[dudeiwin86]

my longtubes make more horsepower and torque off idle than my shorties did.
i have the graphs to prove it too.

green= shorties w/ no cats
red = FRI Custom Stepped longtubes w/ cats

car is running on the same tune in both pulls with no modification to the tune whatsoever.
the car barely ran on the old tune and was updated a few weeks later when i did my head and cam install.

had the custom tune actually been modified for a catted car with longtubes, i can garuntee mor ehorsepoweor and more torque because the car was running WAY stupid rich.
also notice the worse weather conditions during the longtube pull.

longtubes are better for LOW END TORQUE
and TOP END POWER

dacaman had a good thread recently, and its not as much about backpressure, as it is "tuned length" of primaries to match exhaust harmonics.
here is a quote from one of dacaman12's posts (he grinds cams for arrington)

---- Primary Pipe's Harmonics ----
1st Harmonic = 127.5 inches long ... typically never used
2nd Harmonic = 50.2 inches long ... longest recommended
3rd Harmonic = 31.3 inches long ... highly recommended , best Torque Curve
4th Harmonic = 22.7 inches long ... shortest recommended
5th Harmonic = 17.8 inches long ... typically never used
6th Harmonic = 14.7 inches long ... typically never used
7th Harmonic = 12.5 inches long ... typically never used
8th Harmonic = 10.8 inches long ... typically never used

a "primary pipe" would be the one of the 4 coming from the head, to the collector (where the for pipes meet and turn to 1 pipe)
he used some sort of calculator for designing race car exhaust, i really dont know the details.

31" i bet Longtube primaries arent quite that long, but its a lot closer to a "reccomended harmonic" than shorties.

i dont know how much faith can be put into that calculation i quoted, but hey..... i picked up torque and horse...
this "backpressure" nonsensical arguement for shorties doesnt fly.

the only reason to run shorties is if you live in cali

 

[dudeiwin86]

thats are "mopar" midlength headers.
theyre kinda oldschool since they dont make them anymore...
they were originally made by AEM for mopar
and they were for 5.7 cars only, didnt fit 6.1s

now, from what i understand, the Mopar brand midlengths are now being made only for the 6.1, and not the 5.7

they stopped production and made a 180* on what motor they made them for, beats me, they were pretty popular because they were cheaper than most offerings, but still a longer tube than shorties, but not quite as long as true LTs

tbey have always been cheaper because they are a coated mild steel or something (not stainless).

 

[dudeiwin86]

I've never measured exhaust temps myself, but the reason I'm concerned and very interested in your input about this subject is that I have a full SRT exhaust in my RT, and my exhaust ports are matched to the 6.1 exhaust manifolds (not exactly port-matched to this specific combo, but rather they are opened up to be the same shape and size as the SRT manifolds). Given the larger diameter tubes from midpipe to catback, and what I believe to be a more efficient setup than having RT shorty headers, would it make a big diff to go with longtubes at this point? Headwork & cam swap has been done too.

it depends on how you define efficent.
its a good thing you have a H/C swap, or youd probably be too big on the exhaust side.

going to a larger exhaust diameter decreases velocity of the exhaust stream.
velocity is what creates a vacuum effect and pulls more exhaust out of the cylinder head, and more fresh air in.

velocity makes power.

if you dont make enough power/exhaust gas to maintain a high velocity, you will experience power loss.

larger exhaust pipe on a stock vehicle will surely show power loss, on a head cam car, it problem makes enough gas to use SRT sized piping. i would never go any bigger than a 3" pipe tho.

my point is..
"efficiency" is not removing "backpressure" by going to a bigass exhaust.
efficency is having a exhaust pipe *barely* big enough to support XXX ammount of exhaust gas so that you maintain maximum velocity without having an excess ammount of static pressure on the system.

 

[dudeiwin86]

further more, i dont know the first thing about calculating CFM of exhaust gasses in a car.

i do know about using CFM of an airstream to design an HVAC system
*busts out his "ductilator*

using dacamans number of 164cfm of exhaust...
(*edit i was thinking of doing these numbers for the individual exhaust primaries off the head, then i got to wonder.... did you mean 164 cfm per cylinder or 164 cfm per head or 164 cfm total the motor creates? this would actually change some of the numbers below.... and then tehre is factoring in the xpipe and what it would do... but the numbers below still apply, and show a general guideline of what is going on and how things relate/scale*)

using 3" pipe:
Friction Loss (inches water per 100 ft duct): 7.1
Air velocity (ft/min): 3341

using 2.75"
Friction Loss (inches water per 100 ft duct): 10.98
Air velocity (ft/min): 3976

using 2.5"
Friction Loss (inches water per 100 ft duct): 17.72
Air velocity (ft/min): 4811

you can look at the values and see going for 3" to 2.75" only increases static pressure by roughly 3.8", while going from 2.75" to 2.5" sp by 6.8".

while going from 3" to 2.75" increases velocity by 635fpm and going from 2.75" to 2.5" increases velocity by 835fpm

as you can see, when that delta (change in) static pressure from 3.8 to 6.8 is nearly50%, while the increase in velocity is roughly 25%.
you face a lot of dimishing returns.

the most BASIC way i can define static pressure is:
the ammount of pressure that a fan(motor) must overcome to move the airstream(our exhaust).

so at what point does the static pressure become so much that the motor is working harder to move the exhaust out, causing a power loss, than it is gaining from a higher velocity.

to be honest i dont know how to apply these numbers in regards to "what is too much static pressure?" when is the static pressure so much that it is actually preventing you from getting all of the exhaust out of the cylinder head?

i wish i could explain this better, but maybe some one can use these numbers better than i can.

i will continue to research what i can, as i am quite interested myself now that ive put these numbers on paper

 

[dudeiwin86]

Dude, your calculations are in the ballpark, but need to include the secondary vacuum effect as exhaust gas rushes down each runner to the collector and as RPMs increase there's a drop in resistance within each individual runner. Again, push and pull working together.

very true,
im applying what i know about HVAC design, and this is definately a different type of system all together.

i really wish i had more theory to apply,
looking forward to what dacaman has to say

 

[dudeiwin86]

forgot about this thread...

i had an indept conversation with my boss about static pressures in an exhaust system
he is a mechanical engineer with 30+years experience.

one point he made to me was that he cant convince himself that you can actually draw a vacuum in a cars exhaust system

that is to say.. one exhaust pulse does not actually PULL the next exhaust pulse faster thru the pipes

one thing we did cover was that a restriction in your exhaust will travel THRU the head and cause a higher pressure thru that intake runner and limit the ammount of air able to bbe taken in on the intake stroke.

we started to talk some about pipe size vs velocity, but it would literally take days of calculation with firm numbers to determine how to size one pipe in accordance with static pressure vs velocity.

dont have time to elaborate more now, but these ideas make give you something to think about

 

[dudeiwin86]

does JBA make a header that matches the 09 port?
3" on the catback side might be a little large for a stock motor.. then again, it is an 09 car... new animal all together.

aside from that id say your spot on brother!

 

[dudeiwin86]

the only downside is youre losing a lot of velocity in the exhaust stream as soon as you step into that bigger tube,

it may actually be far enough downstream to have minimal effect since the core parts of the system are sized smaller