the new dual pipes back.