Here is my $.02 for what it's worth;
I have my own dyno, 605ci street hemi, through open headers and collectors it made 853hp and 818ftlbs. Back to back pulls 15 minutes apart, engine and oil at same temps. With the full exhaust hooked up,(3-1/2" pipes, X-pipe and 3-1/2" Ultra-Flows that dump right after the mufflers. It made 117hp and 97ftbls. LESS.Dave

People... what you need to consider when talking
exhaust on a car is back pressure.. how much
back pressure do you have.. I know MOST of you
have NEVER measured it.. try it and see what it
is... then try to reduce it.. its simple to do
but MOST dont do it

What about a 2 into 1 system like a Magnum truck exhaust? Using headers of course and 3" piping/muffler .
RT

605cid Hemi...??? Hahahahahaha... Dave... i do not think you and i live on the same planet. HALF ov what you're making will be music to my ears in this roach... Hahahahahahahahaa

I appreciate the answers, but some ov you are not reading. I'm building a SIDE-EXIT exhaust. I could run 5" without drama. Still on the fence here... and to further skew the equation, i'm really starting to lean towards building a 318 instead ov a 360... I cant get a good price on a Magnum 360, and i want to run a lightweight flywheel... Hmmm...

F.M.E....I use to run the 3.5" exhaust with bullet then switch to 3 " flowmaster two chamber both exits in front of the rear end. Same et/mph.

This pic of my A-body 3" is with relocated springs and the car weight supported by the axle, not the frame. Who needs a gas tank anyway...

I appreciate the answers, but some ov you are not reading. I'm building a SIDE-EXIT exhaust.

I understood, that why I posted I run 3.5 collectors into 4 inch side pipes that exit just in front of the rear wheels.

With the inserts out my wife says she can hear me over a mile away at night where we run and test.

I dont think I have any back pressure in my side pipe setup.

Here its a toss up on weather the po po wants to question the side exit exhaust, but most can be made to be confused enough that they wont write a ticket they are not sure about as long as the violation wasnt noise related.

My next mild street car will have 3.5 inch exhaust all the way to the rear bumper, that way if I decide to step the power up I won't need to change the stinking exhaust system again

I run a full 3" system with an X-pipe and thats because I was figuring to make in the 550 to 625 hp range and I knew I would race through the full exh all the time. Honestly a 2.5 system will work fine for your setup. Myself if I planned to make close to 600 hp and I can run any size I want then I would use the 3". If tailpipe room is tight then you can run 2.5 tailpipes on a 3" system and not loose any power. I went with the 3" tailpipes also because my B-body has plenty of room as you can see and my tailpipes have not hit or never rattle at all. They fit fine on my stock B-body with no tubs and the springs in stock location. For what you are doing I would run 2.5 unless you plan to step up alot in the near future. Ron

I went from a 2 1/2 system with x pipe and bullet mufflers, to a 3 inch system, with x pipe and ultra flows.... I saw a bigger gain switching to a 1000cfm Hp series carb, then I did from the exhaust change. This is on a 550hp 526 stroker.

Close to 700 hp and have ran a 3 1/2" side exit exhaust forever and never a problem w/the police not even w/slicks on the street.............They are a bit loud but I don't buy the idea that small pipes make hp in a big motor, just not true and I know of a few 8 second and quicker NOS cars w/some BIG exhaust...........

Again.. I will ask what is your back pressure...
we still dont know crap yet.. if you can say
what your REAL back pressure is we would have
a clue to what the flow of the system is.. everyone
wants to throw out HP numbers but if system X is
this much back pressure and system Y is this much
it will say A LOT.. I measure mine so I do have a
clue to what it is.. a simple gauge hook up will
tell you
EDIT
And state at what RPM

Mr. P Body, What do you use to measure back pressure? I would love to learn this.

You're supposed to use piezometer rings to measure the pressure around the pipe, but a simple tap with a pressure gauge is good enough.

For a horsepower reference, the 2013-14 GT500 made 662 hp out of a supercharged 351" V8. It has 2.75" exhaust to the cats, and 2.5" pipe to the mufflers mounted at the back of the car. The Hellcat is the same- 2.75" exhaust, 707 hp from less than 400 cubes.

Very simple.. a inch of vac gauge( its inches of
merc,HG).. drill it into the exhaust.. I just use
a tap thats 1/8 pipe ad that way I can screw is
a 1/8" pipe fitting.. once your done I just usually
weld it close.. unless I need more readings.. very
simple.. I did a few hundred of these at work

A local dealer tech told me to put a pressure gauge ahead of the catalytic converter and as long as pressure was below 3psi the convertor was good. Will this work on a converterless exhaust?

Without all the crazy do this, do that, whos way is right whose way is wrong, just make your junk as free flowing as possible hit the timers and if your not pleased do more and compare.

How long and how much money does a hellcat designer or gt500 or any oem designer spend on performance ex research alone... id bet a long time and alot of money, our stuff, at least my and others carbed stuff is not rocket science, simply a to b as quick as possible and try to tune it all for the best.

This debate would be different if 90% here had national record holding cars.

They say impersonation is the best form of flattery, copy someones good system and thank them for it.

Where did the converter part come into play? Gutem and run a straight pipe through them.

Thank you! Simple and easy I will be doing this.

I had 3" put on my Duster with turn downs in front of the axle. Why? Because that is what I wanted.