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Exhaust Questions or advice about exhaust systems? This is the place...

 
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post #1 of 9 (permalink) Old 07-25-2010, 11:31 PM Thread Starter
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Exhaust ?

Ok, forgive me, for I am mechanically challened, but when speaking of a "cat-back" system, I am getting that that is the pipe directly behind the converter, through the muffler, then out to the end of the tailpipe. Now, my question is what is the benifit, performance-wise, of having a cat-back system over just replacing the muffler to a free-er flowing muffler? Isn't the only difference the piece of pipe in FRONT of the muffler going backwards to the cat. conv.?
Thanks

'06 silver 4.7 4x4, 2" Rough Country lift, 265/70/17 Goodyear Wrangler Duratracs, 17 x 8.5 MB V-Drive wheels, AEM Brute Force CAI, Optima Red Top, Bilstein HD struts up front, and a couple really cool stickers that add somewhere between 10-13 HP
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post #2 of 9 (permalink) Old 07-26-2010, 06:25 AM
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At risk of starting a firestorm of controversy..........There 'generally' is little performance improvement to be had with any post manifold exaust system.
For the most part, due to the now audible tone, any performance improvement is percieved in the head, but rarely proveable.
Some factory systems may contribute to back pressure at the very highest RPM but in normal driving the engine never sees that type of usage.
The exaust manifolds, again only at the highest RPM, are usually the bottleneck of any exaust system, but IMO, going to a formal header would be a waste of funds for this application.......unless you really did buy a Jeep for drag racing purposes.

So, purely from the performance standpoint, a post catalyst (cat back) system will have similar results to a muffler change, as long as that muffler is a freer flowing design similar to the cat back product.

I could go into 'grand detail' mode, but I'll save that for another time.

Rob


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post #3 of 9 (permalink) Old 07-26-2010, 06:54 AM
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Danny, not to jack your thread but my question is relevant,,,I believe.

Robby - Some of the chip packages state you need a CAI and a Cat Back
system and use premium grade gas to get the full benefits of the performance chips.

Do they improve performance? I guess we have to define what that
performance would be.

Low end speed: Better hole shot
Greater Torque: Better towing ability
Upper end speed: Go faster
Improved gas mileage at nominal operating speeds.
Reduced greenhouse gas effects which reduces global warming.
Makes your butt look smalller.


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post #4 of 9 (permalink) Old 07-26-2010, 07:55 AM
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Quote:
Originally Posted by cico7 View Post
Danny, not to jack your thread but my question is relevant,,,I believe.

Robby - Some of the chip packages state you need a CAI and a Cat Back
system and use premium grade gas to get the full benefits of the performance chips.

Do they improve performance? I guess we have to define what that
performance would be.

Low end speed: Better hole shot
Greater Torque: Better towing ability
Upper end speed: Go faster
Improved gas mileage at nominal operating speeds.
Reduced greenhouse gas effects which reduces global warming.
Makes your butt look smalller.

Fair questions that often come up.
First, the re-programmers (chip)
Basics: Since we are not changing the mechanics of the engine, those being cylinder pressure (cam timing) or compression ratio, the reason they recommend premium fuel is because they are advancing the timing throughout the rpm range.
Regular fuel would detonate with the advanced timing.
Because the timing is advanced and usually the injector pulse width (open time) is a bit longer, the low and mid range power comes on at a lower rpm.
This is that increased 'crispness' off idle you feel and because of the timing and fuel changes, torque will begin to rise at a lower rpm......both timing and increased pulse width will increase power and torque to a small degree.
So, item 1 and 2 are valid from the standpoint of crisper response and a bit more torque.
Remembering that torque is primarily a function of stroke (leverage on the crankshaft), by pushing harder on the piston we can increase it somewhat but the leverage stays the same......kind of like a 150 lb man pushing down on a breaker bar versus a 200 lb man doing the same thing.....one will push harder, but the bar has a limitation before it fails.

Speed: Horsepower is speed.....torque is how fast it gets to that speed.
Since we are feeding a bit more fuel and advancing timing to take advantage of that fuel, the engine can run up a bit faster before it runs out of horsepower, or, in other words, turning at a speed that that is now becomming limited due to cam timing.......it cannot pump any more air.

Improved mileage: Potentally there because if the engine is making more torque, to achieve the same torque that was required to go, say, 60 mph,
you now can maintain that same speed with a lessor throttle opening.

Greenhouse effects eh?
The amount of energy required to create the various components likely negates the greenhouse effect coming out of the tailpipe.....IMO, bogus call to make us feel good about spending money.

The suggestion of Cold Air Intakes and cat back exaust to maximize the effect of the modification applies only to wide open, high RPM operation.
The engine (a air pump in disguise) only pumps air at a given rate based on RPM.
So, if we are cruising at 65 mph and say, 1800 RPM the cfm requirements have not changed....the engine is moving the same amount of air it always has.
But, in a wide open situation, at the VERY TOP of the engines RPM limits there is a possible benifit from reduced pumping losses (intake restriction, exaust restriction) because we are bringing in a bit more fuel that needs to be exausted.....this again assumes the exaust manifolds are not the restriction point (but trust me, they are the culprit).

That may have helped,

Rob


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post #5 of 9 (permalink) Old 07-26-2010, 08:33 AM
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So going back to Danny's question, the cat back would have no effect (other than a little louder) unless you put a performance chip in it, a CAI, and used higher grade fuel....correct?


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post #6 of 9 (permalink) Old 07-26-2010, 09:06 AM
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cico,
The exaust change by itself, generally won't change a thing.

The use of a dynamometer would be necessary to determine what would really change.

No one has and I would be surprised if someone did perform tests on a dyno to see if a restriction or many restrictions are present.
A group of vacuum gauges or manometers would be required.
One in the airbox below the filter, one above the filter, one immediatly before the throttle body blade.
These would help identify a intake flow restriction and again, this would only be seen as the engine approaches redline rpm.
Then, a series of pressure gauges are required to measure downstream restriction.
One in each exaust manifold prior to the outlet flange, one in each exaust pipe immediatly after the flange, one in each exaust pipe just prior to each catalyst, one immediatly after each catalyst, one midpipe prior to the muffler, one at the tailpipe immediatly after the muffler and one just prior to the tailpipe exit.
Then, one can actually see what areas MAY be causing a power loss.
All this testing to address a rpm range that a street engine may see only, what, maybe a hundred times over its entire service life.
So, Danny, you might see an increase with exaust alone, but odds are against it.... no one really knows......alot think they do but with no dyno numbers????? can't say for sure.

But, if you are going to apply other modifications, such as a reprogram, then a freer flowing exaust will help maximize the investment and eliminate one potental restriction point.

Rob


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Last edited by robby; 07-26-2010 at 09:11 AM.
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post #7 of 9 (permalink) Old 07-26-2010, 09:38 AM Thread Starter
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Quote:
Originally Posted by robby View Post
cico,
The exaust change by itself, generally won't change a thing.

The use of a dynamometer would be necessary to determine what would really change.

No one has and I would be surprised if someone did perform tests on a dyno to see if a restriction or many restrictions are present.
A group of vacuum gauges or manometers would be required.
One in the airbox below the filter, one above the filter, one immediatly before the throttle body blade.
These would help identify a intake flow restriction and again, this would only be seen as the engine approaches redline rpm.
Then, a series of pressure gauges are required to measure downstream restriction.
One in each exaust manifold prior to the outlet flange, one in each exaust pipe immediatly after the flange, one in each exaust pipe just prior to each catalyst, one immediatly after each catalyst, one midpipe prior to the muffler, one at the tailpipe immediatly after the muffler and one just prior to the tailpipe exit.
Then, one can actually see what areas MAY be causing a power loss.
All this testing to address a rpm range that a street engine may see only, what, maybe a hundred times over its entire service life.
So, Danny, you might see an increase with exaust alone, but odds are against it.... no one really knows......alot think they do but with no dyno numbers????? can't say for sure.

But, if you are going to apply other modifications, such as a reprogram, then a freer flowing exaust will help maximize the investment and eliminate one potental restriction point.

Rob
Thanks Robby. As always, your answers explain everything very well and always help me. I see now, that a CAI, exhaust, or chip alone may not do a whole lot, but all three together could possibly yield positive reults. Cool. One last question: In terms of adding a cat back "system", or just replacing the muffler alone....any difference at all? From your explanation I would say no, so why does anyone do the cat back systems for 4-600 dollars instead of just replacing the muffler for 150 or so?

'06 silver 4.7 4x4, 2" Rough Country lift, 265/70/17 Goodyear Wrangler Duratracs, 17 x 8.5 MB V-Drive wheels, AEM Brute Force CAI, Optima Red Top, Bilstein HD struts up front, and a couple really cool stickers that add somewhere between 10-13 HP
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post #8 of 9 (permalink) Old 07-26-2010, 01:30 PM
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Quote:
Originally Posted by Danny View Post
Thanks Robby. As always, your answers explain everything very well and always help me. I see now, that a CAI, exhaust, or chip alone may not do a whole lot, but all three together could possibly yield positive reults. Cool. One last question: In terms of adding a cat back "system", or just replacing the muffler alone....any difference at all? From your explanation I would say no, so why does anyone do the cat back systems for 4-600 dollars instead of just replacing the muffler for 150 or so?
Well,
Cat back systems are just that......a system.
The manufacturer of same makes exaust pipes that do not decrease in diameter at the bends and there are no 'accordian' folds at the extreme pipe bends as well.
The muffler is tuned to those pipes and their flow characteristics.
Because of the time and testing it takes to get it right, specifically avoiding 'drone' at steady speeds and the fact that most systems are 100% T-304 stainless (as opposed to 409 used in factory systems) and they have, for the most part a lifetime warranty, so, yep, they are more expensive.

I have 4 vehicles with cat back systems....a 95 Impala, a 96 Impala, a 02 Camaro and a 97 Chev truck.
The truck has a Gibson, the rest have Borla.
All are quiet at cruise with exception to the truck when it is hauling my trailer.
All look like the day I installed them, in the case of the 95, that is 14 years ago.
They are expensive, but I feel I did get what I payed for.
I know I would not be happy with a replacement muffler welded to old pipe because it might give a bit of power......maybe.

You said you're considering a cat back......best bang for the buck.


Rob


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Last edited by robby; 07-26-2010 at 01:34 PM.
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post #9 of 9 (permalink) Old 07-26-2010, 10:24 PM Thread Starter
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Quote:
Originally Posted by robby View Post
Well,
Cat back systems are just that......a system.
The manufacturer of same makes exaust pipes that do not decrease in diameter at the bends and there are no 'accordian' folds at the extreme pipe bends as well.
The muffler is tuned to those pipes and their flow characteristics.
Because of the time and testing it takes to get it right, specifically avoiding 'drone' at steady speeds and the fact that most systems are 100% T-304 stainless (as opposed to 409 used in factory systems) and they have, for the most part a lifetime warranty, so, yep, they are more expensive.

I have 4 vehicles with cat back systems....a 95 Impala, a 96 Impala, a 02 Camaro and a 97 Chev truck.
The truck has a Gibson, the rest have Borla.
All are quiet at cruise with exception to the truck when it is hauling my trailer.
All look like the day I installed them, in the case of the 95, that is 14 years ago.
They are expensive, but I feel I did get what I payed for.
I know I would not be happy with a replacement muffler welded to old pipe because it might give a bit of power......maybe.

You said you're considering a cat back......best bang for the buck.


Rob
Thanks. That'll be my next mod.

'06 silver 4.7 4x4, 2" Rough Country lift, 265/70/17 Goodyear Wrangler Duratracs, 17 x 8.5 MB V-Drive wheels, AEM Brute Force CAI, Optima Red Top, Bilstein HD struts up front, and a couple really cool stickers that add somewhere between 10-13 HP
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